Respiratory Physiology of Freely Diving Harbor Seals (Phoca vitulina)

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Abstract
Two 2-year-old harbor seals (Phova vitulina) were studied during free diving in a circular tank in which they were forced to come to a one-way valve to breathe. Expired gas volume and composition were used to calculate and . End-tidal , and , were continuously recorded during the surface intervals of a series of spontaneous dives when the animals breathed room air or inspired gas mixtures which had either an increased , or a decreased . A smaller tank which restricted activity was used to obtain minimal values for . The seals' ventilation was characterized by rapid frequency (f), 33-30 breaths/min, with a between 0.9 and 1.01 liters. Changing the inspired gas composition did not alter the f and had little effect in increasing the . The major change caused by these gases was to shorten the diving time. Assessment of ventilation involved a consideration of the relationship of the time spent on the surface breathing ( ) to the total time between dives ( ). When the seals were breathing room air, the was constant over a wide range of diving times and spontaneous activity. Changes in inspired gas composition markedly changed the , and it is suggested that this represents a behavioral control of respiration which must be considered in evaluating the responsiveness of diving mammals to , and . The energy expenditure of these seals at rest was greater than predicted and during swimming was about the same as walking for terrestrial animals of this size. Estimates of , between the lung and the blood during the dives were about 20% of those during the surface interval, and the , was even less. These differences are thought to reflect the decrease of the cardiac output which is part of the "diving reflex."