The oceans cover more than two-thirds of the globe, providing a home for at least 118 species of mammals. To make their living there, these mammals have to be able to suspend their breathing for lengthy periods. The capacity of seals and cetaceans for carrying oxygen is higher than land mammals because their blood contains twice the number of red blood cells. This, along with a circulatory system that diverts blood from other parts of the body to the heart and brain, permits them to carry out protracted dives. Cetaceans can also slow down their heart rate during long dives; some whales may remain submerged for almost two hours. To survive in the coldest regions of the ocean, fur-covered marine mammals such as the harp seal make use of air trapped in the fur as an added layer of insulation. Small mammals also employ vasoconstriction, reducing blood flow to the limbs to keep the body core and vital organs at a constant temperature. Large cetaceans such as the sperm whale keep warm with an enormous shell of insulating blubber. When they need to cool down, they draw in ocean water through the blow hole and circulate it through nasal passages, cooling the capillaries in the adjacent spermaceti organ of the head, effectively cooling the whale's whole body. The heated water is then expelled from the blow hole.
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