PHYSIOLOGY

Cetaceans have a variety of adaptations (e.g. respiratory, circulatory, metabolic, etc) that allow them to perform their activities and live in the water.

Sea mammals have a great capacity for holding their breath (apnea), which allows them to travel below the surface of the water.  For instance, sperm whales are known for being able to stay two hours below the water when searching for food. Once a sea mammal starts diving the only source of oxygen is in its muscular tissue and its blood in the pigments that transport (hemoglobin) and store oxygen (mioglobine). A condition known as asphyxia occurs when oxygen is depleted (hypoxia) and the concentration of carbon dioxide increases (hypercapnia).

Aerobic metabolism (oxygen is present) depends on the quantity and availability of the stored oxygen, and can be maintained in short dives and during the initial phase of longer dives. On the contrary, once the available oxygen is used up, the source of energy in longer dives comes from anaerobic metabolism (lack of oxygen). During anaerobic metabolism lactic acid and hydrogen ions are produced and stored, and the metabolic rate decreases.

Another cause of asphyxia is when the blood supply is restricted from the tissues and organs, a process known as ischemia. Blood circulation provides the oxygen that organs require to perform their functions. Therefore, there are organs that need continuous blood supply such us the brain and the heart. Others can stand a decrease in the blood supply such us the visceral organs, some muscles, and the skin. Marine mammals can select the organs that will receive less blood supply, which is known as selective ischemia. This can be done through the constriction of the vessels that surround tissues and organs.

Marine mammals can reduce their heart rate when they dive, which is known as Bradycardia. Bradycardia varies widely between species or within individuals from the same species under different circumstances.