Double circulatory system



The double circulatory system of blood flow refers to the separate systems of pulmonary circulation and the systemic circulation in amphibians, birds and mammals (including humans.) In contrast, fish have a single circulation system.

For instance the adult human heart consists of two separated pumps, the right side with the right atrium and ventricle (which pumps deoxygenated blood into the pulmonary circulation), and the left side with the left atrium and ventricle (which pumps oxygenated blood into the systemic circulation). Blood in one circuit has to go through the heart to enter the other circuit.

Blood circulates through the body at speeds which vary by a factor of ten, from 1.2 m/s in the aorta to approximately 1.1 cm/s in the capillaries. The circulatory system features numerous return paths (out to the kidneys and back, to the liver and back, to the legs and back, etc.), so it is incorrect to think of blood cells traveling the entire circulatory distance. The average heart pumps about 5,040 ml of blood per minute and an average adult has about 5 liters of total blood volume, so the heart is exchanging blood volume at a rate of about ten times per second.