ABSTRACT
For aquatic air-breathing vertebrates, breathhold diving may stretch the physiological capacities of matching energy supply with energy use to the limit, since performance and dive endurance are limited by a strictly confined and declining supply of oxygen. Which tissues serve as primary oxygen stores cuts across phylogenetic lines and relates more to differences in normal behavior. For shallow divers, which would include most sea turtles, the smaller cetaceans, and the sirenians, most oxygen is stored in the lung. These animals typically have normal hematocrits and normal muscle myoglobin concentrations. Deep divers on the other hand, which include the leatherback sea turtle, large cetaceans, and pinnipeds such as the Weddell seal, tend to have smaller lungs, enhanced hematocrits, and high muscle myoglobin concentrations. The nature of the oxygen store also has consequences concerning the oxygen transport properties of blood. Where the lung is the oxygen store, the blood must function to continuously transfer O2 from lung to tissue in the face of declining PO2 and increasing PCO2. Where the blood is the major oxygen store, factors that promote efficient oxygen unloading are primary.
In turtles submergence times are maximized by their having extraordinary high anaerobic capacities for all tissues, including brain. Some marine mammals (the large seals), on the other hand, may prolong submergence times by reducing their metabolic rates while swimming underwater. It is suggested that such metabolic suppression depends upon hormonally mediated perfusion adjustments of vasoconstricted tissues.
