Unsteady-state gas exchange and storage in diving marine mammals: the harbor porpoise and gray seal

Abstract

Breath-by-breath measurements of end-tidal O2 and CO2 concentrations in harbor porpoise reveal that the respiratory gas exchange ratio (RR; CO2 output/O2 uptake) of the first lung ventilation in a breathing bout after a prolonged breath-hold is always well below the animal's metabolic respiratory quotient (RQ) of 0.85. Thus the longest apneic pauses are always followed by an initial breath having a very low RR(0.6–0.7), which thereafter increases with each subsequent breath to values in excess of 1.2. Although the O2 stores of the body are fully readjusted after the first three to four breaths following a prolonged apneic pause, a further three to four ventilations are always needed, not to load more O2 but to eliminate built-up levels of CO2. The slower readjustment of CO2 stores relates to their greater magnitude and to the fact that they must be mobilized from comparatively large and chemically complex HCO[Formula: see text]/CO2 stores that are built up in the blood and tissues during the breath-hold. These data, and similar measurements on gray seals (12), indicate that it is the readjustment of metabolic RQ and not O2 stores per se that governs the amount of time an animal must spend ventilating at the surface after a dive.