Comparative quantitative morphology of the mammalian lung: Trachea

Abstract

Measurements of trachea! dimensions in a variety of mammals, covering the extremes of body size, indicate that tracheal length is nearly proportional to M 0.27, and tracheal radius is proportional to M 0.39. The ratio of ventilation to cross-sectional area of the trachea is a constant for all species, hence average linear flow velocity in the trachea is the same in all mammals. Assuming dead space volume is proportional to tracheal volume, the dead space volume was found to be proportional to M 1.05, in other words, it is proportional to both tidal volume and total pulmonary capacity; and since respiratory frequency is proportional to M −0.28, dead space ventilation is computed to be proportional to M 0.78, and hence, is nearly proportional to expired ventilation. From this it may be inferred that alveolar ventilation is proportional to metabolic CO2 production in all mammals and thus alveolar P CO 2 , is the same in all mammals. Similarly, since alveolar ventilation is proportional to oxygen consumption, alveolar P O 2 will also be nearly the same in all mammals at sea level.