The thickness of avian blood-gas barrier: qualitative and quantitative observations.
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Gas exchange in the avian lung takes place across a blood-gas barrier formed, as in other air-breathing vertebrates, by three essential components, an epithelial cell, a basal lamina, and an endothelial cell. The thickness of the barrier is of functional importance because it constitutes an integral determinant of both the pulmonary diffusing capacity and the tissue metabolism of the lung (Weibel & Knight, 1964; Weibel, 1970/71). Barrier thickness can be expressed either as the harmonic mean thickness which is a measure of the resistance to gas diffusion, or as the arithmetic mean thickness which expresses the tissue density of the barrier and hence its oxygen consumption (Weibel & Knight, 1964; Weibel, 1970/71; Weibel, 1973). The avian lung is regarded as the outstandingly efficient gas exchange system among the air breathing vertebrates (Duncker, 1971; Lasiewski & Calder, 1971; Weibel, 1973; Scheid, 1979). Flight requires an unusually large expenditure of energy (Berger & Hart, 1974). For example, a budgerigar (Melopsittacus undulatus) flying at sea level uses oxygen at almost twice the rate of a mouse of similar weight running hard in a wheel (Tucker, 1968a). In a hypobaric chamber, mice become comatose but house sparrows (Passer domesticus) remain active at a simulated altitude of 6100 m (Tucker, 1968b). The ability of some birds to perform the strenuous work of flapping flight at high altitude is exceptional by mammalian standards (Tucker, 1972; Berger, 1974). Attenuation of the blood-gas barrier without loss of its structural stability could be an evolutionary adaptation in the avian lung favouring a high oxygen transfer from air to blood. A number of authors (King, 1966; Duncker, 1970; King & Molony, 1971; Bouverot, 1978; Meban, 1980) have remarked on the thinness of the avian blood-gas barrier. However, few actual measurements have been published, the only available data being approximate estimates by Schulz (1962), Policard, Collet & Martin (1962) and King & Molony (1971), and a preliminary report of the present work by Maina (1981).