Countercurrent Heat Exchange

Thermoregulation: Marine mammals are well adapted to life in the water. Pinnipeds, sea otters and polar bears are amphibious (able to operate on land and in the water). Sirenians and cetaceans spend all their time in the water.

Keeping a constant body temperature is the most serious challenge facing warm-blooded mammals in an aquatic (watery) environment. Most marine mammals have an insulating layer of fat called blubber that keeps their bodies warm and buoyant. Blubber is rich in lipids (fats or fatty material that cannot dissolve in water) and stores large amounts of energy. Sea otters keep their body temperature constant with a dense (thick) layer of fur that traps a layer of air next to the skin so that their skin never gets wet. Polar bears and some pinnipeds have a thick layer of fur and a blubber layer.

Another way marine mammals control their body temperature is by controlling their blood flow in a process called vasodilation. During vasodilation, blood flow increases to and from peripheral vessels near the surface of the flippers, flukes, and fins. Countercurrent heat exchange allows cold blood returning to the body core to be warmed up by exhanging heat with arteries going to the periphery (flukes and flippers).

Diving: All marine mammals have special traits that allow them to dive deep and stay underwater for a long time. At some point, all must return to the surface to breathe. Whales and dolphins breathe through single or paired blowholes on the dorsal (back) surface of their head. At the surface, they quickly inhale and relax the muscular flap to close it so they can dive.

When diving, blood is directed away from tissues that can handle low oxygen levels and toward the heart and brain where oxygen is needed most. During diving, the heartbeat slows down. Some champion divers are the sperm whales. They can dive more than 1600 meters (over a mile) and may remain submerged for an hour or more! Another champion diver, the elephant seal can dive more than 1500 meters (4920 feet) and stay under for two hours. Bottlenose dolphins can dive to depths of 540 meters (1770 feet) and remain underwater for 8-10 minutes.

A Dolphin breathing through its blowhole while leaping through the air.