Representative vertebrate circulatory systems.

(a) Fish have a single circulation in which blood is pumped from the heart to the gills, from where it circulates to the rest of the body tissues and returns to the heart. (b) Amphibians, such as frogs, have a heart with a single ventricle that is able to separate most of the oxygenated and deoxygenated blood entering it from the two atria. Some mixing does occur, however, as represented in this figure by the white dashed line. A significant advantage of this type of circulation is the ability to redirect blood from the lungs to the skin when underwater. Because the skin is somewhat permeable to oxygen, this partly compensates for the lack of air breathing. Note that blood returning from the skin has picked up oxygen but is returned to the right atrium, not the left atrium as is the case with the lungs. (c) Birds and mammals have a double circulation, in which oxygenated blood is pumped under high pressure to the body's tissues, and deoxygenated blood is pumped under low pressure to the lungs.