Though mammals and birds have higher rates of metabolism, and therefore O2 consumption, and both possess lungs for gas exchange, these taxa have evolved significant differences in their respiratory systems (Figure 11 ). In mammals, influx of air occurs through a trachea, which branches into bronchia of the lungs, and then repeatedly bifurcates into smaller and smaller tubes until reaching alveoli, the numerous (million) blind sacs where gas exchange occurs. After exchange of O2 and CO2, air is exhaled along the same pathways, but lungs are not completely emptied of this air; 88% of air in the lung of humans is carried over from previous breaths and therefore has lower O2 and higher CO2 partial pressures than is found in the atmosphere. For mammals ranging in size from shrews to whales, lung volume averages a constant value of about 6% of body mass, though smaller mammals have a higher rate of metabolism compared with larger ones. Hence, small mammals must breathe more frequently to supply O2 to tissues. For humans, total surface area of the lung is 140 m , equivalent to the floor area of a college lecture hall. Contraction of a diaphragm, a sheet of muscle that separates the thorax and abdomen, increases the volume of the thorax causing a negative pressure and therefore inhalation of air. When the diaphragm relaxes, air is exhaled.
The respiratory system of birds, the most efficient gas-exchange system in air-breathing vertebrates, consists of a rigid inflexible lung together with a series of nine nonvascularized air sacs that occupy both the thorax and abdomen. Bird lungs contain a series of parallel tubes called parabronchi (Figure 11), which have extending from them tiny air capillaries, the principal sites of gas exchange. Although in general bird lungs are 25% smaller than those of mammals, they also have a larger surface area for gas exchange. In addition, birds breathe more frequently than do mammals. A flying bird consumes 2.5 times more O2 than does a similar-sized running mammal. Air sacs expand and contract forcing air along air capillaries in a unidirectional flow, further enhancing O2 extraction. Some birds migrate over the
Himalayan mountains, at an altitude above 30 000 feet, where O2 partial pressures are less than one-third those at sea level. Such low O2 densities would render a small mammal comatose, emphasizing the efficiency of O2 extraction of bird lungs.
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