Blood Oxygen Transport and Delivery in Reptiles
Cenozoic reptiles are characterized by physiological morphological and ecological systems with low energy requirements compared to those of mammals. Ectothermy and low resting rates of metabolism are the primary physiological adaptations of reptiles that produce low energy demand. Adjustments of the oxygen-transport system to different thermoregulatory characteristics among reptiles may be reflected in blood viscosity oxygen capacity oxygen affinity and the temperature sensitivity of oxygenation. Other adaptations reduce the energy cost of oxygen transport. Reptiles have low hematocrits and large, widely spaced capillaries that contribute to a low fluid resistance in the vascular system but also limit the oxygen transport capacity. The low oxygen affinity characteristic of the blood of most reptiles appears to facilitate diffusion of oxygen to the tissues, overcoming the intrinsic limitations imposed by the morphological specializations of the cardiovascular system. The low blood oxygen affinity permits virtually all of the oxygen carried by the blood to be delivered to the tissues during periods of stress. It may also help to maintain a relatively high arterial Po 2 even when a right-to-left shunt occurs in the heart. Reptilian erythrocytes are capable of reducing methemoglobin rapidly. The high concentrations of methemoglobin and polymerized hemoglobin that occur in vivo may indicate that these compounds have a functional role. In their blood physiology as in other aspects of their biology reptiles are specialized animals that reflect selective forces quite different from those that have shaped the evolution of mammals.