The Lung Lobes Of An Avain

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In certain aspects that bear strongly on its function, the structure of the avian respiratory system is exceptional: the lung is practically rigid and inflexible; unlike the mammalian lung that is tidally (bidirectionally=in-and-out) ventilated, the exchange tissue (ET) of the avian lung, specifically the paleopul-monic parabronchi (PPPR; Sect. 3.7), is ventilated unidirectionally and continuously by synchronized action of the air sacs (ASs); the gas exchanger (the lung) has been totally disengaged from the ventilator (the air sacs); compression of the avian lung does not cause significant collapse of the ACs (Macklem et al. 1979); and capacious and transparent, the ASs are totally avascular and play no direct role in gas exchange (e.g. Magnussen et al. 1976). Although deeply marked by the vertebral ribs on the dorsolateral part, i.e. the costal and vertebral surfaces (Fig. 38), in complete contrast to the mammalian lung, the avian lung is never divided into lobes (Fig. 39). From about one-fifth to one-third of the volume of the avian lung is contained between the ribs (e.g. King and Molony 1971). In the more derived birds, where the neopulmonic parabronchi (NPPR) are well developed (Sect. 3.6), the avian lungs are small and fairly wedge-shaped. Such lungs essentially have three surfaces: the dor-solateral surface that contacts the ribs, i.e. the thoracic wall, and is called the costal surface; the dorsomedial surface that contacts the vertebrae and is called the vertebral surface; and the ventromedial surface that is in contact with the tissue of the horizontal septum and is termed the septal surface. Scattered on the septal surface are ostia, openings that connect the bronchi, i.e. PB, SB, and PR, to the ASs (Figs. 40 and 41). With the lungs displaced to the dorsal part of the coelomic cavity (where they are firmly attached to the vertebral ribs; Figs. 38 and 39) and a diaphragm lacking, the liver rather than the lungs, as is the case in mammals, surrounds the heart. In most species of birds, the lungs extend cranially to about the level of the first cervical rib while they terminate caudally near the cranial border of the ilium.

Pertaining to certain general design features that may be purely coincidental or evolutionary (e.g. Perry 1992), the avian respiratory system, the lung-air

Lambdotherium

Fig. 38A,B. Latex cast preparations of the lung and the air sacs of the domestic fowl, Gallus gallus variant domesticus. Tr Trachea; L lungs; arrows costal sulci; circles ostia; 1 cervical air sac; 2 interclavicular air sac; 3 craniothoracic air sac; 4 caudothoracic air sac; 5 abdominal air sac. Scale bars 1 cm. (A from Maina 2002 c; B from Maina and Africa 2000)

Fig. 38A,B. Latex cast preparations of the lung and the air sacs of the domestic fowl, Gallus gallus variant domesticus. Tr Trachea; L lungs; arrows costal sulci; circles ostia; 1 cervical air sac; 2 interclavicular air sac; 3 craniothoracic air sac; 4 caudothoracic air sac; 5 abdominal air sac. Scale bars 1 cm. (A from Maina 2002 c; B from Maina and Africa 2000)

sac system, resembles the reptilian one (Fig. 42): an anterior compartmented space in which most gas exchange occurs (analogous to the avian lung) and a distended, smooth posterior part (analogous to the ASs of the avian respiratory system) occur (e.g. Maina 1989a; Maina et al. 1999). The African chameleon has some saccular extensions of the lung that branch out into the abdominal cavity (e.g. Patt and Patt 1969; Maina 1998). The high level of gasexchange efficiency of the avian lung largely arises from the structural uniqueness of the PR and the pattern and relative directions of flow of air and blood, features that form the basis of the cross-current and counter-current gas-exchange systems (Sect. 3.9). Albeit the remarkable specific diversity in the avian taxon, amazingly, the basic structure of the respiratory system in birds is similar. Differences in fine details, particularly regarding the extent of development of the PR, the arrangement of the SB, and the location, connec-

Avian Lung

Fig. 39A,B. Medial and dorsal views of the lungs of the domestic fowl, Gallus gallus variant domesticus, and the ostrich, Struthio camelus. Arrows Costal sulcae; Tr trachea; Sx location of the syrinx; dashed encircled area hilus; EPPB extrapulmonary primary bronchus. Scale bars A, 1 cm; B, 2 cm. (A from Maina 2002a; B from Maina and Nathaniel 2001)

Fig. 39A,B. Medial and dorsal views of the lungs of the domestic fowl, Gallus gallus variant domesticus, and the ostrich, Struthio camelus. Arrows Costal sulcae; Tr trachea; Sx location of the syrinx; dashed encircled area hilus; EPPB extrapulmonary primary bronchus. Scale bars A, 1 cm; B, 2 cm. (A from Maina 2002a; B from Maina and Nathaniel 2001)

Fig. 40. A Medial view of the lung of the domestic fowl, Gallus gallus variant domesticus, showing air-conducting passages that include the medioventral secondary bronchi (MVSB), lateroventral secondary bronchi (LVSB), paleopulmonic parabronchi (PPPR) and neopulmonic parabronchi (NPPR). Arrow Ostium. Scale bar 1 cm. B The avian lung drawn as transparent to show the airways. mv Medioventral secondary bronchi; md mediodorsal secondary bronchi; lv lateroventral secondary bronchi; p parabronchi; r costal sulci; AbO abdominal ostium; CathO caudal thoracic ostium; Prb primary bronchus; ICrthO cranial thoracis ostium; IclO interclavicular ostium. (A from Maina 2002 c; B from King and McLel-land 1989, redrawn with permission from the publisher)

Fig. 40. A Medial view of the lung of the domestic fowl, Gallus gallus variant domesticus, showing air-conducting passages that include the medioventral secondary bronchi (MVSB), lateroventral secondary bronchi (LVSB), paleopulmonic parabronchi (PPPR) and neopulmonic parabronchi (NPPR). Arrow Ostium. Scale bar 1 cm. B The avian lung drawn as transparent to show the airways. mv Medioventral secondary bronchi; md mediodorsal secondary bronchi; lv lateroventral secondary bronchi; p parabronchi; r costal sulci; AbO abdominal ostium; CathO caudal thoracic ostium; Prb primary bronchus; ICrthO cranial thoracis ostium; IclO interclavicular ostium. (A from Maina 2002 c; B from King and McLel-land 1989, redrawn with permission from the publisher)

Fig. 41. Ventral view of a cast of the lung and air sacs of the domestic fowl, Gallus gallus variant domesticus, showing trachea (Tr), syrinx (S), cervical air sac (CeAs), extrapulmonary primary bronchus (£PP£),interclavicular air sac (JCAS),lung (L),primary bromchi (Pr), craniothoracic air sac (CrTAS), abdominal air sac (AAS), oblique septum (dashed line), and horizontal septum (arrows). Scale bar 1 cm. (Maina and Africa 2000)

Fig. 41. Ventral view of a cast of the lung and air sacs of the domestic fowl, Gallus gallus variant domesticus, showing trachea (Tr), syrinx (S), cervical air sac (CeAs), extrapulmonary primary bronchus (£PP£),interclavicular air sac (JCAS),lung (L),primary bromchi (Pr), craniothoracic air sac (CrTAS), abdominal air sac (AAS), oblique septum (dashed line), and horizontal septum (arrows). Scale bar 1 cm. (Maina and Africa 2000)

Fig. 42. Schematic diagrams of the lung of the monitor lizard, Varanus exanthemati-cus (A), and a mallard duck, Cairina moschata (B), showing the similarities between the designs of the reptilian and avian respiratory systems. (Reproduced from Duncker 1979b)

Fig. 43A,B. Medial and lateral views of the lung of the ostrich, Struthio camelus, showing medioventral secondary bronchi (MVSB), mediodorasal secondary bronchi (MDSB), pulmonary artery (PA), and pulmonary vein (PV). Arrows Costal sulci. Dashed area (A) Primary bronchus. Scale bars 1 cm. (Maina and Nathaniel 2001)

Fig. 43A,B. Medial and lateral views of the lung of the ostrich, Struthio camelus, showing medioventral secondary bronchi (MVSB), mediodorasal secondary bronchi (MDSB), pulmonary artery (PA), and pulmonary vein (PV). Arrows Costal sulci. Dashed area (A) Primary bronchus. Scale bars 1 cm. (Maina and Nathaniel 2001)

Fig. 42. Schematic diagrams of the lung of the monitor lizard, Varanus exanthemati-cus (A), and a mallard duck, Cairina moschata (B), showing the similarities between the designs of the reptilian and avian respiratory systems. (Reproduced from Duncker 1979b)

Fig. 44. Cast of a human lung given here to emphasize the structural differences that exist between it and the avian lung. A Grape-like arrangement of the alveoli (Al). B, C Alveoli (A/) are spherical, terminal gas-exchange units. Arrow Interalveolar pore (pore of Kohn). D Alveoli (Al) interface with blood capillaries (BC) located in the interalveolar septum. (A, D from Maina and van Gils 2001)

era -c tion, number of ASs, however, do occur. While the differences may be of some phylogenetic importance, most of them are of little, if any, functional consequence.

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