A bear's circulatory system is similar to a humans. Like a human, a bear has a four chambered heart that pumps oxygenated blood through the body. Unlike humans, a bear's veins move more blood then the arteries, as a bear has more of them. The veins of a bear also contain valves, which keep the blood flowing the correct way. Humans do not need this as our upright posture allows the blood to flow correctly. Bears walk on all fours so valves are necessary. The polar bear has adapted it's circulatory system so that it's blood holds more oxygen for prolonged periods underwater. It also holds more blood in it's cells so it can survive freezing waters longer.