Circulatory System

Abstract

The circulatory systems of marine mammals follow the general mammalian plan. They are most notable for features associated with the diving response, thermoregulation, and large body mass. Specific features of the circulatory system vary with orders, families, and species. These adaptations include large blood volumes, large capacitance structures (spleens and venous sinuses), venous sphincter muscles, and vascular adaptations for thermoregulation, aortic windkessels, and vascular retia. The basic structure and size of the heart in pinnipeds and cetaceans are typical of mammals. The four-chambered heart, with right ventricular outflow to the lungs, and left ventricular output to the systemic circulation, weighs 0.5% to 1% of body mass in most pinnipeds and small cetaceans. In the great whales, relative heart mass is smaller, about 0.3% to 0.5% of body mass. Chamber size, stroke volume, and resting cardiac output and heart rate (where measured) are also in the general mammalian range and in agreement with mammalian allometric equations. Both the foramen ovale and ductus arteriosus are closed in adult seals and cetaceans as in other mammals. Therefore, utilization of an intermittent fetal circulatory pathway does not appear to be a mechanism to bypass a potential increase in pulmonary vascular resistance during diving.