There are two types of circulatory pathways in the animals, viz. open or closed.
Chambered Heart: A muscular chambered heart is present in all vertebrates. The fishes have a 2-chambered heart; with an atrium and a ventricle. Amphibians and reptiles have a 3-chambered heart; with two atria and one ventricle. But crocodiles have 4-chambered heart. Birds and mammals have 4-chambered heart; with two atria and two ventricles.
Single Circulation: In fishes, the heart pumps out deoxygenated blood to the gills. The oxygenated blood from the gills is supplied to the body parts. Deoxygenated blood is then returned from different body parts to the heart. In single circulation, the blood passes through the heart only once.
Incomplete Double Circulation: In amphibians and reptiles, incomplete double circulation is present. Oxygenated blood is received by the left atrium and deoxygenated blood is received by the right atrium. But, both the bloods get mixed up in the single ventricle which pumps out the mixed blood. In incomplete double circulation, the blood comes to the heart through two different routes, but goes out through a single route.
Complete Double Circulation: Complete double circulation is present in birds and mammals. In this case, the oxygenated blood is received by the left atrium and the deoxygenated blood is received by the right atrium. The oxygenated blood is pumped out through the left ventricle, while the deoxygenated blood is pumped out through the right ventricle. In complete double circulation, there are two separate pathways for oxygenated and deoxygenated bloods.
The human circulatory system is composed of a muscular heart, a network of closed branching blood vessels and blood.
Location and Size: Heart is a mesodermally derived organ. It is situated in the thoracic cavity; in between the two lungs and slightly tilted to the left. It is the size of a clenched fist.
Structure: The heart is protected by a double-walled membrane; called pericardium. The pericardial fluid is filled in this membranous bag.
There are four chambers in the human heart:
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