Respiratory System In Birds

Blood flow

(b) Countercurrent flow

% Saturation fExchange is more complete with countercurrent flow.

Blood flow 20% 25 30 35 40 45 50 55 60 65 70 75 100

48.6 Countercurrent Exchange Is More Efficient than Concurrent Exchange In these models of concurrent and countercurrent gas exchange, the numbers represent the O2 saturation of blood and water. (a) In a concurrent exchanger, the percentages of saturation of blood and water would reach equilibrium even before the water had flowed halfway across the exchange surface. (b) A countercurrent exchanger, such as that found in fish gills, allows more complete exchange because a gradient of O2 saturation is always maintained.

Water flow 25% 30 35 40 45 50 55 60 65 70 75 80 100

Blood flow 20% 25 30 35 40 45 50 55 60 65 70 75 100

In the countercurrent exchanges the water is always more saturated than the blood so that a gradient of O2 saturation exists over the full length of exchange surfaces.

Air Capillaries Bird

Blood capillary

Air capillaries

Blood capillary

Air capillaries

48.7 The Respiratory System of a Bird (a) The air sacs and air spaces in the bones are unique to birds. (b) Air flows through bird lungs unidirectionally in parabronchi. Air capillaries, the site of gas exchange, branch off the parabronchi.

out through the same airways, as it does in mammals. Thus there is little dead space in bird lungs, and the fresh incoming air is not mixed with stale air. In this way, a high Po2 gradient is maintained.

In addition to lungs, birds have air sacs at several locations in their bodies. The air sacs are interconnected with the lungs and with air spaces in some of the bones (Figure 48.7a). The air sacs receive inhaled air, but they are not gas exchange surfaces. As in other air-breathing vertebrates, air enters and leaves a bird's gas exchange system through a trachea (commonly known as the windpipe), which divides into smaller airways called bronchi (singular, bronchus). In air-breathing vertebrates other than birds, the bronchi generate trees of branching airways that become finer and finer until they dead-end in clusters of microscopic, membrane-enclosed air sacs, where gases are exchanged. In bird lungs, however, there are no dead ends; air flows unidirectionally through the lungs (Figure 48.8).

In bird lungs, the bronchi divide into tubelike parabronchi that run parallel to one another through the lungs (Figure 48.7b). Branching off the parabronchi are numerous tiny airways called air capillaries. Air flows through the lungs in the parabronchi and diffuses into the air capillaries, which are

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