Bird respiration: flow patterns in the duck lung.

The respiratory system of birds is structurally and functionally very different from that of mammals and has therefore attracted a great deal of interest. Briefly, the avian respiratory system consists of paired lungs, where gas exchange with the blood occurs, and of several large air sacs (grouped as anterior and posterior sacs) that act as bellows to move gases over the exchange surfaces of the lungs (Fig. 1 A). The gas exchange does not take place in alveoli but in air capillaries, and the finest branches of the bronchi permit through-flow of air. The air sacs are poorly vascularized and apparently do not participate in gas exchange. In spite of a large body of anatomical information the important question of how air flows in the complex system of passageways in the bird lung has remained open.