Physiological Adaptations for Breath-Hold Diving

Abstract

Marine mammals are adapted to long interruptions in breathing that enable them to maintain an oxygen-based metabolism and physiological homeostasis while underwater. The pulmonary and cardiovascular changes associated with the dive response include cessation of breathing (apnea), a decrease in heart rate (bradycardia), a concurrent reduction in cardiac output, and peripheral vasoconstriction that maintains central arterial blood pressure. A dive response during aerobic dives enables marine mammals to balance the conflicting demands of (1) optimizing the distribution and use of blood and muscle oxygen stores to maximize the ADL over the normal range of diving metabolic rates and (2) ensuring that active muscle receives adequate oxygen as exertion increases. Blood volume and concentrations of blood hemoglobin and muscle myoglobin are elevated and serve as a significant oxygen store that increases aerobic dive duration. Hepatic and renal function along with digestion and assimilation continue during aerobic dives to maintain physiological homeostasis. Carnivorous marine mammals rely primarily on fat for ATP synthesis under resting, normoxic conditions, and during dives within their ADL. Herbivorous Sirenia have more carbohydrate in their diet than carnivorous marine mammals but probably rely on the aerobic metabolism of a mixture of carbohydrate, amino acids, and short-chain fatty acids, the latter of which result from the fermentation of structural polysaccharides such as cellulose.