Organs and special tissue
Polar Bears have a closed circulatory system like humans. This means their blood flows through veins and ateries to deliver oxygen to the body. Polar Bears have a four-chambered heart, veins, arteries, and capillaries connecting these. However, one difference that polar bears have between humans is that when they hibernate, their blood pressure and heart rate drops. Also, the blood cells are adapted to hold more air, which allows a longer period of time to be spent underwater. Blood vessels have dilated in order to accommodate more blood, also allowing more freedom in the frigid water.
Rushman, Michael. "MM: Polar Bears." MM: Polar Bears. Destination: Marine Mammals, 05 Apr. 2003. Web. 20 Apr. 2013. <http://www.rushman.org/michael/polarb.html>.