Structure and Function of Avian Pulmonary Capillaries: Comparison with Mammals

Abstract

The avian pulmonary circulation has many interesting features and much information can be gained by comparing the avian and mammalian systems. Two features are primarily responsible for the unique features of avian capillaries. First, the avian lung has separated the ventilation and gas exchange functions. Second, the avian lung uses a flow-through process for ventilation rather than the reciprocal pattern adopted by mammals. As a consequence, the environment of the pulmonary capillaries is very different in the avian compared with the mammalian lung. The avian capillaries are nested in a syncytium of air capillaries, whereas the capillaries of the mammalian lung are strung out along the alveolar walls. This means that the mechanical support of the categories is very different in birds compared with mammals. A consequence of this is that the walls of the capillaries are very different. In the avian lung, the blood–gas barrier is extremely thin and uniform throughout the capillaries. Contrast this with the mammalian lung where a type I collagen cable is required for the support of the capillaries, and as a result the blood–gas barrier is much thicker in places and the diffusion characteristics are therefore inferior. Another striking difference is that avian pulmonary capillaries are remarkably rigid unlike those in mammals, which undergo recruitment and distention when the cardiac output rises. The implications of this for pulmonary vascular resistance during intense exercise such as flying are still unclear.