Gas Exchange in Birds

The gas exchange system in birds exhibits many unique features that endow them with the enhanced capacity for oxygen transport that is believed to underlie the evolution of the increased cardiorespiratory performance required to support flight. Bird lungs are rigid and are ventilated by an extensive air sac system that isolates the expansible bellows from the gas exchange surface. This reduces the mechanical forces acting on the lung itself allowing the gas-exchange tissue to be exceptionally thin with a relatively large surface area. The avian blood-gas barrier is also mechanically stronger and more resistant to stress failure than that of mammals. Airflow through the lung is unidirectional, unlike the tidal flow in the lungs of most other vertebrates. Blood in the pulmonary capillaries flows perpendicular to the air diffusing into the air capillaries in a cross-current fashion that is inherently more effective than the alveolar exchange mechanism of mammalian lungs. When the demand for gas exchange increases, birds are thought to be capable of higher ventilation rates than mammals and many are also capable of large increases in the volume of each breath (reducing dead space ventilation). The latter is accommodated by a more compliant respiratory system associated with the presence of the large air sacs. CO₂ excretion also increases as a secondary consequence of large increases in ventilation, which restrains the ventilatory response. Birds can ventilate more than mammals in severe hypoxia because of an enhanced capacity to restore blood pH rapidly when blood PCO₂ changes and because the brain vasculature is insensitive to hypocapnia. Compared to mammals, birds have larger hearts and cardiac stroke volumes, and can sustain heart rates during free flight that are similar to or greater than those of mammals during maximal exercise. The bird heart also has a higher capillarity than the mammalian heart with a resulting increase in oxygen diffusion capacity. Finally, the capacity for O₂ diffusion into the peripheral tissues appears to be higher in birds than in mammals and other vertebrates largely because there is a mesh of branching capillaries that surrounds avian muscle fibres, which are themselves smaller in size when compared to mammals. The unique respiratory and cardiovascular physiology of birds enhances hypoxia tolerance and exercise capacity by improving the overall capacity for O₂ transport. Birds that fly at high altitude appear to have evolved further specialized traits that set them apart from most other birds.