Birds have a respiratory system which is functionally comparable to that of mammals but anatomically quite different. Birds don't breathe the same way mammals do. They have an incomplete diaphragm and the arrangements of the chest musculature and the sternum don't lend themselves to expansion in the same way that the chest of mammals does. Consequently they can't inflate and deflate the lungs in the same way. They are nevertheless "negative pressure breathers" and it's worth examining the mechanism in a little detail, as it will help make more sense of the histology.
The principles of gas exchange are the same in all vertebrates (and invertebrates, for that matter) since they're based on fundamental physics and follow the laws governing the behavior of gases. In mammals there's a thin overlying surface epithelium and capillaries below that. In birds, the structural arrangements are a little different, but the net result is the same: to place blood filled capillaries in close proximity to an epithelial surface, on the other side of which there is oxygen. The oxygen diffuses through the epithelium, through the capillary wall, and into the blood.
The birds' upper respiratory tract is histologically similar to mammals'. The same sort of respiratory epithelium is found in the nasal cavity, and a demonstration of a chicken face cut in frontal section, is available for you to examine. This slide is comparable to slide 115. Conchae are present, and you may see olfactory areas on this demonstration.
The trachea of birds is very similar to that of mammals, too; the principal difference in slide preparations which should tip you off is that the hyaline cartilage rings are complete in birds. Histologically, the trachea, the primary bronchi, and the mesobronchus for each lung (see below) are lined with typical respiratory epithelium. There is usually lymphatic tissue in the lamina propria of the bronchi, and smooth muscle around the outside.
It's in the structure of the lung that birds differ greatly from mammals, as you will see on slide 114. Avian lungs are very small, when considered as a proportion of the whole body size, and they are very stiff—they don't undergo appreciable volume changes during use, because, as discussed above, they don't have to be inflated.
The air sacs don't participate in gas exchange; they simply shuffle the air around. On slide 114, you'll see the straight through air passage called the mesobronchus passing through one lung. As it does so, you'll be able to see where it gives off secondary bronchi, and in turn where these give rise to the parabronchi (also called tertiary bronchi). The parabronchi can easily be distinguished because their wall is interrupted by the air vesicles projecting outwards from the lumen, which gives the wall a "scalloped" appearance.
For an overview of the histology of the avian lung, click here.
Within the parabronchi, smaller structures, the air vesicles are visible as little "bays" or extensions of the lumen of the parabronchus. The air vesicles themselves are composed of simple squamous epithelium, with an underlying CT to support it, and this epithelium is continuous with that lining the air capillaries. In the wall of each air vesicle are found numerous air capillaries, continuous loops from and back to the parabronchus, through which air passes. The air capillaries are laid alongside blood capillaries in the wall, in such a way as to establish a countercurrent flow.
To see some details of the lung exchange surface, click here.
Exchange of gases between the air capillaries and the blood is enhanced by this arrangement as the blood flowing through the blood capillaries is always being brought into contact with air fully saturated with oxygen. So you can see that despite the differences in their gross anatomy and histologic appearance, mammalian and avian lungs use the same basic strategies of an increase in surface area, and minimizing the physical barrier to gaseous diffusion. The physical and physiologic barrier to gas exchange is the same as mammals: lung epithelium, basal lamina(e), capillary wall.
In terms of overall operational efficiency, the feather wearing bipeds have us featherless ones beat all hollow. Their lungs are much more efficient than ours, thanks to the flow through and countercurrent mechanisms. They have to be, to meet the extreme demands of flight muscles for oxygenation.
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