During Inspiration Intrapulmonary Pressure Is
The avian respiratory system delivers oxygen from the air to the tissues and likewise removes carbon dioxide. In addition, the respiratory system plays an of import role in thermoregulation (maintaining normal torso temperature). The avian respiratory system is different from that of other vertebrates, with birds having relatively small lungs plus ix air sacs that play an important role in respiration (but are non directly involved in the exchange of gases).
 | (A). Dorsal view of the trachea (circled) and the lung of the Ostrich (Struthio camelus). The lungs are deeply entrenched into the ribs on the dorsolateral aspects (arrowhead). Filled circle is on the right primary bronchus. Annotation that the right primary bronchus is relatively longer, rather horizontal and relatively narrower than the left chief bronchus. Scale bar, ane cm. (B) Close up of the dorsal attribute of the lung showing the deep costal sulci (s). Trachea indicated by an open circle; filled circle = right primary bronchus. Scale bar, 2 cm (Maina and Nathaniel 2001). |
Avian respiratory arrangement
(hd = humeral diverticulum of the clavicular air sac; adapted from Sereno et al. 2008)
The air sacs let a unidirectional flow of air through the lungs. Unidirectional flow means that air moving through bird lungs is largely 'fresh' air & has a higher oxygen content. In contrast, air flow is 'bidirectional' in mammals, moving back and forth into and out of the lungs. As a result, air coming into a mammal's lungs is mixed with 'old' air (air that has been in the lungs for a while) & this 'mixed air' has less oxygen. So, in bird lungs, more oxygen is available to diffuse into the blood (avian respiratory system).
Pulmonary air-sac system of a Common Teal (Anas crecca). a. Latex injection (blue) highlighting the location of air sacs.
b, Chief components of the avian flow-through arrangement. Abd, intestinal aire sac; Cdth, caudal thoracic alveolus; Cl, clavicular
air sac; Crth, cranial thoracic air sac; Cv, cervical air sac; Fu, furcula; Hu, humerus; Lu, lung; Lvd, lateral vertebral diverticula;
Pv, pelvis; and Tr, trachea (From: O'Connor and Claessens 2005).
The alveolar lungs of mammals (Rhesus monkey; A) and parabronchial lungs of birds (pigeon; B) are subdivided into large
numbers of extremely small alveoli (A, inset) or air capillaries (radiating from the parabronchi; B, inset). The mammalian respiratory
system is partitioned homogeneously, and then the functions of ventilation and gas exchange are shared by alveoli and much of the lung volume.
The avian respiratory system is partitioned heterogeneously, so the functions of ventilation and gas exchange are separate in the air sacs
(shaded in gray) and the parabronchial lung, respectively. Air sacs act as bellows to ventilate the tube-like parabronchi (Powell and Hopkins 2004).
Comparison of the avian 'unidirectional' respiratory system (a) where gases are exchanged betwixt the lungs and the blood in the parabronchi, and the bidirectional respiratory arrangement of mammals (b) where gas exchange occurs in small-scale expressionless-end sacs chosen alveoli (From: West et al. 2007).
Animated gif created by Eleanor Lutz (Eleanor's website: http://tabletopwhale.com/2014/x/24/iii-different-ways-to-exhale.html)
Credit: Zina Deretsky, National Science Foundation
Bird-like respiratory systems in dinosaurs -- A recent analysis showing the presence of a very bird-like pulmonary, or lung, system in predatory dinosaurs provides more evidence of an evolutionary link between dinosaurs and birds. First proposed in the late xixthursday century, theories about the animals' relatedness enjoyed brief support but presently fell out of favor. Show gathered over the past xxx years has breathed new life into the hypothesis. O'Connor and Claessens (2005) brand clear the unique pulmonary system of birds, which has fixed lungs and air sacs that penetrate the skeleton, has an older history than previously realized. Information technology also dispels the theory that predatory dinosaurs had lungs similar to living reptiles, like crocodiles.
The avian pulmonary system uses "flow-through ventilation," relying on a set of 9 flexible air sacs that act like bellows to move air through the almost completely rigid lungs. Air sacs do not take part in the actual oxygen exchange, but do profoundly enhance its efficiency and allow for the loftier metabolic rates found in birds. This system too keeps the book of air in the lung about constant. O'Connor says the presence of an extensive pulmonary air sac system with period-through ventilation of the lung suggests this group of dinosaurs could have maintained a stable and high metabolism, putting them much closer to a warm-blooded existence. "More and more than characteristics that once defined birds--feathers, for example--are now known to accept been present in dinosaurs, so, many avian features may really be dinosaurian," said O'Connor. A portion of the air sac really integrates with the skeleton, forming air pockets in otherwise dumbo os. The exact office of this skeletal modification is non completely understood, but one explanation theorizes the skeletal air pockets evolved to lighten the os structure, assuasive dinosaurs to walk upright and birds to wing.
Some hollow bones are providing solid new evidence of how birds evolved from dinosaurs.
Most birds have 9 ai r sacs:
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Air sacs and centric pneumatization in an extant avian. The body of bird in left lateral view, showing the cervical (C), interclavicular (I), anterior thoracic (AT), posterior thoracic (PT), and abdominal (AB) air sacs. The hatched area shows the volume change during exhalation. The cervical and anterior thoracic vertebrae are pneumatized by diverticula of the cervical air sacs. The posterior thoracic vertebrae and synsacrum are pneumatized past the abdominal air sacs in most taxa. Diverticula of the abdominal air sacs normally invade the vertebral cavalcade at several points. Diverticula oft unite when they come into contact, producing a organization of continuous vertebral airways extending from the tertiary cervical vertebra to the terminate of the synsacrum. Modified from Duncker 1971 (Wedel 2003).
| Computerized centric tomogram of an awake, spontaneously breathing goose; air is darkest. A large percentage of the bird'south body is filled with the several air sacs. Upper left: At the level of the shoulder joints (hh, humeral head) is the intraclavicular alveolus (ICAS), which extends from the center cranially to the clavicles (i.due east., furcula or wishbone). Southward, sternum; FM, large flight muscles with enclosed air sac diverticula, arrowheads; t, trachea. Upper right: At the level of the caudal middle (H) is the paired cranial thoracic air sacs (TAS). Arrowhead points to the medial wall of the air sac (contrast enhanced with aerosolized tantalum powder). The dorsal body cavity is filled with the lungs, which are tightly fastened to the dorsal and lateral torso wall. 5, thoracic vertebrae. Lower left: At the level of the knees (M) is the paired caudal thoracic air sacs (PTAS) and paired intestinal air sacs, with the abdominal viscera (AV) filling the ventral body cavity. The membrane separating the intestinal air sacs from ane another (arrowhead) and from the caudal thoracic air sacs (arrows) can be seen. Lower right: At the level of the caudal pelvis, the abdominal air sacs, which extend to the bird'due south tail, tin can be seen. Pointer, membrane separating intestinal air sacs (Dark-brown et al. 1997). |  |
Birds can breathe through the mouth or the nostrils (nares). Air entering these openings (during inspiration) passes through the pharynx & then into the trachea (or windpipe). The trachea is generally every bit long as the neck. However, some birds, such as cranes, have an exceptionally long (up to ane.5 one thousand) trachea that is coiled within the hollowed keel of the breastbone (shown below). This organisation may give boosted resonance to their loud calls (check this brusque video of calling Sandhill Cranes).
Sandhill Cranes calling in flying
The typical bird trachea is ii.7 times longer and i.29 times wider than that of similarly-sized mammals. The net event is that tracheal resistance to air catamenia is like to that in mammals, just the tracheal dead infinite book is most 4.5 times larger. Birds recoup for the larger tracheal dead space by having a relatively larger tidal volume and a lower respiratory frequency, approximately 1-third that of mammals. These two factors lessen the impact of the larger tracheal expressionless space volume on ventilation. Thus, infinitesimal tracheal ventilation is just most 1.5 to 1.9 times that of mammals (Ludders 2001).
Examples of tracheal loops found in Blackness Swans (Cygnus atratus), Whooper
Swans (Cygnus cygnus), White Spoonbills (Platalea leucorodia), Helmeted Curassow (Crax pauxi),
and Whooping Cranes (Grus americana).
Source: http://world wide web.ivis.org/advances/Anesthesia_Gleed/ludders2/chapter_frm.asp
The trachea bifurcates (or splits) into two master bronchi at the syrinx. The syrinx is unique to birds & is their 'voicebox' (in mammals, sounds are produced in the larynx). The main bronchi enter the lungs & are then called mesobronchi. Branching off from the mesobronchi are smaller tubes called dorsobronchi. The dorsobronchi, in turn, pb into the still smaller parabronchi. Parabronchi tin be several millimeters long and 0.5 - ii.0 mm in diameter (depending on the size of the bird) (Maina 1989) and their walls contain hundreds of tiny, branching, & anastomosing 'air capillaries' surrounded by a profuse network of blood capillaries (Welty and Baptista 1988). It is within these 'air capillaries' that the exchange of gases (oxygen and carbon dioxide) between the lungs and the blood occurs. Later on passing through the parabronchi, air moves into the ventrobronchi.
Semi-schematic drawing of the lung-air sac organisation in situ. The cranial half of the dorsobronchi (four) and the parabronchi (half dozen) has been removed. 1 = trachea, 2 = master bronchus, 3 = ventrobronchi with the connections into (A) cervical, (B) interclavicular and (C) cranial thoracic air sacs, 5 = laterobronchi and the caudal primary bronchus open into the (D) posterior thoracic and (Due east) abdominal air sacs (From: Duncker 2004).
Avian respiratory arrangement showing the bronchi located within the lungs. Dorsobronchi and ventrobronchi branch off of the primary bronchus; parabronchi extend from the dorsobronchi to the ventrobronchi. Light blue arrows indicate the direction of air menstruum through the parabronchi. The primary bronchus continues through the lung and opens into the abdominal air sac. (Source: http://world wide web.ivis.org/advances/Anesthesia_Gleed/ludders2/chapter_frm.asp)
Birds exhibit some variation in lung structure and, specifically, in the arrangement of parabronchi. Nearly birds have two sets of parabronchi, the paleopulmonic ('aboriginal lung') and neopulmonic ('new lung') parabronchi. However, the neopulmonic region is absent in some birds (east.1000., penguins) and poorly adult in others (e.g., storks [Ciconiidae] and ducks [Anatidae]). In songbirds (Passeriformes), pigeons (Columbiformes), and gallinaceous birds (Galliformes), the neopulmonic region of the lung is well-developed (Maina 2008). In these latter groups, the neopulmonic parabronchi contain about 15 to 20% of the gas exchange surface of the lungs (Fedde 1998). Whereas airflow through the paleopulmonic parabronchi is unidirectional, airflow through the neopulmonic parabronchi is bidirectional. Parabronchi tin exist several millimeters long and 0.5 - 2.0 mm in diameter (depending on the size of the bird) (Maina 1989) and their walls contain hundreds of tiny, branching, and anastomosing air capillaries surrounded by a profuse network of blood capillaries.
Differences amongst different birds in the development of the neopulmonic region of the lung. (a) Penguin lungs are entirely paleopulmonic.
(b) Some birds, such as ducks, take a relatively small neopulmonic region. (c) Songbirds have a well-adult neopulmonic region.
1, trachea, 2, primary bronchus, 3, ventrobronchus, 4, dorsobronchus, 5, lateral bronchus, half dozen, paleopulmonic parabronchi,
7, neopulmonic parabronchi; A, cervical alveolus, B, interclavicular air sac, C, cranial thoracic alveolus, D, caudal thoracic air sac,
E, abdominal air sac. The white arrows signal changes in volume of the air sacs during the respiratory bicycle (From: McLelland 1989).
So, how does air flow through the avian lungs & air sacs during respiration?
Air menstruation through the avian respiratory system during inspiration (a) and expiration (b).
1 - interclavicular alveolus, ii - cranial thoracic air sac, 3 - caudal thoracic air sac, 4 - abdominal air sac
(From: Reese et al. 2006).
A schematic of the avian respiratory system, illustrating the major air sacs and their connections to the lung. (A) The lateral and dorsal management of movement of the rib cage during exhalation is indicated past arrows. (B) The management of airflow during inspiration. (C) The direction of flow during expiration (From: Plummer and Goller 2008).
Respiratory airflow in avian lungs. Filled and open up arrows denote direction of air catamenia 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. In bird lungs (A), well-nigh air directly enters the caudal air sacs during inspiration (thick black arrow), whereas a lesser part flows through the parabronchi/air capillaries into cranial air sacs (thin black arrows). During expiration the major function of inspired air streams from the reservoirs (caudal air sacs, thick open arrows) through the parabronchi/air capillaries into major distal airways, where it mixes with the deoxygenated respiratory gas stored in cranial air sacs during the inspiratory phase. Consequently, respiratory gas flow through the parabronchi, atria, and the gas-exchanging air capillaries is unidirectional and continuous during both inspiration and expiration. This principle is achieved past cranio-caudal pressure gradients in the respiratory system changing between inspiration and expiration and the consecutive opening and closing of valve systems between mesobronchi/air sacs and the parabronchi (not indicated in the figure). Hence, airflow is constant and loftier in the parabronchi, atria, and the gas-exchanging air capillaries (From: Bernhard et al. 2004).
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 organization. Surfactant SP-A has only been detected in the mesobronchi of birds. SP-A plays an of import part in innate host defense and regulation of inflammatory processes and may exist important in the mesobronchi considering air flow is slower and small particles could tend to accrue in that location (run into figure below). Surfactant SP-C is not found in the avian respiratory system (or, if so, in very small quantities), but is found in the alveoli of mammals along with SP-A and SP-B. Because the mammalian respiratory arrangement (beneath) includes structures that are collapsible (alveoli) and areas with low airflow, all 3 surfactants are important for reducing surface tension and innate host defense (Bernhard et al. 2004).
Airflow in mammalian lungs is bidirectional during the respiratory wheel, with highly reduced airflow
in peripheral structures, i.eastward., bronchioles and, particularly, the gas-exchanging alveoli. Consequently, minor particles (< 1 µm)
that enter the alveoli may sediment, making a system of start line of defence necessary, comprising alveolar macrophages
(white blood cells), SP-A, and (phospholipid) regulators of inflammatory processes (From: Bernhard et al. 2004).
A: A high-ability view of a foreign particle (p) existence engulfed past an epithelial cell (e) in an avian lung.
Arrows, elongated microvilli. B: Surface of an atrium of the lung of the domestic fowl showing red blood
cells with i of them (r) being engulfed by the underlying epithelial cell (pointer): eastward, epithelial surface; m, a free
(surface) macrophage. Scale bars: A = 0.5 µm; B = 10 µm (From: Nganpiep and Maina 2002).
Air menses is driven by changes in pressure within the respiratory system:
- During inspiration:
- the sternum moves forrad and down while the vertebral ribs movement cranially to aggrandize the sternal ribs and the thoracoabdominal cavity (come across diagrams below). This expands the posterior and inductive air sacs and lowers the pressure, causing air to move into those air sacs.
- Air from the trachea and bronchi moves into the posterior air sacs and, simultaneously,
- air from the lungs moves into the anterior air sacs.
Changes in the position of the thoracic skeleton during animate in a bird. The solid lines represent
thoracic position at the end of expiration while the dotted lines show the thoracic position
at the end of inspiration (Source: http://www.ivis.org/advances/Anesthesia_Gleed/ludders2/chapter_frm.asp).
Cartoon 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 crenel (PC) where the lungs are located from the subpulmonary cavity (SP) where almost
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 intestinal 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, just 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 2006). The increase in volume lowers air pressure and draws air
into the air sacs.
Schematic representation of the lungs and air sacs of a bird and the pathway of
gas flow through the pulmonary system during inspiration and expiration. For purposes of clarity, the neopulmonic lung
is not shown. The intrapulmonary bronchus is as well known every bit the mesobronchus. A - Inspiration. B - Expiration
Source: http://www.ivis.org/advances/Anesthesia_Gleed/ludders2/chapter_frm.asp
- During expiration:
- the sternum moves backward and up & the vertebral ribs move caudally to retract the sternal ribs and reduce the book of the thoracoabdominal cavity. The reduces the volume of the anterior & posterior air sacs, causing air to move out of those sacs.
- Air from the posterior sacs moves into the lungs &, simultaneously,
- air from the anterior sacs moves into the trachea & out of the body.
So, air always moves unidirectionally through the lungs and, equally a result, is higher in oxygen content than, for example, air in the alveoli of humans and other mammals.
| | Role of uncinate processes and associated muscles in avian respiration -- Codd et al. (2005) examined the activity of three muscles associated with the uncinate processes, (1) external intercostal, (2) appendicocostalis and (iii) external oblique (labeled in drawing to the left) examined using electrodes during sitting, standing and moderate speed treadmill running in a Giant Canada Goose. The external intercostal muscles demonstrated no respiratory activity, beingness agile merely during running, suggesting they play some role in trunk stabilization. The appendicocostalis and external oblique muscles are respiratory muscles, beingness agile during inspiration and expiration, respectively. The activity of the appendicocostalis musculus increased when sternal movements were restricted, suggesting that activity of these muscles may exist particularly important during prolonged sitting such as during egg incubation. Codd et al. (2005) suggested that the uncinate processes in birds facilitate movements of the ribs and sternum during breathing and therefore are integral to the breathing mechanics of birds. |
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, Hawkeye 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 courage and sternum. The insertion of the appendicocostales muscles nigh the end of the uncinate processes may provide a mechanical advantage for moving the elongated ribs during animate (Tickle et al. 2007).
substantial temperature increases because of greenhouse warming sparked 2 major mass-extinction events. In addition, he believes the atmospheric condition spurred the development of an unusual breathing system in Saurischian dinosaurs. Rather than having a diaphragm to force air in and out of lungs, the Saurischians had lungs attached to a series of thin-walled air sacs that announced to have functioned something similar bellows to move air through the trunk. This breathing system, nevertheless plant in today'due south birds, fabricated the Saurischian dinosaurs better equipped than mammals to survive the harsh conditions in which oxygen content of air at the Earth'south surface was simply about half of today's 21%. "The literature always said that the reason birds had sacs was and so they could exhale when they fly. But I don't know of whatsoever brontosaurus that could fly," Ward said. "Yet, when we considered that birds fly at altitudes where oxygen is significantly lower, we finally put it all together with the fact that the oxygen level at the surface was only x - 11% at the time the dinosaurs evolved. That's the same as trying to breathe at 14,000 feet. If y'all've ever been at 14,000 feet, you know information technology's non easy to breathe," he said. Ward presented his ideas at the 2003 annual meeting of the American Geological Club in Seattle. See: http://www.nature.com/nsu/031103/031103-seven.html
Exchange of gases:
In the avian lung, oxygen diffuses (by unproblematic improvidence) from the air capillaries into the blood & carbon dioxide from the claret into the air capillaries (shown in this effigy and in figures beneath ). This exchange is very efficient in birds for a number of reasons. Outset, the complex arrangement of blood and air capillaries in the avian lung creates a substantial expanse through which gases tin lengthened. The surface expanse available for commutation (SAE) varies with bird size. For instance, the ASE is well-nigh 0.17 m 2 for Firm Sparrows (virtually 30 gms; Passer domesticus), 0.nine m two for Stone Pigeons (about 350 gms; Columba livia), 3.0 one thousand ii for a Mallard (about 1150 gms; Anas platyrhynchos), and viii.9 thousand 2 for a male Graylag Goose (about 3.7 kg; Anser anser) (Maina 2008). All the same, smaller birds take a greater SAE per unit mass than do larger birds. For example, the SAE is about 90 cm 2/gm for Violet-eared Hummingbirds (Colibri coruscans; Dubach 1981), about 26 cm 2/gm for Mallards, and most 5.4 cm 2/gm for Emus (Dromaius novaehollandiae; Maina and Rex 1989). Among mammals, there is likewise a negative relationship betwixt SAE and body size, with smaller mammals like shrews having a greater SAE per unit mass than larger mammals. However, for birds and mammals of like size, the SAE of birds is more often than not about 15% greater (Maina et al. 1989).
A second reason why gas exchange in avian lungs is so efficient is that the blood-gas bulwark 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. Amongst terrestrial vertebrates, the blood-gas barrier is thinnest in birds. Natural selection has favored thinner claret-gas barriers in birds and mammals because endotherms use oxygen at higher rates than ectotherms like amphibians and reptiles. Amidst birds, the thickness of the blood-gas bulwark varies, with smaller birds generally having thinner claret-gas barriers than larger birds. For example, the blood-gas barrier is 0.099 μm thick in Violet-eared Hummingbirds and 0.56 μm thick in Ostriches (West 2009).
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 (W 2009).
Also contributing to the efficiency of gas exchange in avian lungs is a process called cross-current substitution. Air passing through air capillaries and blood moving through blood capillaries more often than not travel at right angles to each other in what is chosen cross-current menstruum (Figure beneath; Makanya and Djonov 2009). As a outcome, 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.e., higher partial pressures) in the claret leaving the lungs than is possible in the alveolar lungs of mammals (Figures below).
| | Diagram of parabronchial anatomy, gas-exchange region of the bird'south lung-air-sac respiratory system. The few hundred to thou parabronchi, one of which is fully shown here, are packed tightly into a hexagonal array. The fundamental parabronchial lumen, through which gas flows unidirectionally during both inspiration and expiration is surrounded by gas-exchange tissue composed of an intertwined network of blood and air capillaries. On the left side of this diagram, the lumen of the parabronchus leads into multiple chambers called atria (A) that, in turn, pb into smaller chambers chosen infundibulae (I). Branching from the infundibulae are numerous air capillaries. On the correct side of this diagram are the blood vessels. Arteries (a) lead into the capillaries that are closely associated with the air capillaries. It is here (air and blood capillaries) where oxygen and carbon dioxide are exchanged. After flowing through the capillaries, blood then moves into the veins (v) that volition have the claret out of the lungs (From: Duncker 1971 as reprinted in Powell 2000). |
(A) Micrograph of lung tissue from a Brown Honeyeater (Lichmera indistincta) showing (a) parabronchi, (b) blood vessel, and (c) exchange tissue (bar, 200 micrometers). (B) Electron micrograph from the lung of a Welcome Swallow (Hirundo neoxena) showing (a) blood-air barrier, (b) air capillary, (c) claret capillary, and (d) blood-red blood prison cell in the claret capillary (bar, 2 micrometers). (From: Vitali and Richardson 1998).
A) Medial view of the lung of a domestic chicken (Gallus gallus domesticus). p, primary bronchus; v, ventrobronchus; d, dorsobronchus; r, parabronchi. Scale bar, 1 cm. (B) An intraparabronchial artery (i) giving rise to blood capillaries (c) in the lung of an Emu (Dromiceus novaehollandiae). a, air capillaries. Calibration bar, 15 μm. (C) Air capillaries closely associated with claret capillaries (arrows) in a craven lung. Scale bar, x μm. (D) Blood capillaries (c) closely associated with air capillaries (spaces) in a chicken lung. Scale bar, 12 μm. (From: Maina 2002).
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). Notation that the general direction
of blood flow through the blood capillaries is perpendicular to the flow of air through
the air capillaries, i.e., cross-current flow (From: Makanya and Djonov 2009).
| | Morphology of a chicken lung. Lite microscopy (top prototype) and electron microscopy (bottom two images) of a chicken lung depicting the respiratory system of birds. In the bird lung, air capillaries (Ac) run along with blood capillaries forming the blood-air bulwark that is typically < 0.two µm in thickness. The barrier (shown in the bottom image) separates the lumen of the Ac (*) from the red blood cells (RBC) in the blood capillaries and consists of a mostly continuous surfactant layer (arrows), thin cytoplasmic processes of epithelial cells (Ep), a common basal membrane (Bm), and the endothelial cells of the blood capillary (En). Surfactant is a mixture of lipids and proteins that acts in the air capillaries of avian lungs both as an "antiglue" (preventing the adhesion of respiratory surfaces that may occur when the lungs collapse, e.chiliad., during diving, swallowing of prey or on expiration) and to forestall liquid influx into the lungs (Daniels et al. 1998). Magnifications: superlative image - ×270; centre image - ×1,600; bottom epitome - ×88,000 (Image from Bernhard et al. 2001). |
In birds, the thickness of the blood-gas barrier in the 7.3-grand Violet-eared Hummingbird (Colibri coruscans) is 0.099 µm, whereas that of an immature 40-kg Ostrich (Struthio camelus) is 0.56 µm (Maina and Due west 2005).
Human relationship between the harmonic mean thickness of the blood-gas barrier (the thickness of the barrier that affects the improvidence of oxygen from air capillaries into blood capillaries) against body mass in the lungs of bats, birds, and non-flying mammals. Birds accept especially thinner barriers than bats and not-flying mammals
(Maina 2000).
Lite micrographs of a portion of the lung of a craven (A) and rabbit (B).
Note the small-scale diameter of the air capillaries in the chicken lung vs. that of the rabbit alveoli (aforementioned magnification).
(A) In the chicken lung, pulmonary capillaries are supported past 'struts' of epithelium (arrows). (B) In the rabbit lung,
pulmonary capillaries are suspended in the big spaces between alveoli (Watson et al. 2007).
Cross-current exchange:
Top: Air flow (large arrows) and claret menstruum (small arrows) illustrating the cross-current gas-exchange mechanism operating
in the avian lung (between the claret capillaries and air capillaries). Note the serial organisation of blood capillaries running from the periphery to the lumen of the parabronchus and the air capillaries radially extending from the parabronchial lumen. The substitution of gases (elementary diffusion of O2 and COtwo) occurs simply between blood capillaries and air capillaries. As air moves through a parabronchus and each successive air capillary, the partial pressure of oxygen (PO2) declines (as indicated by the decreased density of the stippling) because oxygen is diffusing into the blood capillaries associated with each air capillary. Every bit a upshot of this diffusion, the fractional pressure of oxygen in the blood leaving the lungs (pulmonary vein) is college than that in blood entering the lungs (pulmonary artery) (equally indicated by the increased density of the stippling).
Bottom: Relative fractional pressures of O2 and CO2 (1) for air entering a parabronchus (initial-parabronchial, PI) and air leaving a parabronchus (end-parabronchial, PE), and (2) for claret before entering claret capillaries in the lungs (pulmonary artery, PA) and for blood after leaving the blood capillaries in the lungs (pulmonary vein, P5). The partial pressure of oxygen (PO2) of venous blood (PV) is derived from a mixture of all serial air capillary-claret capillary units. Because of this cross-current substitution the partial pressure of oxygen in avian pulmonary veins (P5) is greater than that of the air leaving the parabronchus (PDue east); air that will be exhaled. In mammals, the fractional pressure level of oxygen in veins leaving the lungs cannot exceed that of exhaled air (terminate-expiratory gas, or PE) (Figure adapted from Scheid and Piiper 1987). Importantly, the partial pressure level of oxygen in claret leaving the avian lung is the event of 'mixing'; blood from a series of capillaries associated with successive air capillaries along the length of a parabronchus is mixed equally the claret leaves the capillaries and enters pocket-size veins. Equally a issue, the management of air flow through a parabronchus does not effect the efficiency of the cross-electric current exchange (because gases are only exchanged between blood capillaries and air capillaries, non between the parabronchus and the blood). And then, in above diagram, reversing the management of air flow would obviously mean that the air capillary on the far left would accept the highest partial pressure of oxygen rather than the air capillary on the far right (and so the stippling pattern that indicates the amount of oxygen in each air capillary would exist reversed). Even so, because of the 'mixing' of blood only mentioned, this reversal would accept petty effect on the PFive, the fractional force per unit area of oxygen in blood leaving via pulmonary veins (the PO2 would probable be a bit lower because some oxygen would accept been lost the first time air passed through the neopulmonic parabronchi). This is important considering well-nigh birds take neopulmonic parabronchi as well as paleopulmonic parabronchi and, although air flow through paleopulmonic parabronchi is unidirectional, air menses through neopulmonic parabronchi is bidirectional.
Diagram showing the menstruation 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 forth 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 claret
by interparabronchial arteries (IPA) that give ascension to arterioles (stars) that terminate in claret capillaries. Subsequently passing through the capillaries, blood flows
into the intraparabronchial venules (asterisks) that bleed into interparabronchial veins (IPV). These in turn empty into the pulmonary vein which returns the
blood to the heart. (From: Maina and Woodward 2009).
Command of Ventilation:
Ventilation and respiratory rate are regulated to run across the demands imposed by changes in metabolic activity (e.g., rest and flight) every bit well every bit other sensory inputs (e.g., oestrus and cold). There is likely a central respiratory control center in the avian brain, but this has not been unequivocally demonstrated. Equally 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 encephalon. It also appears that the chemical drive on respiratory frequency and inspiratory and expiratory elapsing depend on feedback from receptors in the lung also as on extrapulmonary chemoreceptors, mechanoreceptors, and thermoreceptors (Ludders 2001).
Central chemoreceptors affect ventilation in response to changes in arterial PCO 2 and hydrogen ion concentration. Peripheral extrapulmonary chemoreceptors, specifically the carotid bodies (located in the carotid arteries), are influenced by PO 2 and increase their belch rate as PO 2 decreases, thus increasing ventilation; they decrease their rate of belch as PO 2 increases or PCO ii decreases. These responses are the same as those observed in mammals. Dissimilar 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 jiff-to-breath footing by acting as the afferent limb of an inspiratory-inhibitory reflex that is sensitive to the timing, rate, and extent of CO2 washout from the lung during inspiration (Ludders 2001).
Respiration by Avian Embryos
| During avian development at that place are iii sequential stages of respiration (Tazawa 1987): prenatal (embryonic), paranatal (hatching), and postnatal (posthatching). During the prenatal phase respiratory gas exchange occurs via improvidence between the external environment and the initial gas exchanger (i.due east., the expanse vasculosa, or the region of blood island formation and forerunner of the chorioallantoic membrane) in early on embryonic life and later the vascular bed of the chorioallantois. The paranatal phase starts when the bill penetrates into the air pocket (air cell) between the inner and outer shell membranes (both internal to shell; i.east., internal pipping) this occurs during the last two-3 days of incubation. During this stage, the lungs brainstorm to replace the chorioallantois equally the gas exchanger, nonetheless improvidence remains the major mechanism moving gas across the crush. The postnatal phase begins when the neb penetrates the beat (i.e., external pipping) (Brown et al. 1997). |  Source: world wide web.ece.utexas.edu/~bevans/courses/. . . |
Chicken embryo
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Useful links:
How Animals Work: Avian Respiratory Dynamics Animation
More lecture notes:
Energy Remainder & Thermoregulation
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During Inspiration Intrapulmonary Pressure Is,
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