Most of the higher vertebrates have double circulation system. It consist of two separate circulation:
(a) Systemic circulation – between heart and body
(b) Pulmonary circulation – between heart and lungs
In systemic circulation the oxygenated blood from the left ventricle of heart is pumped to all the body parts (except lungs) through aorta. Then the deoxygenated blood from various parts comes back to heart by superior and inferior vena cava into the right atria. This completes the 1st circulation.
In pulmonary circulation the deoxygenated blood brought back from body parts is pumped to lungs by the right ventricle through the pulmonary artery. In lungs deoxygenated blood is again converted back to oxygenated blood and sent back to the heart in left atria by pulmonary veins. This completes the 2nd circulation.
The most important significance of this system is that the oxygenated and deoxygenated blood does not mix anywhere in the body thus improves the oxygen supplying capacity of the heart.
