Oxygen Storage

Since many cetacean species are required to make deep dives for long periods of time to hunt for food, they must efficiently store large amounts of oxygen. Inflating their lungs with large amounts of air before a dive would be counter-intuitive since they would need to use energy and oxygen to overcome their buoyant lungs. Instead, cetaceans primarily store oxygen in their blood and muscle, where it can be easily accessed when needed during cellular respiration (Marshall 2002). In the blood oxygen binds to hemoglobin and in the muscle it binds to myoglobin.

Hemoglobin and myoglobin are both iron-proteins with an attached heme group. They bind to molecular oxygen, carbon-dioxide and carbon-monoxide and are instrumental in the cardiovascular oxygen transport. When they are saturated with vascular oxygen they are known as oxyhemoglobin and oxymyoglobin. Each protein is more strongly attracted to CO2 than oxygen, so when CO2 begins to accumulate in the blood oxyhemoglobin releases its oxygen to be used metabolically. When both the blood and peripheral tissues are oxygen deprived, the myoglobin in the muscles releases its stored oxygen (King 2005). This is the final metabolic option before anaerobic oxidation begins. Cetaceans must use both oxyhemoglobin and oxymyoglobin during physically-taxing deep dives. Oxygen-saturated hemoglobin gives blood its red color while myoglobin-saturated muscle is characteristically brown.

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Brix et. al conducted a study to test the oxygen affinity of hemoglobin in a Lesser Rorqual whale. They looked at the affect of increasing concentrations of CO2 and lactate on the ability of hemoglobin to bind to oxygen. The results reveal that like terrestrial mammals, Lesser Rorqual whale hemoglobin becomes less and less attracted to oxygen as CO2 and lactate levels increase (Brix et. al. 1990). We can therefore conclude that cetaceans can be forced to surface and breath either because of intense physical activity that produces lactic acid in the blood and muscles or because of prolonged submergence leading to the depletion of oxygen and the increase of concentrations of CO2.

Porpoises with different behaviors and environments have different volumes of blood and hemocrit per kilogram of body weight. Figure adapted from Ridgway and Johnston 1966.

The ability of a cetacean species to successfully store oxygen may also be affected by its behavior and environment. Ridgway and Johnston (1966) compared the blood volumes and hemoglobin concentrations of three genera of porpoise. They found that the Dall porpoise (Phocoenoides dalli), the most pelagic and active swimmer, had the highest volume of red blood cells per body weight, while the Montagu porpoise (Tursiops truncates), which lives in shallower waters, had almost half the volume of red blood cells per body weight as the Dall porpoise (Ridway and Johnston 1966). Since the hemoglobin is found in the red blood cells, it comes as no surprise that the Dall porpoise is able to be more active and dive deeper because of its increased ability to store oxygen.