respiration, human


respiration, human

Introduction

      the process by which oxygen is taken up and carbon dioxide discharged.

The design of the respiratory system
 The human gas exchanging organ, the lung, is located in the thorax, where its delicate tissues are protected by the bony and muscular thoracic cage. The lung provides the organism with a continuous flow of oxygen and clears the blood of the gaseous waste product, carbon dioxide. Atmospheric air is pumped in and out regularly through a system of pipes, called conducting airways, which join the gas exchange region with the outside of the body. The airways can be divided into upper and lower airway systems. The transition between the two systems is located where the pathways of the respiratory and digestive systems cross, just at the top of the larynx.

 The upper airway system comprises the nose and the paranasal cavities, called sinuses, the pharynx, or throat, and partly also the oral cavity, since it may be used for breathing. The lower airway system consists of the larynx, the trachea, the stem bronchi, and all the airways ramifying intensively within the lungs, such as the intrapulmonary bronchi, the bronchioles, and the alveolar ducts. For respiration, the collaboration of other organ systems is clearly essential. The diaphragm, as the main respiratory muscle, and the intercostal muscles of the chest wall play an essential role by generating, under the control of the central nervous system, the pumping action on the lung. The muscles expand and contract the internal space of the thorax, whose bony framework is formed by the ribs and the thoracic vertebrae. The contribution of the lung and chest wall (ribs and muscles) to respiration is described below in The mechanics of breathing (respiration, human). The blood, as a carrier for the gases, and the circulatory system (i.e., the heart and the blood vessels) are mandatory elements of a working respiratory system (see blood and cardiovascular system (circulation)).

Morphology of the upper airways
The nose
 The nose is the external protuberance of an internal space, the nasal cavity. It is subdivided into a left and right canal by a thin medial cartilaginous and bony wall, the nasal septum. Each canal opens to the face by a nostril and into the pharynx by the choana. The floor of the nasal cavity is formed by the palate, which also forms the roof of the oral cavity. The complex shape of the nasal cavity is due to projections of bony ridges, the superior, middle, and inferior turbinate (nasal concha) bones (or conchae), from the lateral wall. The passageways thus formed below each ridge are called the superior, middle, and inferior nasal meatuses.

      On each side, the intranasal space communicates with a series of neighbouring air-filled cavities within the skull (the paranasal sinuses (sinus)) and also, via the nasolacrimal duct (tear duct and glands), with the lacrimal apparatus in the corner of the eye. The duct drains the lacrimal fluid into the nasal cavity. This fact explains why nasal respiration can be rapidly impaired or even impeded during weeping: the lacrimal fluid is not only overflowing into tears, it is also flooding the nasal cavity.

      The paranasal sinuses are sets of paired single or multiple cavities of variable size. Most of their development takes place after birth, and they reach their final size toward the age of 20 years. The sinuses are located in four different skull bones—the maxilla, the frontal, the ethmoid, and the sphenoid bones. Correspondingly, they are called the maxillary sinus, which is the largest cavity; the frontal sinus; the ethmoid sinuses; and the sphenoid sinus, which is located in the upper posterior wall of the nasal cavity. The sinuses have two principal functions: because they are filled with air, they help keep the weight of the skull within reasonable limits, and they serve as resonance chambers for the human voice.

      The nasal cavity with its adjacent spaces is lined by a respiratory mucosa (mucous membrane). Typically, the mucosa of the nose contains mucus-secreting glands and venous plexuses; its top cell layer, the epithelium, consists principally of two cell types, ciliated and secreting cells. This structural design reflects the particular ancillary functions of the nose and of the upper airways in general with respect to respiration. They clean, moisten, and warm the inspired air, preparing it for intimate contact with the delicate tissues of the gas-exchange area. During expiration through the nose, the air is dried and cooled, a process that saves water and energy.

      Two regions of the nasal cavity have a different lining. The vestibule, at the entrance of the nose, is lined by skin that bears short thick hairs called vibrissae. In the roof of the nose, the olfactory organ with its sensory epithelium checks the quality of the inspired air. About two dozen olfactory nerves convey the sensation of smell from the olfactory cells through the bony roof of the nasal cavity to the central nervous system.

 For the anatomical description, the pharynx can be divided into three floors. The upper floor, the nasopharynx, is primarily a passageway for air and secretions from the nose to the oral pharynx. It is also connected to the tympanic cavity of the middle ear through the auditory tubes that open on both lateral walls. The act of swallowing opens briefly the normally collapsed auditory tubes and allows the middle ears to be aerated and pressure differences to be equalized. In the posterior wall of the nasopharynx is located a lymphatic organ, the pharyngeal tonsil. When it is enlarged (as in tonsil hypertrophy or adenoid (adenoids) vegetation), it may interfere with nasal respiration and alter the resonance pattern of the voice.

      The middle floor of the pharynx connects anteriorly to the mouth and is therefore called the oral pharynx or oropharynx. It is delimited from the nasopharynx by the soft palate, which roofs the posterior part of the oral cavity.

      The lower floor of the pharynx is called the hypopharynx. Its anterior wall is formed by the posterior part of the tongue. Lying directly above the larynx, it represents the site where the pathways of air and food cross each other: Air from the nasal cavity flows into the larynx, and food from the oral cavity is routed to the esophagus directly behind the larynx. The epiglottis, a cartilaginous, leaf-shaped flap, functions as a lid to the larynx and, during the act of swallowing, controls the traffic of air and food.

Morphology of the lower airways
The larynx
      The larynx is an organ of complex structure that serves a dual function: as an air canal to the lungs and a controller of its access, and as the organ of phonation. sound is produced by forcing air through a sagittal slit formed by the vocal cords (vocal cord), the glottis. This causes not only the vocal cords but also the column of air above them to vibrate. As evidenced by trained singers (singing), this function can be closely controlled and finely tuned. Control is achieved by a number of muscles innervated by the laryngeal nerves. For the precise function of the muscular apparatus, the muscles must be anchored to a stabilizing framework. The laryngeal skeleton consists of almost a dozen pieces of cartilage, most of them very small, interconnected by ligaments (ligament) and membranes. The largest cartilage of the larynx, the thyroid cartilage, is made of two plates fused anteriorly in the midline. At the upper end of the fusion line is an incision, the thyroid notch; below it is a forward projection, the laryngeal prominence. Both of these structures are easily felt through the skin. The angle between the two cartilage plates is sharper and the prominence more marked in men than in women, which has given this structure the common name of Adam's apple. Behind the shieldlike thyroid cartilage, the vocal cords span the laryngeal lumen. They correspond to elastic ligaments attached anteriorly in the angle of the thyroid shield and posteriorly to a pair of small pyramidal pieces of cartilage, the arytenoid cartilages. The vocal ligaments are part of a tube, resembling an organ pipe, made of elastic tissue. Just above the vocal cords, the epiglottis is also attached to the back of the thyroid plate by its stalk. The cricoid, another large cartilaginous piece of the laryngeal skeleton, has a signet-ring shape. The broad plate of the ring lies in the posterior wall of the larynx and the narrow arch in the anterior wall. The cricoid is located below the thyroid cartilage, to which it is joined in an articulation reinforced by ligaments. The transverse axis of the joint allows a hingelike rotation between the two cartilages. This movement tilts the cricoid plate with respect to the shield of the thyroid cartilage and hence alters the distance between them. Because the arytenoid cartilages rest upright on the cricoid plate, they follow its tilting movement. This mechanism plays an important role in altering length and tension of the vocal cords. The arytenoid cartilages articulate with the cricoid plate and hence are able to rotate and slide to close and open the glottis.

      Viewed frontally, the lumen of the laryngeal tube has an hourglass shape, with its narrowest width at the glottis. Just above the vocal cords there is an additional pair of mucosal folds called the false vocal cords or the vestibular folds. Like the true vocal cords, they are also formed by the free end of a fibroelastic membrane. Between the vestibular folds and the vocal cords, the laryngeal space enlarges and forms lateral pockets extending upward. This space is called the ventricle of the larynx. Because the gap between the vestibular folds is always larger than the gap between the vocal cords, the latter can easily be seen from above with the laryngoscope, an instrument designed for visual inspection of the interior of the larynx.

      The muscular (muscle) apparatus of the larynx comprises two functionally distinct groups. The intrinsic muscles act directly or indirectly on the shape, length, and tension of the vocal cords. The extrinsic muscles act on the larynx as a whole, moving it upward (e.g., during high-pitched phonation or swallowing) or downward. The intrinsic muscles attach to the skeletal components of the larynx itself; the extrinsic muscles join the laryngeal skeleton cranially to the hyoid bone or to the pharynx and caudally to the sternum (breastbone).

The trachea and the stem bronchi
 Below the larynx lies the trachea, a tube about 10 to 12 centimetres long and two centimetres wide. Its wall is stiffened by 16 to 20 characteristic horseshoe-shaped, incomplete cartilage rings that open toward the back and are embedded in a dense connective tissue. The dorsal wall contains a strong layer of transverse smooth muscle fibres that spans the gap of the cartilage. The interior of the trachea is lined by the typical respiratory epithelium. The mucosal layer contains mucous glands.

      At its lower end, the trachea divides in an inverted Y into the two stem (or main) bronchi, one each for the left and right lung. The right main bronchus has a larger diameter, is oriented more vertically, and is shorter than the left main bronchus. The practical consequence of this arrangement is that foreign bodies passing beyond the larynx will usually slip into the right lung. The structure of the stem bronchi closely matches that of the trachea.

Structural design of the airway tree
      The hierarchy of the dividing airways, and partly also of the blood vessels penetrating the lung, largely determines the internal lung structure. Functionally the intrapulmonary airway system can be subdivided into three zones, a proximal, purely conducting zone, a peripheral, purely gas-exchanging zone, and a transitional zone in between, where both functions grade into one another. From a morphological point of view, however, it makes sense to distinguish the relatively thick-walled, purely air-conducting tubes from those branches of the airway tree structurally designed to permit gas exchange.

      The structural design of the airway tree is functionally important because the branching pattern plays a role in determining air flow and particle deposition. In modeling the human airway tree, it is generally agreed that the airways branch according to the rules of irregular dichotomy. Regular dichotomy means that each branch of a treelike structure gives rise to two daughter branches of identical dimensions. In irregular dichotomy, however, the daughter branches may differ greatly in length and diameter. The models calculate the average path from the trachea to the lung periphery as consisting of about 24–25 generations of branches. Individual paths, however, may range from 11 to 30 generations. The transition between the conductive and the respiratory portions of an airway lies on average at the end of the 16th generation, if the trachea is counted as generation 0. The conducting airways comprise the trachea, the two stem bronchi, the bronchi, and the bronchioles. Their function is to further warm, moisten, and clean the inspired air and distribute it to the gas-exchanging zone of the lung. They are lined by the typical respiratory epithelium with ciliated cells and numerous interspersed mucus-secreting goblet cells. Ciliated cells are present far down in the airway tree, their height decreasing with the narrowing of the tubes, as does the frequency of goblet cells. In bronchioles the goblet cells are completely replaced by another type of secretory cells named Clara cells. The epithelium is covered by a layer of low-viscosity fluid, within which the cilia (cilium) exert a synchronized, rhythmic beat directed outward. In larger airways, this fluid layer is topped by a blanket of mucus of high viscosity. The mucus layer is dragged along by the ciliary action and carries the intercepted particles toward the pharynx, where they are swallowed. This design can be compared to a conveyor belt for particles, and indeed the mechanism is referred to as the mucociliary escalator.

      Whereas cartilage rings or plates provide support for the walls of the trachea and bronchi, the walls of the bronchioles, devoid of cartilage, gain their stability from their structural integration into the gas-exchanging tissues. The last purely conductive airway generations in the lung are the terminal bronchioles. Distally, the airway structure is greatly altered by the appearance of cuplike outpouchings from the walls. These form minute air chambers and represent the first gas-exchanging alveoli (pulmonary alveolus) on the airway path. In the alveoli, the respiratory epithelium gives way to a very flat lining layer that permits the formation of a thin air–blood barrier. After several generations (Z) of such respiratory bronchioles, the alveoli are so densely packed along the airway that an airway wall proper is missing; the airway consists of alveolar ducts. The final generations of the airway tree end blindly in the alveolar sacs.

The lungs (lung)
 The organ lung is parted into two slightly unequal portions, a left lung and a right lung, which occupy most of the intrathoracic space. The space between them is filled by the mediastinum, which corresponds to a connective tissue space containing the heart, major blood vessels, the trachea with the stem bronchi, the esophagus, and the thymus gland. The right lung represents 56 percent of the total lung volume and is composed of three lobes, a superior, middle, and inferior lobe, separated from each other by a deep horizontal and an oblique fissure. The left lung, smaller in volume because of the asymmetrical position of the heart, has only two lobes separated by an oblique fissure. In the thorax, the two lungs rest with their bases on the diaphragm, while their apexes extend above the first rib. Medially, they are connected with the mediastinum at the hilum, a circumscribed area where airways, blood and lymphatic vessels, and nerves enter or leave the lungs. The inside of the thoracic cavities and the lung surface are covered with serous membranes, respectively the parietal pleura and the visceral pleura, which are in direct continuity at the hilum. Depending on the subjacent structures, the parietal pleura can be subdivided into three portions: the mediastinal, costal, and diaphragmatic pleurae. The lung surfaces facing these pleural areas are named accordingly, since the shape of the lungs is determined by the shape of the pleural cavities. Because of the presence of pleural recesses, which form a kind of reserve space, the pleural cavity is larger than the lung volume.

      During inspiration, the recesses are partly opened by the expanding lung, thus allowing the lung to increase in volume. Although the hilum is the only place where the lungs are secured to surrounding structures, the lungs are maintained in close apposition to the thoracic wall by a negative pressure between visceral and parietal pleurae. A thin film of extracellular fluid between the pleurae enables the lungs to move smoothly along the walls of the cavity during breathing. If the serous membranes become inflamed ( pleurisy), respiratory movements can be painful. If air enters a pleural cavity ( pneumothorax), the lung immediately collapses owing to its inherent elastic properties, and breathing is abolished on this side.

Pulmonary segments
      The lung lobes are subdivided into smaller units, the pulmonary segments. There are 10 segments in the right lung and, depending on the classification, eight to 10 segments in the left lung. Unlike the lobes, the pulmonary segments are not delimited from each other by fissures but by thin membranes of connective tissue containing veins and lymphatics; the arterial supply follows the segmental bronchi. These anatomical features are important because pathological processes may be limited to discrete units, and the surgeon can remove single diseased segments instead of whole lobes.

The intrapulmonary conducting airways: bronchi and bronchioles
      In the intrapulmonary bronchi, the cartilage rings of the stem bronchi are replaced by irregular cartilage plates; furthermore, a layer of smooth muscle is added between the mucosa and the fibrocartilaginous tunic. The bronchi are ensheathed by a layer of loose connective tissue that is continuous with the other connective tissue elements of the lung and hence is part of the fibrous skeleton spanning the lung from the hilum to the pleural sac. This outer fibrous layer contains, besides lymphatics and nerves, small bronchial vessels to supply the bronchial wall with blood from the systemic circulation. Bronchioles are small conducting airways ranging in diameter from three to less than one millimetre. The walls of the bronchioles lack cartilage and seromucous glands. Their lumen is lined by a simple cuboidal epithelium with ciliated cells and Clara cells, which produce a chemically ill-defined secretion. The bronchiolar wall also contains a well-developed layer of smooth muscle cells, capable of narrowing the airway. Abnormal spasms of this musculature cause the clinical symptoms of bronchial asthma.

The gas-exchange region
 The gas-exchange region comprises three compartments: air, blood, and tissue. Whereas air and blood are continuously replenished, the function of the tissue compartment is twofold: it provides the stable supporting framework for the air and blood compartments, and it allows them to come into close contact with each other (thereby facilitating gas exchange) while keeping them strictly confined. The respiratory gases diffuse from air to blood, and vice versa, through the 140 square metres of internal surface area of the tissue compartment. The gas-exchange tissue proper is called the pulmonary parenchyma, while the supplying structures, conductive airways, lymphatics, and non-capillary blood vessels belong to the non-parenchyma.

      The gas-exchange region begins with the alveoli of the first generation of respiratory bronchioles. Distally, the frequency of alveolar outpocketings increases rapidly, until after two to four generations of respiratory bronchioles, the whole wall is formed by alveoli. The airways are then called alveolar ducts and, in the last generation, alveolar sacs. On average, an adult human lung has about 300,000,000 alveoli. They are polyhedral structures, with a diameter of about 250 to 300 micrometres, and open on one side, where they connect to the airway. The alveolar wall, called the interalveolar septum, is common to two adjacent alveoli. It contains a dense network of capillaries, the smallest of the blood vessels, and a skeleton of connective tissue fibres. The fibre system is interwoven with the capillaries and particularly reinforced at the alveolar entrance rings. The capillaries are lined by flat endothelial cells with thin cytoplasmic extensions. The interalveolar septum is covered on both sides by the alveolar epithelial (epithelium) cells. A thin, squamous cell type, the type I pneumocyte, covers between 92 and 95 percent of the gas-exchange surface; a second, more cuboidal cell type, the type II pneumocyte, covers the remaining surface. The type I cells form, together with the endothelial cells, the thin air–blood barrier for gas exchange; the type II cells are secretory cells. Type II pneumocytes produce a surface-tension-reducing material, the pulmonary surfactant (surface-active agent), which spreads on the alveolar surface and prevents the tiny alveolar spaces from collapsing. Before it is released into the airspaces, pulmonary surfactant is stored in the type II cells in the form of lamellar bodies. These granules are the conspicuous ultrastructural features of this cell type. On top of the epithelium, alveolar macrophages creep around within the surfactant fluid. They are large cells, and their cell bodies abound in granules of various content, partly foreign material that may have reached the alveoli, or cell debris originating from cell damage or normal cell death. Ultimately, the alveolar macrophages are derived from the bone marrow, and their task is to keep the air–blood barrier clean and unobstructed. The tissue space between the endothelium of the capillaries and the epithelial lining is occupied by the interstitium. It contains connective tissue and interstitial fluid. The connective tissue comprises a system of fibres, amorphous ground substance, and cells (mainly fibroblasts (fibroblast)), which seem to be endowed with contractile properties. The fibroblasts are thought to control capillary blood flow or, alternatively, to prevent the accumulation of extracellular fluid in the interalveolar septa. If for some reason the delicate fluid balance of the pulmonary tissues is impaired, an excess of fluid accumulates in the lung tissue and within the airspaces. This pathological condition is called pulmonary edema. As a consequence, the respiratory gases must diffuse across longer distances, and proper functioning of the lung is severely jeopardized.

Blood vessels (blood vessel), lymphatic vessels, and nerves
      With respect to blood circulation, the lung is a complex organ. It has two distinct though not completely separate vascular systems: a low-pressure pulmonary (pulmonary circulation) system and a high-pressure bronchial system. The pulmonary (or lesser) circulation is responsible for the oxygen supply of the organism. Blood, low in oxygen content but laden with carbon dioxide, is carried from the right heart through the pulmonary arteries to the lungs. On each side, the pulmonary artery enters the lung in the company of the stem bronchus and then divides rapidly, following relatively closely the course of the dividing airway tree. After numerous divisions, small arteries accompany the alveolar ducts and split up into the alveolar capillary networks. Because intravascular pressure determines the arterial wall structure, the pulmonary arteries, which have on average a pressure five times lower than systemic arteries, are much flimsier than systemic arteries of corresponding size. The oxygenated blood from the capillaries is collected by venules and drained into small veins. These do not accompany the airways and arteries but run separately in narrow strips of connective tissue delimiting small lobules. The interlobular veins then converge on the intersegmental septa. Finally, near the hilum the veins merge into large venous vessels that follow the course of the bronchi. Generally, four pulmonary veins drain blood from the lung and deliver it to the left atrium of the heart.

      The bronchial circulation has a nutritional function for the walls of the larger airways and pulmonary vessels. The bronchial arteries originate from the aorta or from an intercostal artery. They are small vessels and generally do not reach as far into the periphery as the conducting airways. With a few exceptions, they end several generations short of the terminal bronchioles. They split up into capillaries surrounding the walls of bronchi and vessels and also supply adjacent airspaces. Most of their blood is naturally collected by pulmonary veins. Small bronchial veins exist, however; they originate from the peribronchial venous plexuses and drain the blood through the hilum into the azygos and hemiazygos veins of the posterior thoracic wall.

      The lymph is drained from the lung through two distinct but interconnected sets of lymphatic vessels. The superficial, subpleural lymphatic network collects the lymph from the peripheral mantle of lung tissue and drains it partly along the veins toward the hilum. The deep lymphatic system originates around the conductive airways and arteries and converges into vessels that mostly follow the bronchi and arterial vessels into the mediastinum.

      Within the lung and the mediastinum, lymph nodes (lymph node) exert their filtering action on the lymph before it is returned into the blood through the major lymphatic vessels, called bronchomediastinal trunks. Lymph drainage paths from the lung are complex. The precise knowledge of their course is clinically relevant, because malignant tumours of the lung spread via the lymphatics.

      The pleurae, the airways, and the vessels are innervated by afferent and efferent fibres of the autonomic nervous system. Parasympathetic nerve fibres from the vagus nerve (10th cranial nerve) and sympathetic branches of the sympathetic nerve trunk meet around the stem bronchi to form the pulmonary autonomic nerve plexus, which penetrates into the lung along the bronchial and vascular walls. The sympathetic fibres mediate a vasoconstrictive action in the pulmonary vascular bed and a secretomotor activity in the bronchial glands. The parasympathetic fibres stimulate bronchial constriction. Afferent fibres to the vagus nerve transmit information from stretch receptors, and those to the sympathetic centres carry sensory information (e.g., pain) from the bronchial mucosa.

Lung development
      After early embryogenesis, during which the lung primordium is laid down, the developing human lung undergoes four consecutive stages of development, ending after birth. The names of the stages describe the actual morphology of the prospective airways. The pseudoglandular stage exists from five to 17 weeks; the canalicular stage, from 16 to 26 weeks; the saccular stage, from 24 to 38 weeks; and finally the alveolar stage, from 36 weeks of fetal (fetus) age to about 11/2 to two years after birth.

      The lung appears around the 26th day of intrauterine life as a ventral bud of the prospective esophagus. The bud separates distally from the gut, divides, and starts to grow into the surrounding mesenchyme. The epithelial components of the lung are thus derived from the gut (i.e., they are of endodermal origin), and the surrounding tissues and the blood vessels are derivatives of the mesoderm.

      Following rapid successive dichotomous divisions, the lung begins to look like a gland, giving the first stage of development (pseudoglandular) its name. At the same time the vascular connections also develop and form a capillary plexus around the lung tubules. Toward week 17, all the conducting airways of the lung are preformed, and it is assumed that, at the outermost periphery, the tips of the tubules represent the first structures of the prospective gas-exchange region.

      During the canalicular stage, the future lung periphery develops further. The prospective airspaces enlarge at the expense of the intervening mesenchyme, and their cuboidal epithelium differentiates into type I and type II epithelial cells or pneumocytes. Toward the end of this stage, areas with a thin prospective air–blood barrier have developed, and surfactant production has started. These structural and functional developments give a prematurely born fetus a small chance to survive at this stage.

      During the saccular stage, further generations of airways are formed. The tremendous expansion of the prospective respiratory airspaces causes the formation of saccules and a marked decrease in the interstitial tissue mass. The lung looks more and more “aerated,” although it is filled with fluid originating from the lungs and from the amniotic fluid surrounding the fetus. Some weeks before birth (parturition), alveolar formation begins by a septation process that subdivides the saccules into alveoli. At this stage of lung development, the infant is born.

      At birth the intrapulmonary fluid is rapidly evacuated and the lung fills with air with the first breaths. Simultaneously, the pulmonary circulation, which before was practically bypassed and very little perfused, opens up to accept the full cardiac output.

      The newborn lung is far from being a miniaturized version of the adult lung. It has only about 20,000,000 to 50,000,000 alveoli, or 6 to 15 percent of the full adult complement. Therefore, alveolar formation is completed in the early postnatal period. Although it was previously thought that alveolar formation could continue to the age of eight years and beyond, it is now accepted that the bulk of alveolar formation is concluded much earlier, probably before the age of two years. Even with complete alveolar formation, the lung is not yet mature. The newly formed interalveolar septa still contain a double capillary network instead of the single one of the adult lungs. This means that the pulmonary capillary bed must be completely reorganized during and after alveolar formation; it has to mature. Only after full microvascular maturation, which is terminated sometime between the ages of two and five years, is the lung development completed, and the lung can enter a phase of normal growth.

Peter H. Burri

Control of breathing
      Breathing is an automatic and rhythmic act produced by networks of neurons (neuron) in the hindbrain (the pons and medulla). The neural networks direct muscles that form the walls of the thorax and abdomen and produce pressure gradients that move air into and out of the lungs. The respiratory rhythm and the length of each phase of respiration are set by reciprocal stimulatory and inhibitory interconnection of these brain-stem neurons.

      An important characteristic of the human respiratory system is its ability to adjust breathing patterns to changes in both the internal milieu and the external environment. Ventilation increases and decreases in proportion to swings in carbon dioxide production and oxygen consumption caused by changes in metabolic rate. The respiratory system is also able to compensate for disturbances that affect the mechanics of breathing, such as the airway narrowing that occurs in an asthmatic attack. Breathing also undergoes appropriate adjustments when the mechanical advantage of the respiratory muscles is altered by postural changes or by movement.

      This flexibility in breathing patterns in large part arises from sensors distributed throughout the body that send signals to the respiratory neuronal networks in the brain. Chemoreceptors detect changes in blood oxygen levels and change the acidity of the blood and brain. Mechanoreceptors (mechanoreception) monitor the expansion of the lung, the size of the airway, the force of respiratory muscle contraction, and the extent of muscle shortening.

      Although the diaphragm is the major muscle of breathing, its respiratory action is assisted and augmented by a complex assembly of other muscle groups. Intercostal muscles (intercostalis muscle) inserting on the ribs, the abdominal muscles, and muscles such as the scalene and sternocleidomastoid that attach both to the ribs and to the cervical spine at the base of the skull also play an important role in the exchange of air between the atmosphere and the lungs. In addition, laryngeal muscles and muscles in the oral and nasal pharynx adjust the resistance of movement of gases through the upper airways during both inspiration and expiration. Although the use of these different muscle groups adds considerably to the flexibility of the breathing act, they also complicate the regulation of breathing. These same muscles are used to perform a number of other functions, such as speaking, chewing and swallowing, and maintaining posture. Perhaps because the “respiratory” muscles are employed in performing nonrespiratory functions, breathing can be influenced by higher brain centres and even controlled voluntarily to a substantial degree. An outstanding example of voluntary control is the ability to suspend breathing by holding one's breath. Input into the respiratory control system from higher brain centres may help optimize breathing so that not only are metabolic demands satisfied by breathing but ventilation also is accomplished with minimal use of energy.

Central organization of respiratory neurons
      The respiratory rhythm is generated within the pons and medulla. Three main aggregations of neurons are involved: a group consisting mainly of inspiratory neurons in the dorsomedial medulla, a group made up of inspiratory and expiratory neurons in the ventrolateral medulla, and a group in the rostral pons consisting mostly of neurons that discharge in both inspiration and expiration. It is currently thought that the respiratory cycle of inspiration and expiration is generated by synaptic interactions within these groups of neurons.

      The inspiratory and expiratory medullary neurons are connected to projections from higher brain centres and from chemoreceptors and mechanoreceptors; in turn they drive cranial motor neurons, which govern the activity of muscles in the upper airways and the activity of spinal motor neurons, which supply the diaphragm and other thoracic and abdominal muscles. The inspiratory and expiratory medullary neurons also receive input from nerve cells responsible for cardiovascular and temperature regulation, allowing the activity of these physiological systems to be coordinated with respiration.

      Neurally, inspiration is characterized by an augmenting discharge of medullary neurons that terminates abruptly. After a gap of a few milliseconds, inspiratory activity is restarted, but at a much lower level, and gradually declines until the onset of expiratory neuron activity. Then the cycle begins again. The full development of this pattern depends on the interaction of several types of respiratory neurons: inspiratory, early inspiratory, off-switch, post-inspiratory, and expiratory.

      Early inspiratory neurons trigger the augmenting discharge of inspiratory neurons. This increase in activity, which produces lung expansion, is caused by self-excitation of the inspiratory neurons and perhaps by the activity of an as yet undiscovered upstream pattern generator. Off-switch neurons in the medulla terminate inspiration, but pontine neurons and input from stretch receptors in the lung help control the length of inspiration. When the vagus nerves are sectioned or pontine centres are destroyed, breathing is characterized by prolonged inspiratory activity that may last for several minutes. This type of breathing, which occasionally occurs in persons with diseases of the brain stem, is called apneustic breathing.

      Post-inspiratory neurons are responsible for the declining discharge of the inspiratory muscles that occurs at the beginning of expiration. Mechanically, this discharge aids in slowing expiratory flow rates and probably assists the efficiency of gas exchange. It is believed by some that these post-inspiratory neurons have inhibitory effects on both inspiratory and expiratory neurons and therefore play a significant role in determining the length of the respiratory cycle and the different phases of respiration.

      As the activity of the post-inspiratory neurons subsides, expiratory neurons discharge and inspiratory neurons are strongly inhibited. There may be no peripheral manifestation of expiratory neuron discharge except for the absence of inspiratory muscle activity, although in upright humans the lower expiratory intercostal muscles and the abdominal muscles may be active even during quiet breathing. Moreover, as the demand to breathe increases (for example, with exercise), more expiratory intercostal and abdominal muscles contract. As expiration proceeds, the inhibition of the inspiratory muscles gradually diminishes and inspiratory neurons resume their activity.

Chemoreceptors (chemoreception)
      One way in which breathing is controlled is through feedback by chemoreceptors. There are two kinds of respiratory chemoreceptors: arterial chemoreceptors, which monitor and respond to changes in the partial pressure of oxygen and carbon dioxide in the arterial blood, and central chemoreceptors in the brain, which respond to changes in the partial pressure of carbon dioxide in their immediate environment. Ventilation levels behave as if they were regulated to maintain a constant level of carbon dioxide partial pressure and to ensure adequate oxygen levels in the arterial blood. Increased activity of chemoreceptors caused by hypoxia or an increase in the partial pressure of carbon dioxide augments both the rate and depth of breathing, which restores partial pressures of oxygen and carbon dioxide to their usual levels. On the other hand, too much ventilation depresses the partial pressure of carbon dioxide, which leads to a reduction in chemoreceptor activity and a diminution of ventilation. During sleep and anesthesia, lowering carbon dioxide levels three to four millimetres of mercury below values occurring during wakefulness can cause a total cessation of breathing (apnea).

Peripheral chemoreceptors
       hypoxia, or the reduction of oxygen supply to tissues to below physiological levels (produced, for example, by a trip to high altitudes), stimulates the carotid and aortic bodies, the principal arterial chemoreceptors. The two carotid bodies are small organs located in the neck at the bifurcation of each of the two common carotid arteries into the internal and external carotid arteries. This organ is extraordinarily well perfused and responds to changes in the partial pressure of oxygen in the arterial blood flowing through it rather than to the oxygen content of that blood (the amount of oxygen chemically combined with hemoglobin). The sensory nerve from the carotid body increases its firing rate hyperbolically as the partial pressure of oxygen falls. In addition to responding to hypoxia, the carotid body increases its activity linearly as the partial pressure of carbon dioxide in arterial blood is raised. This arterial blood parameter rises and falls as air enters and leaves the lungs, and the carotid body senses these fluctuations, responding more to rapid than to slow changes in the partial pressure of carbon dioxide. Larger oscillations in the partial pressure of carbon dioxide occur with breathing as metabolic rate is increased. The amplitude of these fluctuations, as reflected in the size of carotid body signals, may be used by the brain to detect changes in the metabolic rate and to produce appropriate adjustment in ventilation.

      The carotid body communicates with medullary respiratory neurons through sensory fibres that travel with the carotid sinus nerve, a branch of the glossopharyngeal nerve. Microscopically, the carotid body consists of two different types of cells (cell). The type I cells are arranged in groups and are surrounded by type II cells. The type II cells are generally not believed to have a direct role in chemoreception. Fine sensory nerve fibres are found in juxtaposition to type I cells, which, unlike type II cells, contain electron-dense vesicles. Acetylcholine, catecholamines, and neuropeptides such as enkephalins, vasoactive intestinal peptide, and substance P, are located within the vesicles. It is believed that hypoxia and hypercapnia (excessive carbon dioxide in the blood) cause the release of one or more of these neuroactive substances from the type I cells, which then act on the sensory nerve. It is possible to interfere independently with the responses of the carotid body to carbon dioxide and oxygen, which suggests that the same mechanisms are not used to sense or transmit changes in oxygen or carbon dioxide. The aortic bodies located near the arch of the aorta also respond to acute changes in the partial pressure of oxygen, but less well than the carotid body responds to changes in the partial pressure of carbon dioxide. The aortic bodies are responsible for many of the cardiovascular effects of hypoxia.

Central chemoreceptors
      Carbon dioxide is one of the most powerful stimulants of breathing. As the partial pressure of carbon dioxide in arterial blood rises, ventilation increases nearly linearly. Ventilation normally increases by two to four litres per minute with each one millimetre of mercury increase in the partial pressure of carbon dioxide. Carbon dioxide increases the acidity of the fluid surrounding the cells but also easily passes into cells and thus can make the interior of cells more acid. It is not clear whether the receptors respond to the intracellular or extracellular effects of carbon dioxide or acidity.

      Even if both the carotid and aortic bodies are removed, inhaling gases that contain carbon dioxide stimulates breathing. This observation shows that there must be additional receptors that respond to changes in the partial pressure of carbon dioxide. Current thinking places these receptors near the undersurface (ventral part) of the medulla. However, microscopic examination has not conclusively identified specific chemoreceptor cells in this region. The same areas of the ventral medulla also contain vasomotor neurons that are concerned with the regulation of blood pressure. Some investigators argue that respiratory responses produced at the ventral medullary surface are direct and are caused by interference with excitatory and inhibitory inputs to respiration from these vasomotor neurons. They believe that respiratory chemoreceptors that respond to carbon dioxide are more diffusely distributed in the brain.

Muscle and lung receptors (receptor)
      Receptors in the respiratory muscles and in the lung can also affect breathing patterns. These receptors are particularly important when lung function is impaired, since they can help maintain tidal volume and ventilation at normal levels.

      Changes in the length of a muscle affect the force it can produce when stimulated. Generally there is a length at which the force generated is maximal. Receptors, called spindles, in the respiratory muscles measure muscle length and increase motor discharge to the diaphragm and intercostal muscles when increased stiffness of the lung or resistance to the movement of air caused by disease impedes muscle shortening. tendon organs, another receptor in muscles, monitor changes in the force produced by muscle contraction. Too much force stimulates tendon organs and causes decreasing motor discharge to the respiratory muscles and may prevent the muscles from damaging themselves.

      Inflation of the lungs in animals stops breathing by a reflex described by the German physiologist Ewald Hering (Hering, (Karl) Ewald (Konstantin)) and the Austrian physiologist Josef Breuer (Breuer, Josef). The Hering-Breuer reflex is initiated by lung expansion, which excites stretch receptors in the airways. Stimulation of these receptors, which send signals to the medulla by the vagus nerve, shortens inspiratory times as tidal volume (the volume of air inspired) increases, accelerating the frequency of breathing. When lung inflation is prevented, the reflex allows inspiratory time to be lengthened, helping to preserve tidal volume.

      There are also receptors in the airways and in the alveoli that are excited by rapid lung inflations and by chemicals such as histamine, bradykinin, and prostaglandins. The most important function of these receptors, however, may be to defend the lung against noxious material in the atmosphere. When stimulated, these receptors constrict the airways and cause rapid shallow breathing, which inhibits the penetration of injurious agents into the bronchial tree. These receptors are supplied, like the stretch receptors, by the vagus nerve. Some of these receptors (called irritant receptors) are innervated by myelinated nerve fibres, others (the J receptors) by unmyelinated fibres. Stimulation of irritant receptors also causes coughing.

Variations in breathing
      One of the remarkable features of the respiratory control system is that ventilation increases sufficiently to keep the partial pressure of carbon dioxide in arterial blood nearly unchanged despite the large increases in metabolic (metabolism) rate that can occur with exercise, thus preserving acid–base homeostasis. A number of signals arise during exercise that can augment ventilation. Sources of these signals include mechanoreceptors in the exercising limbs; the arterial chemoreceptors, which can sense breath-by-breath oscillations in the partial pressure of carbon dioxide; and thermal receptors, because body temperature rises as metabolism increases. The brain also seems to anticipate changes in the metabolic rate caused by exercise, because parallel increases occur in the output from the motor cortex to the exercising limbs and to respiratory neurons. Changes in the concentration of potassium and lactic acid in the exercising muscles acting on unmyelinated nerve fibres may be another mechanism for stimulation of breathing during exercise. It remains unclear, however, how these various mechanisms are adjusted to maintain acid–base balance.

      During sleep, body metabolism is reduced, but there is an even greater decline in ventilation so that the partial pressure of carbon dioxide in arterial blood rises slightly and arterial partial pressure of oxygen falls. The effects on ventilatory pattern vary with sleep stage. In slow-wave sleep, breathing is diminished but remains regular, while in rapid eye movement sleep, breathing can become quite erratic. Ventilatory responses to inhaled carbon dioxide and to hypoxia are less in all sleep stages than during wakefulness. Sufficiently large decreases in the partial pressure of oxygen or increases in the partial pressure of carbon dioxide will cause arousal and terminate sleep.

      During sleep, ventilation may swing between periods when the amplitude and frequency of breathing are high and periods in which there is little attempt to breathe, or even apnea (cessation of breathing). This rhythmic waxing and waning of breathing, with intermittent periods of apnea, is called Cheyne-Stokes breathing, after the physicians who first described it. The mechanism that produces the Cheyne-Stokes ventilation pattern is still argued, but it may entail unstable feedback regulation of breathing. Similar swings in ventilation sometimes occur in persons with heart failure or with central nervous system disease.

      In addition, ventilation during sleep may intermittently fall to low levels or cease entirely because of partial or complete blockage of the upper airways. In some individuals, this intermittent obstruction occurs repeatedly during the night, leading to severe drops in the levels of blood oxygenation. The condition, termed sleep apnea syndrome, occurs most commonly in the elderly, in the newborn, in males, and in the obese. Because arousal is often associated with the termination of episodes of obstruction, sleep is of poor quality, and complaints of excessive daytime drowsiness are common. Snoring and disturbed behaviour during sleep may also occur.

      In some persons with sleep apnea syndrome, portions of the larynx and pharynx may be narrowed by fat deposits or by enlarged tonsils and adenoids, which increase the likelihood of obstruction. Others, however, have normal upper airway anatomy, and obstruction may occur because of discoordinated activity of upper airway and chest wall muscles. Many of the upper airway muscles, like the tongue and laryngeal adductors, undergo phasic changes in their electrical activity synchronous with respiration, and the reduced activity of these muscles during sleep may lead to upper airway closure.

Neil S. Cherniack

The mechanics of breathing
 Air moves in and out of the lungs in response to differences in pressure. When the air pressure within the alveolar spaces falls below atmospheric pressure, air enters the lungs (inspiration), provided the larynx is open; when the air pressure within the alveoli exceeds atmospheric pressure, air is blown from the lungs (expiration). The flow of air is rapid or slow in proportion to the magnitude of the pressure difference. Because atmospheric pressure remains relatively constant, flow is determined by how much above or below atmospheric pressure the pressure within the lungs rises or falls.

      Alveolar pressure fluctuations are caused by expansion and contraction of the lungs resulting from tensing and relaxing of the muscles of the chest and abdomen (abdominal cavity). Each small increment of expansion transiently increases the space enclosing lung air. There is, therefore, less air per unit of volume in the lungs and pressure falls. A difference in air pressure between atmosphere and lungs is created, and air flows in until equilibrium (proprioception) with atmospheric pressure is restored at a higher lung volume. When the muscles of inspiration relax, the volume of chest and lungs decreases, lung air becomes transiently compressed, its pressure rises above atmospheric pressure, and flow into the atmosphere results until pressure equilibrium is reached at the original lung volume. This, then, is the sequence of events during each normal respiratory cycle: lung volume change leading to pressure difference, resulting in flow of air into or out of the lung and establishment of a new lung volume.

The lung–chest system
      The forces that normally cause changes in volume of the chest and lungs stem not only from muscle contraction but from the elastic (elasticity) properties of both the lung and the chest. A lung is similar to a balloon in that it resists stretch, tending to collapse almost totally unless held inflated by a pressure difference between its inside and outside. This tendency of the lung to collapse or pull away from the chest is measurable by carefully placing a blunt needle between the outside of the lung and the inside of the chest wall, thereby allowing the lung to separate from the chest at this particular spot. The pressure measured in the small pleural space so created is substantially below atmospheric pressure at a time when the pressure within the lung itself equals atmospheric pressure. This negative (below-atmospheric) pressure is a measure, therefore, of the force required to keep the lung distended. The force increases (pleural pressure becomes more negative) as the lung is stretched and its volume increases during inspiration. The force also increases in proportion to the rapidity with which air is drawn into the lung and decreases in proportion to the force with which air is expelled from the lungs. In summary, the pleural pressure reflects primarily two forces: (1) the force required to keep the lung inflated against its elastic recoil and (2) the force required to cause airflow in and out of the lung. Because the pleural pressure is below atmospheric pressure, air is sucked into the chest and the lung collapses ( pneumothorax) when the chest wall is perforated, as by a wound or by a surgical incision.

      The force required to maintain inflation of the lung and to cause airflow is provided by the chest and diaphragm (the muscular partition between chest and abdomen), which are in turn stretched inward by the pull of the lungs. The lung–chest system thus acts as two opposed coiled springs, the length of each of which is affected by the other. Were it not for the outward traction of the chest on the lungs, these would collapse; and were it not for the inward traction of the lungs on the chest and diaphragm, the chest would expand to a larger size and the diaphragm would fall from its dome-shaped position within the chest.

The role of muscles (muscle)
      The respiratory muscles displace the equilibrium of elastic forces in the lung and chest in one direction or the other by adding muscular contraction. During inspiration, muscle contraction is added to the outward elastic force of the chest to increase the traction on the lung required for its additional stretch. When these muscles relax, the additional retraction of lung returns the system to its equilibrium position.

      Contraction of the abdominal muscles (abdominal muscle) displaces the equilibrium in the opposite direction by adding increased abdominal pressure to the retraction of lungs, thereby further raising the diaphragm and causing forceful expiration. This additional muscular force is removed on relaxation and the original lung volume is restored. During ordinary breathing, muscular contraction occurs only on inspiration, expiration being accomplished “passively” by elastic recoil of the lung.

      At total relaxation of the muscles of inspiration and expiration, the lung is distended to a volume—called the functional residual capacity—of about 40 percent of its maximum volume at the end of full inspiration. Further reduction of the lung volume results from maximal contraction of the expiratory muscles of chest and abdomen. The volume in these circumstances is known as the residual volume; it is about 20 percent of the volume at the end of full inspiration (known as the total lung capacity). Additional collapse of the lung to its “minimal air” can be accomplished only by opening the chest wall and creating a pneumothorax.

      The membranes of the surface of the lung (visceral pleura) and on the inside of the chest (parietal pleura) are normally kept in close proximity (despite the pull of lung and chest in opposite directions) by surface tension of the thin layer of fluid covering these surfaces. The strength of this bond can be appreciated by the attempt to pull apart two smooth surfaces, such as pieces of glass, separated by a film of water.

The respiratory pump and its performance
      The energy expended on breathing is used primarily in stretching the lung–chest system and thus causing airflow. It normally amounts to 1 percent of the basal energy requirements of the body but rises substantially during exercise or illness. The respiratory pump is versatile, capable of increasing its output 25 times, from a normal resting level of about six litres (366 cubic inches) per minute to 150 litres per minute in adults. Pressures within the lungs can be raised to 130 centimetres of water (about 1.8 pounds per square inch) by the so-called Valsalva maneuver—i.e., a forceful contraction of the chest and abdominal muscles against a closed glottis (i.e., with no space between the vocal cords). Airflow velocity, normally reaching 30 litres per minute in quiet breathing, can be raised voluntarily to 400 litres per minute. cough is accomplished by suddenly opening the larynx during a brief Valsalva maneuver. The resultant high-speed jet of air is an effective means of clearing the airways of excessive secretions or foreign particles. The beating of cilia (cilium) (hairline projections) from cells lining the airways normally maintains a steady flow of secretions toward the nose, cough resulting only when this action cannot keep pace with the rate at which secretions are produced.

      An infant (childhood disease and disorder) takes 33 breaths per minute with a tidal volume (the amount of air breathed in and out in one cycle) of 15 millilitres, totaling about 0.5 litre—approximately one pint—per minute as compared to adult values of 14 breaths, 500 millilitres, and seven litres, respectively.

      If the force of surface tension is responsible for the adherence of parietal and visceral pleurae, it is reasonable to question what keeps the lungs' alveolar walls (also fluid-covered) from sticking together and thus eliminating alveolar airspaces. In fact, such adherence occasionally does occur and is one of the dreaded complications of premature births. Normal lungs, however, contain a substance—a phospholipid surfactant (surface-active agent)—that reduces surface tension and keeps alveolar walls separated.

Arthur A. Siebens

Gas exchange
      Respiratory gases (gas)— oxygen and carbon dioxide—move between the air and the blood across the respiratory exchange surfaces in the lungs. The structure of the human lung provides an immense internal surface that facilitates gas exchange between the alveoli and the blood in the pulmonary capillaries. The area of the alveolar surface in the adult human is about 100 square metres. Gas exchange across the membranous barrier between the alveoli and capillaries is enhanced by the thin nature of the membrane, about 0.5 micrometre, or 1/100 of the diameter of a human hair.

      Respiratory gases move between the environment and the respiring tissues (tissue) by two principal mechanisms, convection and diffusion. Convection, or mass flow, is responsible for movement of air from the environment into the lungs and for movement of blood between the lungs and the tissues. Respiratory gases also move by diffusion across tissue barriers such as membranes. Diffusion is the primary mode of transport of gases between air and blood in the lungs and between blood and respiring tissues in the body. The process of diffusion is driven by the difference in partial pressures of a gas between two locales. In a mixture of gases, the partial pressure of each gas is directly proportional to its concentration. The partial pressure of a gas in fluid is a measure of its tendency to leave the fluid when exposed to a gas or fluid that does not contain that gas. A gas will diffuse from an area of greater partial pressure to an area of lower partial pressure regardless of the distribution of the partial pressures of other gases. There are large changes in the partial pressures of oxygen and carbon dioxide as these gases move between air and the respiring tissues. The partial pressure of carbon dioxide in this pathway is lower than the partial pressure of oxygen, due to differing modes of transport in the blood, but almost equal quantities of the two gases are involved in metabolism and gas exchange.

      Oxygen and carbon dioxide are transported between tissue cells and the lungs by the blood. The quantity transported is determined both by the rapidity with which the blood circulates and the concentrations of gases in blood. The rapidity of circulation is determined by the output of the heart, which in turn is responsive to overall body requirements. Local flows can be increased selectively, as occurs, for example, in the flow through skeletal muscles during exercise. The performance of the heart and circulatory regulation are, therefore, important determinants of gas transport.

      Oxygen and carbon dioxide are too poorly soluble in blood to be adequately transported in solution. Specialized systems for each gas have evolved to increase the quantities of those gases that can be transported in blood. These systems are present mainly in the red cells, which make up 40 to 50 percent of the blood volume in most mammals. Plasma, the cell-free, liquid portion of blood, plays little role in oxygen exchange but is essential to carbon dioxide exchange.

Transport of oxygen
      Oxygen is poorly soluble in plasma, so that less than 2 percent of oxygen is transported dissolved in plasma. The vast majority of oxygen is bound to hemoglobin, a protein contained within red cells. Hemoglobin is composed of four iron-containing ring structures (hemes) chemically bonded to a large protein (globin). Each iron atom can bind and then release an oxygen molecule. Enough hemoglobin is present in normal human blood to permit transport of about 0.2 millilitre of oxygen per millilitre of blood. The quantity of oxygen bound to hemoglobin is dependent on the partial pressure of oxygen in the lung to which blood is exposed. The curve representing the content of oxygen in blood at various partial pressures of oxygen, called the oxygen-dissociation curve, is a characteristic S-shape because binding of oxygen to one iron atom influences the ability of oxygen to bind to other iron sites. In alveoli at sea level, the partial pressure of oxygen is sufficient to bind oxygen to essentially all available iron sites on the hemoglobin molecule.

      Not all of the oxygen transported in the blood is transferred to the tissue cells. The amount of oxygen extracted by the cells depends on their rate of energy expenditure. At rest, venous blood returning to the lungs still contains 70 to 75 percent of the oxygen that was present in arterial blood; this reserve is available to meet increased oxygen demands. During extreme exercise the quantity of oxygen remaining in venous blood decreases to 10 to 25 percent. At the steepest part of the oxygen-dissociation curve (the portion between 10 and 40 millimetres of mercury partial pressure), a relatively small decline in the partial pressure of oxygen in the blood is associated with a relatively large release of bound oxygen.

      Hemoglobin binds not only to oxygen but to other substances such as hydrogen ions (hydrogen ion) (which determine the acidity, or pH, of the blood), carbon dioxide, and 2,3-diphosphoglycerate (2,3-DPG; a salt in the red blood cells that plays a role in liberating oxygen from hemoglobin in the peripheral circulation). These substances do not bind to hemoglobin at the oxygen-binding sites; however, with the binding of oxygen, changes in the structure of the hemoglobin molecule occur that affect its ability to bind other gases or substances. Conversely, binding of these substances to hemoglobin affects the affinity of hemoglobin for oxygen. (Affinity denotes the tendency of molecules of different species to bind to one another.) Increases in hydrogen ions, carbon dioxide, or 2,3-DPG decrease the affinity of hemoglobin for oxygen, and the oxygen-dissociation curve shifts to the right. Because of this decreased affinity, an increased partial pressure of oxygen is required to bind a given amount of oxygen to hemoglobin. A rightward shift of the curve is thought to be of benefit in releasing oxygen to the tissues when needs are great in relation to oxygen delivery, as occurs with anemia or extreme exercise. Reductions in normal concentrations of hydrogen ions, carbon dioxide, and 2,3-DPG result in an increased affinity of hemoglobin for oxygen, and the curve is shifted to the left. This displacement increases oxygen binding to hemoglobin at any given partial pressure of oxygen and is thought to be beneficial if the availability of oxygen is reduced, as occurs at extreme altitude.

      Temperature changes affect the oxygen-dissociation curve similarly. An increase in temperature shifts the curve to the right (decreased affinity; enhanced release of oxygen); a decrease in temperature shifts the curve to the left (increased affinity). The range of body temperature usually encountered in humans is relatively narrow, so that temperature-associated changes in oxygen affinity have little physiological importance.

Transport of carbon dioxide
      Transport of carbon dioxide in the blood is considerably more complex. A small portion of carbon dioxide, about 5 percent, remains unchanged and is transported dissolved in blood. The remainder is found in reversible chemical combinations in red blood cells or plasma. Some carbon dioxide binds to blood proteins, principally hemoglobin, to form a compound known as carbamate. About 88 percent of carbon dioxide in the blood is in the form of bicarbonate ion. The distribution of these chemical species between the interior of the red blood cell and the surrounding plasma varies greatly, with the red blood cells containing considerably less bicarbonate and more carbamate than the plasma.

      Less than 10 percent of the total quantity of carbon dioxide carried in the blood is eliminated during passage through the lungs. Complete elimination would lead to large changes in acidity between arterial and venous blood. Furthermore, blood normally remains in the pulmonary capillaries less than a second, an insufficient time to eliminate all carbon dioxide.

      Carbon dioxide enters blood in the tissues because its local partial pressure is greater than its partial pressure in blood flowing through the tissues. As carbon dioxide enters the blood, it combines with water to form carbonic acid (H2CO3), a relatively weak acid, which dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). Blood acidity is minimally affected by the released hydrogen ions because blood proteins, especially hemoglobin, are effective buffering agents. (A buffer solution resists change in acidity by combining with added hydrogen ions and, essentially, inactivating them.) The natural conversion of carbon dioxide to carbonic acid is a relatively slow process; however, carbonic anhydrase, a protein enzyme present inside the red blood cell, catalyzes this reaction with sufficient rapidity that it is accomplished in only a fraction of a second. Because the enzyme is present only inside the red blood cell, bicarbonate accumulates to a much greater extent within the red cell than in the plasma. The capacity of blood to carry carbon dioxide as bicarbonate is enhanced by an ion transport system inside the red blood cell membrane that simultaneously moves a bicarbonate ion out of the cell and into the plasma in exchange for a chloride ion. The simultaneous exchange of these two ions, known as the chloride shift, permits the plasma to be used as a storage site for bicarbonate without changing the electrical charge of either the plasma or the red blood cell. Only 26 percent of the total carbon dioxide content of blood exists as bicarbonate inside the red blood cell, while 62 percent exists as bicarbonate in plasma; however, the bulk of bicarbonate ions is first produced inside the cell, then transported to the plasma. A reverse sequence of reactions occurs when blood reaches the lung, where the partial pressure of carbon dioxide is lower than in the blood.

      Hemoglobin acts in another way to facilitate the transport of carbon dioxide. Amino groups of the hemoglobin molecule react reversibly with carbon dioxide in solution to yield carbamates. A few amino sites on hemoglobin are oxylabile, that is, their ability to bind carbon dioxide depends on the state of oxygenation of the hemoglobin molecule. The change in molecular configuration of hemoglobin that accompanies the release of oxygen leads to increased binding of carbon dioxide to oxylabile amino groups. Thus, release of oxygen in body tissues enhances binding of carbon dioxide as carbamate. Oxygenation of hemoglobin in the lungs has the reverse effect and leads to carbon dioxide elimination.

      Only 5 percent of carbon dioxide in the blood is transported free in physical solution without chemical change or binding, yet this pool is important, because only free carbon dioxide easily crosses biologic membranes. Virtually every molecule of carbon dioxide produced by metabolism must exist in the free form as it enters blood in the tissues and leaves capillaries in the lung. Between these two events, most carbon dioxide is transported as bicarbonate or carbamate.

Gas exchange in the lung
      The introduction of air into the alveoli allows the removal of carbon dioxide and the addition of oxygen to venous blood. Because ventilation is a cyclic phenomenon that occurs through a system of conducting airways, not all inspired air participates in gas exchange. A portion of the inspired breath remains in the conducting airways and does not reach the alveoli where gas exchange occurs. This portion is approximately one-third of each breath at rest but decreases to as little as 10 percent during exercise, due to the increased size of inspired breaths.

      In contrast to the cyclic nature of ventilation, blood flow through the lung is continuous, and almost all blood entering the lungs participates in gas exchange. The efficiency of gas exchange is critically dependent on the uniform distribution of blood flow and inspired air throughout the lungs. In health, ventilation and blood flow are extremely well matched in each exchange unit throughout the lungs. The lower parts of the lung receive slightly more blood flow than ventilation because gravity has a greater effect on the distribution of blood than on the distribution of inspired air. Under ideal circumstances, partial pressures of oxygen and carbon dioxide in alveolar gas and arterial blood are identical. Normally there is a small difference between oxygen tensions in alveolar gas and arterial blood because of the effect of gravity on matching and the addition of a small amount of venous drainage to the bloodstream after it has left the lungs. These events have no measurable effect on carbon dioxide partial pressures because the difference between arterial and venous blood is so small.

Abnormal gas exchange
      Lung disease can lead to severe abnormalities in blood gas composition. Because of the differences in oxygen and carbon dioxide transport, impaired oxygen exchange is far more common than impaired carbon dioxide exchange. Mechanisms of abnormal gas exchange are grouped into four categories—hypoventilation, shunting, ventilation–blood flow imbalance, and limitations of diffusion.

      If the quantity of inspired air entering the lungs is less than is needed to maintain normal exchange—a condition known as hypoventilation—the alveolar partial pressure of carbon dioxide rises and the partial pressure of oxygen falls almost reciprocally. Similar changes occur in arterial blood partial pressures because the composition of alveolar gas determines gas partial pressures in blood perfusing the lungs. This abnormality leads to parallel changes in both gas and blood and is the only abnormality in gas exchange that does not cause an increase in the normally small difference between arterial and alveolar partial pressures of oxygen.

      In shunting, venous blood enters the bloodstream without passing through functioning lung tissue. Shunting of blood may result from abnormal vascular (blood vessel) communications or from blood flowing through unventilated portions of the lung (e.g., alveoli filled with fluid or inflammatory material). A reduction in arterial blood oxygenation is seen with shunting, but the level of carbon dioxide in arterial blood is not elevated even though the shunted blood contains more carbon dioxide than arterial blood.

      The differing effects of shunting on oxygen and carbon dioxide partial pressures are the result of the different configurations of the blood-dissociation curves of the two gases. As noted above, the oxygen-dissociation curve is S-shaped and plateaus near the normal alveolar oxygen partial pressure, but the carbon dioxide-dissociation curve is steeper and does not plateau as the partial pressure of carbon dioxide increases. When blood perfusing the collapsed, unventilated area of the lung leaves the lung without exchanging oxygen or carbon dioxide, the content of carbon dioxide is greater than the normal carbon dioxide content. The remaining healthy portion of the lung receives both its usual ventilation and the ventilation that normally would be directed to the abnormal lung. This lowers the partial pressure of carbon dioxide in the alveoli of the normal area of the lung. As a result, blood leaving the healthy portion of the lung has a lower carbon dioxide content than normal. The lower carbon dioxide content in this blood counteracts the addition of blood with a higher carbon dioxide content from the abnormal area, and the composite arterial blood carbon dioxide content remains normal. This compensatory mechanism is less efficient than normal carbon dioxide exchange and requires a modest increase in overall ventilation, which is usually achieved without difficulty. Because the carbon dioxide-dissociation curve is steep and relatively linear, compensation for decreased carbon dioxide exchange in one portion of the lung can be counterbalanced by increased excretion of carbon dioxide in another area of the lung.

      In contrast, shunting of venous blood has a substantial effect on arterial blood oxygen content and partial pressure. Blood leaving an unventilated area of the lung has an oxygen content that is less than the normal content (indicated by the square). In the healthy area of the lung, the increase in ventilation above normal raises the partial pressure of oxygen in the alveolar gas and, therefore, in the arterial blood. The oxygen-dissociation curve, however, reaches a plateau at the normal alveolar partial pressure, and an increase in blood partial pressure results in a negligible increase in oxygen content. Mixture of blood from this healthy portion of the lung (with normal oxygen content) and blood from the abnormal area of the lung (with decreased oxygen content) produces a composite arterial oxygen content that is less than the normal level. Thus, an area of healthy lung cannot counterbalance the effect of an abnormal portion of the lung on blood oxygenation because the oxygen-dissociation curve reaches a plateau at a normal alveolar partial pressure of oxygen. This effect on blood oxygenation is seen not only in shunting but in any abnormality that results in a localized reduction in blood oxygen content.

      Mismatching of ventilation and blood flow is by far the most common cause of a decrease in partial pressure of oxygen in blood. There are minimal changes in blood carbon dioxide content unless the degree of mismatch is extremely severe. Inspired air and blood flow normally are distributed uniformly, and each alveolus receives approximately equal quantities of both. As matching of inspired air and blood flow deviates from the normal ratio of 1 to 1, alveoli become either overventilated or underventilated in relation to their blood flow. In alveoli that are overventilated, the amount of carbon dioxide eliminated is increased, which counteracts the fact that there is less carbon dioxide eliminated in the alveoli that are relatively underventilated. Overventilated alveoli, however, cannot compensate in terms of greater oxygenation for underventilated alveoli because, as is shown in the oxygen-dissociation curve, a plateau is reached at the alveolar partial pressure of oxygen, and increased ventilation will not increase blood oxygen content. In healthy lungs there is a narrow distribution of the ratio of ventilation to blood flow throughout the lung that is centred around a ratio of 1 to 1. In disease, this distribution can broaden substantially so that individual alveoli can have ratios that markedly deviate from the ratio of 1 to 1. Any deviation from the usual clustering around the ratio of 1 to 1 leads to decreased blood oxygenation—the more disparate the deviation, the greater the reduction in blood oxygenation. Carbon dioxide exchange, on the other hand, is not affected by an abnormal ratio of ventilation and blood flow as long as the increase in ventilation that is required to maintain carbon dioxide excretion in overventilated alveoli can be achieved.

      A fourth category of abnormal gas exchange involves limitation of diffusion of gases across the thin membrane separating the alveoli from the pulmonary capillaries. A variety of processes can interfere with this orderly exchange; for oxygen, these include increased thickness of the alveolar–capillary membrane, loss of surface area available for diffusion of oxygen, a reduction in the alveolar partial pressure of oxygen required for diffusion, and decreased time available for exchange due to increased velocity of flow. These factors are usually grouped under the broad description of “diffusion limitation,” and any can cause incomplete transfer of oxygen with a resultant reduction in blood oxygen content. There is no diffusion limitation of the exchange of carbon dioxide because this gas is more soluble than oxygen in the alveolar–capillary membrane, which facilitates carbon dioxide exchange. The complex reactions involved in carbon dioxide transport proceed with sufficient rapidity to avoid being a significant limiting factor in exchange.

Robert A. Klocke

Interplay of respiration, circulation, and metabolism
      The interplay of respiration, circulation, and metabolism is the key to the functioning of the respiratory system as a whole. Cells set the demand for oxygen uptake and carbon dioxide discharge, that is, for gas exchange in the lungs. The circulation of the blood links the sites of oxygen utilization and uptake. The proper functioning of the respiratory system depends on both the ability of the system to make functional adjustments to varying needs and the design features of the sequence of structures involved, which set the limit for respiration.

      The main purpose of respiration is to provide oxygen to the cells at a rate adequate to satisfy their metabolic needs. This involves transport of oxygen from the lung to the tissues by means of the circulation of blood. In antiquity and the medieval period, the heart was regarded as a furnace where the “fire of life” kept the blood boiling. Modern cell biology has unveiled the truth behind the metaphor. Each cell maintains a set of furnaces, the mitochondria, where, through the oxidation of foodstuffs such as glucose, the energetic needs of the cells are supplied. The precise object of respiration therefore is the supply of oxygen to the mitochondria.

      Cell metabolism depends on energy derived from high-energy phosphates such as adenosine triphosphate (ATP), whose third phosphate bond can release a quantum of energy to fuel many cell processes, such as the contraction of muscle fibre proteins or the synthesis of protein molecules. In the process, ATP is degraded to adenosine diphosphate (ADP), a molecule with only two phosphate bonds. To recharge the molecule by adding the third phosphate group requires energy derived from the breakdown of foodstuffs, or substrates. Two pathways are available: (1) anaerobic glycolysis, or fermentation, which operates in the absence of oxygen; and (2) aerobic metabolism, which requires oxygen and involves the mitochondria (mitochondrion). The anaerobic pathway leads to acid waste products and is wasteful of resources: The breakdown of one molecule of glucose generates only two molecules of ATP. In contrast, aerobic metabolism has a higher yield (36 molecules of ATP per molecule of glucose) and results in “clean wastes”—water and carbon dioxide, which are easily eliminated from the body and are recycled by plants in the process of photosynthesis. For any sustained high-level cell activity, the aerobic metabolic pathway is therefore preferable. Since oxidative phosphorylation occurs only in mitochondria, and since each cell must produce its own ATP (it cannot be imported), the number of mitochondria in a cell reflects its capacity for aerobic metabolism, or its need for oxygen.

      The supply of oxygen to the mitochondria at an adequate rate is a critical function of the respiratory system, because the cells maintain only a limited store of high-energy phosphates and of oxygen, whereas they usually have a reasonable supply of substrates in stock. If oxygen supply is interrupted for a few minutes, many cells, or even the organism, will die.

      Oxygen is collected from environmental air, transferred to blood in the lungs, and transported by blood flow to the periphery of the cells where it is discharged to reach the mitochondria by diffusion. The transfer of oxygen to the mitochondria involves several structures and different modes of transports. It begins with ventilation of the lung, which is achieved by convection or mass flow of air through an ingeniously branched system of airways; in the most peripheral airways ventilation of alveoli is completed by diffusion of oxygen through the air to the alveolar surface. The transfer of oxygen from alveolar air into the capillary blood occurs by diffusion across the tissue barrier; it is driven by the oxygen partial pressure difference between alveolar air and capillary blood and depends on the thickness (about 0.5 micrometre) and the surface area of the barrier (about 130 square metres in humans). Convective transport by the blood depends on the blood flow rate (cardiac output) and on the oxygen capacity of the blood, which is determined by its content of hemoglobin in the red blood cells. The last step is the diffusive discharge of oxygen from the capillaries into the tissue and cells, which is driven by the oxygen partial pressure difference and depends on the quantity of capillary blood in the tissue. In this process the blood plays a central role and affects all transport steps: oxygen uptake in the lung, transport by blood flow, and discharge to the cells. Blood also serves as carrier for both respiratory gases: oxygen, which is bound to hemoglobin in the red blood cells, and carbon dioxide, which is carried by both plasma and red blood cells and which also serves as a buffer for acid-base balance in blood and tissues.

      Metabolism, or, more accurately the metabolic rate of the cells, sets the demand for oxygen. At rest, a human consumes about 250 millilitres of oxygen each minute. With exercise this rate can be increased more than 10-fold in a normal healthy individual, but a highly trained athlete may achieve a more than 20-fold increase. As more and more muscle cells become engaged in doing work, the demand for ATP and oxygen increases linearly with work rate. This is accompanied by an increased cardiac output, essentially due to a higher heart rate, and by increased ventilation of the lungs; as a consequence, the oxygen partial pressure difference across the air–blood barrier increases and oxygen transfer by diffusion is augmented. These dynamic adjustments to the muscles' needs occur up to a limit that is twice as high in the athlete as in the untrained individual. This range of possible oxidative metabolism from rest to maximal exercise is called the aerobic scope. The upper limit to oxygen consumption is not conferred by the ability of muscles to do work, but rather by the limited ability of the respiratory system to provide or utilize oxygen at a higher rate. Muscle can do more work, but beyond the aerobic scope they must revert to anaerobic metabolism, with the result that waste products, mainly lactic acid, accumulate and limit the duration of work.

      The limit to oxidative metabolism is therefore set by some features of the respiratory system, from the lung to the mitochondria. Knowing precisely what sets the limit is important for understanding respiration as a key vital process, but it is not straightforward, because of the complexity of the system. Much has been learned from comparative physiology and morphology, based on observations that oxygen consumption rates differ significantly among species. For example, the athletic species in nature, such as dogs or horses, have an aerobic scope more than twofold greater than that of other animals of the same size; this is called adaptive variation. Then, oxygen consumption per unit body mass increases as animals become smaller, so that a mouse consumes six times as much oxygen per gram of body mass as a cow, a feature called allometric variation. Furthermore, the aerobic scope can be increased by training in an individual, but this induced variation achieves at best a 50 percent difference between the untrained and the trained state, well below interspecies differences.

      Within the aerobic scope the adjustments are due to functional variation. For example, cardiac output is augmented by increasing heart rate. Mounting evidence indicates that the limit to oxidative metabolism is related to structural design features of the system. The total amount of mitochondria in skeletal muscle is strictly proportional to maximal oxygen consumption, in all types of variation. In training, the mitochondria increase in proportion to the augmented aerobic scope. Mitochondria set the demand for oxygen, and they seem able to consume up to five millilitres of oxygen per minute and gram of mitochondria. If energy (ATP) needs to be produced at a higher rate, the muscle cells make more mitochondria. It is thus possible that oxygen consumption is limited at the periphery, at the last step of aerobic metabolism. But it is also possible that more central parts of the respiratory system may set the limit to oxygen transport, mainly the heart, whose capacity to pump blood reaches a limit, both in terms of rate and of the size of the ventricles, which determines the volume of blood that can be pumped with each stroke. The issue of peripheral versus central limitation is still under debate. It appears, however, that the lung as a gas-exchanging organ has sufficient redundancy that it does not limit aerobic metabolism at the site of oxygen uptake. But, whereas the mitochondria, the blood, the blood vessels, and the heart can increase in number, rate, or volume to augment their capacity when energy needs increase, such as in training, the lung lacks this capacity to adapt. If this proves true, the lung may well constitute the ultimate limit for the respiratory system, beyond which oxidative metabolism cannot be increased by training.

Ewald R. Weibel

Adaptations

High altitudes
      Ascent from sea level to high altitude has well-known effects upon respiration. The progressive fall in barometric pressure (atmospheric pressure) is accompanied by a fall in the partial pressure of oxygen, both in the ambient air and in the alveolar spaces of the lung; and it is this fall that poses the major respiratory challenge to humans at high altitude. Humans and some mammalian species like cattle adjust to the fall in oxygen pressure through the reversible and non-inheritable process of acclimatization, which, whether undertaken deliberately or not, commences from the time of exposure to high altitudes. Indigenous mountain species like the llama, on the other hand, exhibit an adaptation that is heritable and has a genetic basis.

      Respiratory acclimatization in humans is achieved through mechanisms that heighten the partial pressure of oxygen at all stages, from the alveolar spaces in the lung to the mitochondria in the cells, where oxygen is needed for the ultimate biochemical expression of respiration. The decline in the ambient partial pressure of oxygen is offset to some extent by greater ventilation, which takes the form of deeper breathing rather than a faster rate at rest. Diffusion of oxygen across the alveolar walls into the blood is facilitated, and in some experimental animal studies the alveolar walls are thinner at altitude than at sea level. The scarcity of oxygen at high altitudes stimulates increased production of hemoglobin and red blood cells, which increases the amount of oxygen transported to the tissues. The extra oxygen is released by increased levels of inorganic phosphates in the red blood cells, such as 2,3-diphosphoglycerate. With a prolonged stay at altitude, the tissues develop more blood vessels, and, as capillary density is increased, the length of the diffusion path along which gases must pass is decreased—a factor augmenting gas exchange. In addition, the size of muscle fibres decreases, which also shortens the diffusion path of oxygen.

      The initial response of respiration to the fall of oxygen partial pressure in the blood on ascent to high altitude occurs in two small nodules, the carotid bodies, attached to the division of the carotid arteries on either side of the neck. As the oxygen deprivation persists, the carotid bodies enlarge but become less sensitive to the lack of oxygen. The low oxygen partial pressure in the lung is associated with thickening of the small blood vessels in pulmonary alveolar walls and a slight increase in pulmonary blood pressure, thought to enhance oxygen perfusion of the lung apices.

      Indigenous mountain animals like the llama, alpaca, and vicuña in the Andes or the yak in the Himalayas are adapted rather than acclimatized to the low oxygen partial pressures of high altitude. Their hemoglobin has a high oxygen affinity, so that full saturation of the blood with oxygen occurs at a lower partial pressure of oxygen. In contrast to acclimatized humans, these indigenous, adapted mountain species do not have increased levels of hemoglobin or of organic phosphates in the red cells; they do not develop small muscular blood vessels or an increased blood pressure in the lung; and their carotid bodies remain small.

      Native human highlanders are acclimatized rather than genetically adapted to the reduced oxygen pressure. After living many years at high altitude, some highlanders lose this acclimatization and develop chronic mountain sickness, sometimes called Monge's disease, after the Peruvian physician who first described it. This disease is characterized by greater levels of hemoglobin. In Tibet some infants of Han origin never achieve satisfactory acclimatization on ascent to high altitude. A chemodectoma, or benign tumour, of the carotid bodies may develop in native highlanders in response to chronic exposure to low levels of oxygen.

Donald Albert Heath

      Fluid is not a natural medium for sustaining human life after the fetal stage; human respiration requires ventilation with air. Nevertheless, all vertebrates, including humans, exhibit a set of responses that may be called a “diving reflex,” which involves cardiovascular and metabolic adaptations to conserve oxygen during diving into water. Other physiological changes are also observed, either artificially induced (as by hyperventilation) or resulting from pressure changes in the environment at the same time that a diver is breathing from an independent gas supply.

      Hyperventilation, a form of overbreathing that increases the amount of air entering the pulmonary alveoli, may be used intentionally by swimmers to prolong the time they are able to hold their breath under water. Hyperventilation can be dangerous, and this danger is greatly increased if the swimmer descends to depth, as sometimes happens in snorkeling (snorkel). The increased ventilation prolongs the duration of the breath-hold by reducing the carbon dioxide pressure in the blood, but it cannot provide an equivalent increase in oxygen. Thus the carbon dioxide that accumulates with exercise takes longer to reach the threshold at which the swimmer is forced to take another breath, but concurrently the oxygen content of the blood falls to unusually low levels. The increased environmental pressure of the water around the breath-holding diver increases the partial pressures of the pulmonary gases. This allows an adequate oxygen partial pressure to be maintained in the setting of reduced oxygen content, and consciousness remains unimpaired. When the accumulated carbon dioxide at last forces the swimmer to return to the surface, however, the progressively diminishing pressure of the water on his ascent reduces the partial pressure of the remaining oxygen. Unconsciousness may then occur in or under the water.

      Divers who breathe from an apparatus that delivers gas at the same pressure as that of the surrounding water need not return to the surface to breathe and can remain at depth for prolonged periods. But this apparent advantage introduces additional hazards, many of them unique in human physiology. Most of the hazards result from the environmental pressure of water. Two factors are involved. At the depth of a diver, the absolute pressure, which is approximately one additional atmosphere for each 10-metre increment of depth, is one factor. The other factor, acting at any depth, is the vertical hydrostatic pressure gradient across the body. The effects of pressure are seen in many processes at the molecular and cellular level and include the physiological effects of the increased partial pressures of the respiratory gases, the increased density of the respiratory gases, the effect of changes of pressure upon the volumes of the gas-containing spaces in the body, and the consequences of the uptake of respiratory gases into, and their subsequent elimination from, the blood and tissues of the diver, often with the formation of bubbles. The multiple effects of submersion upon respiration are not easily separated from one another or clearly distinguishable from related effects of pressure upon other bodily systems.

      The increased work of breathing, rather than cardiac or muscular performance, is the limiting factor for hard physical work underwater. Although the increased work of breathing may be largely due to the effects of increased respiratory gas density upon pulmonary function, the use of underwater breathing apparatus adds significant external breathing resistance to the diver's respiratory burden.

      Arterial carbon dioxide pressure should remain unchanged during changes of ambient pressure, but the impaired alveolar ventilation at depth leads to some carbon dioxide retention (hypercapnia). This may be compounded by an increased inspiratory content of carbon dioxide, especially if the diver uses closed-circuit and semiclosed-circuit rebreathing equipment or wears an inadequately ventilated helmet. Alveolar oxygen levels can also be disturbed in diving. hypoxia may result from failure of the gas supply and may occur without warning. More commonly, the levels of inspired oxygen are increased. Oxygen in excess can be a poison; at a partial pressure greater than 1.5 bar (“surface equivalent value” = 150 percent), it may cause the rapid onset of convulsions, and after prolonged exposures at somewhat lower partial pressures it may cause pulmonary oxygen toxicity with reduced vital capacity and later pulmonary edema. In mixed-gas diving, inspired oxygen is therefore maintained at a partial pressure somewhere between 0.2 and 0.5 bar, but at great depths the inhomogeneity of alveolar ventilation and the limitations of gas diffusion appear to require oxygen provision at greater than normal levels.

      The maximum breathing capacity and the maximum voluntary ventilation of a diver breathing compressed air diminish rapidly with depth, approximately in proportion to the reciprocal of the square root of the increasing gas density. Thus the practice of using an inert gas such as helium as the oxygen diluent at depths where nitrogen becomes narcotic, like an anesthetic, has the additional advantage of providing a breathing gas of lesser density. The use of hydrogen, which in a mixture with less than 4 percent oxygen is noncombustible, provides a greater respiratory advantage for deep diving.

      At the extreme depths now attainable by humans—some 500 metres in the sea and more than 680 metres in the laboratory—direct effects of pressure upon the respiratory centre may be part of the “high-pressure neurological syndrome” and may account for some of the anomalies of breathlessness (dyspnea) and respiratory control that occur with exercise at depth.

      The term carbon dioxide retainer is commonly applied to a diver who fails to eliminate carbon dioxide in the normal manner. An ability to tolerate carbon dioxide may increase the work capacity of a diver at depth but also may predispose him to other consequences that are less desirable. High values of end-tidal carbon dioxide with only moderate exertion may be associated with a diminished tolerance to oxygen neurotoxicity, a condition that, if it occurs underwater, places the diver at great risk. nitrogen narcosis is enhanced by the presence of excess carbon dioxide, and the physical properties of carbon dioxide facilitate the nucleation and growth of bubbles on decompression.

      Independent of the depth of the dive are the effects of the local hydrostatic pressure gradient upon respiration. The supporting effect of the surrounding water pressure upon the soft tissues promotes venous return from vessels no longer solely influenced by gravity; and, whatever the orientation of the diver in the water, this approximates the effects of recumbency upon the cardiovascular and respiratory systems. Also, the uniform distribution of gas pressure within the thorax contrasts with the hydrostatic pressure gradient that exists outside the chest. Intrathoracic pressure may be effectively lower than the pressure of the surrounding water, in which case more blood will be shifted into the thorax, or it may be effectively greater, resulting in less intrathoracic blood volume. The concept of a hydrostatic balance point within the chest, which represents the net effect of the external pressures and the effects of chest buoyancy, has proved useful in designing underwater breathing apparatuses.

      Intrapulmonary gas expands exponentially during the steady return of a diver toward the surface. Unless vented, the expanding gas may rupture alveolar septa and escape into interstitial spaces. The extra-alveolar gas may cause a “burst lung” ( pneumothorax) or the tracking of gas into the tissues of the chest ( mediastinal emphysema), possibly extending into the pericardium or into the neck. More seriously, the escaped alveolar gas may be carried by the blood circulation to the brain (arterial gas embolism). This is a major cause of death among divers. Failure to exhale during ascent causes such accidents and is likely to occur if the diver makes a rapid emergency ascent, even from depths as shallow as two metres. Other possible causes of pulmonary barotrauma include retention of gas by a diseased portion of lung and gas trapping due to dynamic airway collapse during forced expiration at low lung volumes.

       decompression sickness may be defined as the illness, following a reduction of pressure, that is caused by the formation of bubbles from gases that were dissolved in the tissues while the diver was at an increased environmental pressure. The causes are related to the inadequacy of the diver's decompression, perhaps failure to follow a correct decompression protocol, or occasionally a diver's idiosyncratic response to an apparently safe decompression procedure. The pathogenesis begins both with the mechanical effects of bubbles and their expansion in the tissues and blood vessels and with the surface effects of the bubbles upon the various components of the blood at the blood–gas interface. The lung plays a significant role in the pathogenesis and natural history of this illness and may contribute to the clinical picture. Shallow, rapid respiration, often associated with a sharp retrosternal pain on deep inspiration, signals the onset of pulmonary decompression sickness, the “chokes.” Whether occurring alone or as part of a more complex case of decompression sickness, this respiratory pattern constitutes an acute emergency. It usually responds rapidly to treatment by recompression in a compression chamber.

David H. Elliott

Additional Reading
Basic information about the respiratory system and the process of respiration is included in Andrew Davies and Carl Moores, The Respiratory System (2003); and Michael P. Hlastala and Albert J. Berger, Physiology of Respiration, 2nd. ed. (2001). Comprehensive coverage of the diseases of the human respiratory system is provided by Alfred P. Fishman and Jack A. Elias, Fishman's Pulmonary Diseases and Disorders, 3rd ed. (2008).Control of breathing is described in Murray D. Altose and Yoshikazu Kawakami, Control of Breathing in Health and Disease (1999); and Jerome A. Dempsey and Allan I. Pack, Regulation of Breathing, 2nd ed. (1995). Abnormal breathing during sleep is covered by Nicholas A. Saunders and Colin E. Sullivan (eds.), Sleep and Breathing Disorders (1994).Adaptations of the human respiratory system to high altitude are described in a comprehensive but readable manner in Donald Heath and David Reid Williams, High-Altitude Medicine and Pathology, 2nd ed. (1995).The effects of swimming and diving on respiration are detailed in Peter B. Bennett and David H. Elliott (eds.), The Physiology and Medicine of Diving, 4th ed. (1993).Peter H. Burri Michael F. Beers Neil S. Cherniack Donald Albert Heath David H. Elliott

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Universalium. 2010.

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