Respiration and the Effects of Pressure

Abstract

Respiration refers to gas exchange between an animal and the environment. Except for sea otters, which have unusually large lungs, the lung volumes of marine and terrestrial mammals scale isometrically with body mass. Compared with terrestrial mammals, marine mammals have a larger tidal volume (relative to total lung volume), exhibit high tidal flows (especially in small Odontoceti), and breathe apneustically in which there are periods of apnea. When averaged over periods of eupnea and apnea, marine mammals have a resting respiratory rate that is much less than predicted for but an average tidal volume that is threefold larger than predicted. Barotrauma is physical damage to tissues caused by a difference in pressure between gas in the respiratory system and the surrounding fluid or tissue, and this can be avoided if the pressure equalizes. Marine mammals have a very compliant thorax that enables their lungs (primarily the alveoli) to collapse under pressure. To prevent a high partial pressures of nitrogen in blood and tissues resulting in decompression sickness and nitrogen narcosis, reinforcement of the terminal airways in the lungs of pinnipeds and Cetacea allows the alveoli to collapse eventually isolating residual gas in the non-gas-exchanging airways. There is no clear effect of pressure on enzyme and membrane structure and function in marine mammals, but how they tolerate high pressures remains unknown.