The Lung-Air Sac System of Birds


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Development, Structure, and Function

A The lateral and dorsal direction of motion of the rib cage during exhalation is indicated by arrows. B The direction of airflow during inspiration. C The direction of flow during expiration From: Plummer and Goller Avian respiratory cycle This Flash diagram shows the paths that air takes through the respiratory system when a bird breathes. During inhalation, air moves into the posterior air sacs and, simultaneously, into the lungs and through the parabronchi and into the anterior air sacs. During exhalation, air moves out of the posterior air sacs into and through the parabronchi and, simultaneously, out of the anterior air sacs and out of the body via the trachea.

During inhalation, all air sacs expand as inhaled air enters the posterior air sacs and lungs and, simultaneously, air moves out of the lungs and into the anterior air sacs. During exhalation, the air sacs diminish in volume as air moves 1 from the posterior air sacs through the lungs and 2 from the anterior air sacs and out of the body via the trachea. Respiratory airflow in avian lungs. Filled and open arrows denote direction of air flow during inspiration filled arrows and expiration open arrows , respectively.

Relative thickness of the arrows indicates the proportion of air streaming through the different areas of the respiratory system during the respiratory cycle. Dotted arrows indicate the volume changes of air sacs. Consequently, respiratory gas flow through the parabronchi, atria, and the gas-exchanging air capillaries is unidirectional and continuous during both inspiration and expiration. Hence, airflow is constant and high in the parabronchi, atria, and the gas-exchanging air capillaries From: Bernhard et al. Surfactant SP-B in the figure above is mixture of phospholipids and specific proteins that functions to maintain airflow through the 'tubes' of the avian respiratory system.

Surfactant SP-A has only been detected in the mesobronchi of birds. SP-A plays an important role in innate host defense and regulation of inflammatory processes and may be important in the mesobronchi because air flow is slower and small particles could tend to accumulate there see figure below. Because the mammalian respiratory system below includes structures that are collapsible alveoli and areas with low airflow, all three surfactants are important for reducing surface tension and innate host defense Bernhard et al.

Airflow in mammalian lungs is bidirectional during the respiratory cycle, with highly reduced airflow in peripheral structures, i. A: A high-power view of a foreign particle p being engulfed by an epithelial cell e in an avian lung.

Bird Respiration

Arrows, elongated microvilli. B: Surface of an atrium of the lung of the domestic fowl showing red blood cells with one of them r being engulfed by the underlying epithelial cell arrow : e, epithelial surface; m, a free surface macrophage. Air flow is driven by changes in pressure within the respiratory system:.

Changes in the position of the thoracic skeleton during breathing in a bird. Drawing of a bird coelom in transverse section during expiration gray bones and inspiration white bones. Dashed lines illustrate the horizontal septum that separates the pleural cavity PC where the lungs are located from the subpulmonary cavity SP where most of the air sacs are located except the abdominals that are in the peritoneal cavity , and the oblique septum that separates the air sacs from the abdominal cavity AC and digestive viscera. Both septa insert on the ventral keel of vertebrae. The volume of the pleural cavity changes very little with respiratory rib movements, but the volume of the subpulmonary cavity and the air sacs is greatly increased when the oblique septum is stretched during inspiration Adapted from: Klein and Owerkowicz The increase in volume lowers air pressure and draws air into the air sacs.

The intrapulmonary bronchus is also known as the mesobronchus.

Alligators' bird breath may explain dinosaurs' triumph

A - Inspiration. So, air always moves unidirectionally through the lungs and, as a result, is higher in oxygen content than, for example, air in the alveoli of humans and other mammals. Variation in length of uncinate processes -- Birds with different forms of locomotion exhibit morphological differences in their rib cages: A terrestrial walking species, Cassowary Casuaris casuaris ; B a typical flying bird, Eagle Owl Bubo bubo ; and C an aquatic, diving species, Razorbill Alca torda.

Uncinate processes are shorter in walking species, of intermediate length in typical birds, and relatively long in diving species scale bar, 5 cm. Muscles attached to uncinate processes appendicocostales muscles help rotate the ribs forwards, pushing the sternum down and inflating the air sacs during inspiration.

Another muscle external oblique attached to uncinate processes pulls the ribs backward, moving the sternum upward during expiration. The longer uncinate processes of diving birds are probably related to the greater length of the sternum and the lower angle of the ribs to the backbone and sternum. The insertion of the appendicocostales muscles near the end of the uncinate processes may provide a mechanical advantage for moving the elongated ribs during breathing Tickle et al.

This exchange is very efficient in birds for a number of reasons. First, the complex arrangement of blood and air capillaries in the avian lung creates a substantial surface area through which gases can diffuse. The surface area available for exchange SAE varies with bird size. For example, the ASE is about 0. However, smaller birds have a greater SAE per unit mass than do larger birds. Among mammals, there is also a negative relationship between SAE and body size, with smaller mammals like shrews having a greater SAE per unit mass than larger mammals.

A second reason why gas exchange in avian lungs is so efficient is that the blood-gas barrier through which gases diffuse is extremely thin. This is important because the amount of gas diffusing across this barrier is inversely proportional to its thickness. Among terrestrial vertebrates, the blood-gas barrier is thinnest in birds. Natural selection has favored thinner blood-gas barriers in birds and mammals because endotherms use oxygen at higher rates than ectotherms like amphibians and reptiles.

Among birds, the thickness of the blood-gas barrier varies, with smaller birds generally having thinner blood-gas barriers than larger birds. For example, the blood-gas barrier is 0.

Comparison of the mean thickness of the blood-gas barrier of 34 species of birds, 37 species of mammals, 16 species of reptiles, and 10 species of amphibians revealed that birds had significantly thinner blood-gas barriers than the other taxa West Also contributing to the efficiency of gas exchange in avian lungs is a process called cross-current exchange. Air passing through air capillaries and blood moving through blood capillaries generally travel at right angles to each other in what is called cross-current flow Figure below; Makanya and Djonov As a result, oxygen diffuses from the air capillaries into the blood at many points along the length of the parabronchi, resulting in a greater concentration of oxygen i.

A Micrograph of lung tissue from a Brown Honeyeater Lichmera indistincta showing a parabronchi, b blood vessel, and c exchange tissue bar, micrometers. B Electron micrograph from the lung of a Welcome Swallow Hirundo neoxena showing a blood-air barrier, b air capillary, c blood capillary, and d red blood cell in the blood capillary bar, 2 micrometers. From: Vitali and Richardson A Medial view of the lung of a domestic chicken Gallus gallus domesticus. Scale bar, 1 cm. B An intraparabronchial artery i giving rise to blood capillaries c in the lung of an Emu Dromiceus novaehollandiae.

C Air capillaries closely associated with blood capillaries arrows in a chicken lung. D Blood capillaries c closely associated with air capillaries spaces in a chicken lung. From: Maina An individual air capillary AC surrounded by a dense network of blood capillaries asterisk in a chicken lung. The blood capillaries drain into a larger vein V6 adjacent to an infundibulum IF.

Note that the general direction of blood flow through the blood capillaries is perpendicular to the flow of air through the air capillaries, i. In birds, the thickness of the blood-gas barrier in the 7. Relationship between the harmonic mean thickness of the blood-gas barrier the thickness of the barrier that affects the diffusion of oxygen from air capillaries into blood capillaries against body mass in the lungs of bats, birds, and non-flying mammals.

Birds have particularly thinner barriers than bats and non-flying mammals Maina Light micrographs of a portion of the lung of a chicken A and rabbit B. Note the small diameter of the air capillaries in the chicken lung vs. A In the chicken lung, pulmonary capillaries are supported by 'struts' of epithelium arrows. B In the rabbit lung, pulmonary capillaries are suspended in the large spaces between alveoli Watson et al.

Top: Air flow large arrows and blood flow small arrows illustrating the cross-current gas-exchange mechanism operating in the avian lung between the blood capillaries and air capillaries. Note the serial arrangement of blood capillaries running from the periphery to the lumen of the parabronchus and the air capillaries radially extending from the parabronchial lumen. The exchange of gases simple diffusion of O 2 and CO 2 occurs only between blood capillaries and air capillaries. As air moves through a parabronchus and each successive air capillary, the partial pressure of oxygen P O2 declines as indicated by the decreased density of the stippling because oxygen is diffusing into the blood capillaries associated with each air capillary.

As a result of this diffusion, the partial pressure of oxygen in the blood leaving the lungs pulmonary vein is higher than that in blood entering the lungs pulmonary artery as indicated by the increased density of the stippling. Bottom: Relative partial pressures of O 2 and CO 2 1 for air entering a parabronchus initial-parabronchial, P I and air leaving a parabronchus end-parabronchial, P E , and 2 for blood before entering blood capillaries in the lungs pulmonary artery, P A and for blood after leaving the blood capillaries in the lungs pulmonary vein, P V. The partial pressure of oxygen P O2 of venous blood P V is derived from a mixture of all serial air capillary-blood capillary units.

Because of this cross-current exchange the partial pressure of oxygen in avian pulmonary veins P V is greater than that of the air leaving the parabronchus P E ; air that will be exhaled. In mammals, the partial pressure of oxygen in veins leaving the lungs cannot exceed that of exhaled air end-expiratory gas, or P E Figure adapted from Scheid and Piiper Importantly, the partial pressure of oxygen in blood leaving the avian lung is the result of 'mixing'; blood from a series of capillaries associated with successive air capillaries along the length of a parabronchus is mixed as the blood leaves the capillaries and enters small veins.

As a result, the direction of air flow through a parabronchus does not effect the efficiency of the cross-current exchange because gases are only exchanged between blood capillaries and air capillaries, not between the parabronchus and the blood.

How birds breathe: Did evolution finally get it right? – Atlas of Science

So, in above diagram, reversing the direction of air flow would obviously mean that the air capillary on the far left would have the highest partial pressure of oxygen rather than the air capillary on the far right so the stippling pattern that indicates the amount of oxygen in each air capillary would be reversed. However, because of the 'mixing' of blood just mentioned, this reversal would have little effect on the P V , the partial pressure of oxygen in blood leaving via pulmonary veins the P O2 would likely be a bit lower because some oxygen would have been lost the first time air passed through the neopulmonic parabronchi.

This is important because most birds have neopulmonic parabronchi as well as paleopulmonic parabronchi and, although air flow through paleopulmonic parabronchi is unidirectional, air flow through neopulmonic parabronchi is bidirectional. Diagram showing the flow of air from the parabronchial lumen PL into the air capillaries not shown and arterial blood from the periphery of the parabronchus into the area of gas exchange exchange tissue, ET.

The orientation between the flow of air along the parabronchus and that of blood into the exchange tissue ET from the periphery is perpendicular or cross-current dashed arrows. The exchange tissue is supplied with arterial blood by interparabronchial arteries IPA that give rise to arterioles stars that terminate in blood capillaries. After passing through the capillaries, blood flows into the intraparabronchial venules asterisks that drain into interparabronchial veins IPV.

These in turn empty into the pulmonary vein which returns the blood to the heart. From: Maina and Woodward Ventilation and respiratory rate are regulated to meet the demands imposed by changes in metabolic activity e. There is likely a central respiratory control center in the avian brain, but this has not been unequivocally demonstrated.

As in mammals, the central control area appears to be located in the pons and medulla oblongata with facilitation and inhibition coming from higher regions of the brain. It also appears that the chemical drive on respiratory frequency and inspiratory and expiratory duration depend on feedback from receptors in the lung as well as on extrapulmonary chemoreceptors, mechanoreceptors, and thermoreceptors Ludders Central chemoreceptors affect ventilation in response to changes in arterial P CO 2 and hydrogen ion concentration.

Peripheral extrapulmonary chemoreceptors, specifically the carotid bodies located in the carotid arteries , are influenced by P O 2 and increase their discharge rate as P O 2 decreases, thus increasing ventilation; they decrease their rate of discharge as P O 2 increases or P CO 2 decreases. These responses are the same as those observed in mammals. Unlike mammals, birds have a unique group of peripheral receptors located in the lung called intrapulmonary chemoreceptors IPC that are acutely sensitive to carbon dioxide and insensitive to hypoxia.

The IPC affect rate and volume of breathing on a breath-to-breath basis by acting as the afferent limb of an inspiratory-inhibitory reflex that is sensitive to the timing, rate, and extent of CO 2 washout from the lung during inspiration Ludders Source: www. Chicken embryo.


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  4. Rau1, J. Hohlfeld, A. They serve to increase respiratory efficiency by providing a large surface area for gas exchange. See also pulmonary alveolus. Air sac. Article Media.

    Types of Breathing

    Info Print Cite. Submit Feedback. In addition to providing a very efficient supply of oxygen, the rapid airflow would have been an effective cooling mechanism, which is essential for animals that are active but too large to get rid of all the excess heat through their skins. From Wikipedia, the free encyclopedia. This article is about a component of dinosaur and bird skeletal systems. For the structures in mammalian lungs, see pulmonary alveolus.

    The Vertebrate Body. Holt-Saunders International. Kemp, Tom ed. Bibcode : PLoSO Bibcode : Natur. November Bibcode : Sci January November—December Physiological and Biochemical Zoology. September Respiration Physiology.

    Alligators' bird breath may explain dinosaurs' triumph

    Archived from the original on December Canadian Journal of Earth Sciences. Bibcode : CaJES.. July Full text currently online at "Findarticles.

    The Lung-Air Sac System of Birds The Lung-Air Sac System of Birds
    The Lung-Air Sac System of Birds The Lung-Air Sac System of Birds
    The Lung-Air Sac System of Birds The Lung-Air Sac System of Birds
    The Lung-Air Sac System of Birds The Lung-Air Sac System of Birds
    The Lung-Air Sac System of Birds The Lung-Air Sac System of Birds
    The Lung-Air Sac System of Birds The Lung-Air Sac System of Birds
    The Lung-Air Sac System of Birds The Lung-Air Sac System of Birds

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