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In the first circuit, the blood is pumped to the lungs, where it acquires oxygen. It then returns to the heart and enters the second circuit, going to the rest of the body, eventually, after having been deoxygenated, returning to the heart.

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 because they do not have lungs. All animals with lungs have a double circulatory 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 one hundred, 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 adult heart pumps approximately 5 L/min of blood at rest (the cardiac output); with a total blood volume of approximately 5L in an average adult, a blood cell will run through the complete circuit about once a minute, though this depends on what tissue it circulates to. For example, blood supplying the coronary circulation of the heart muscle itself will return to the pulmonary loop much faster than blood supplying the toes. During intense exercise, the cardiac output can increase fivefold.

See also

• Circulatory system

Throughout the circulatory system there are many valves. There are 3 main types. The first is the bicuspid valve (mitral valve) which is found between the left atrium (auricle) and the left ventricle. This valve allows oxygenated blood to flow from the left atrium into the left ventricle during ventricular relaxation (diastole). The bicuspid valve also prevents the backflow of blood into the left atrium during ventricular contraction (systole). Then there is the tricuspid valve which is found between the right atrium and the right ventricle. This valve allows deoxygenated blood to flow from the right atrium into the right ventricle during ventricular relaxation. It prevents the backflow of blood into the right atrium during ventricular contraction. Finally there are the two semilunar valves, found at the beginning of the arteries leaving the heart. It is called a double circulatory system because it has two loops, one from the heart to the lungs and one from the heart to the rest of the body.

The advantage of a double circulatory system is that blood can be pumped to the rest of the body at a higher pressure.

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