The mammalian heart has four chambers: the right atrium, right ventricle, left atrium, and left ventricle. Atria are for receiving blood; ventricles are for pumping blood to the lungs and body. Theventricles are larger than the atria and their walls are thick, because muscular walls are needed to forcefully pump the blood from the heart to the body and lungs. Deoxygenated blood from the bodyenters the right atrium, which pumps it to the right ventricle. The right ventricle pumps blood to the lungs, where carbon dioxide diffuses out, and oxygen diffuses in. From the lungs, oxygenated bloodenters the left atrium, where it is pumped to the left ventricle (the largest and strongest of the 4 chambers), which pumps it out to the rest of the body, including the heart's own blood supply.
Themain components of the circulatory system are the heart, the blood, and the blood vessels.
The circulatory systems of all vertebrates, as well as of annelids (for example, earthworms) and cephalopods(squid and octopus) are closed, meaning that the blood never leaves the system of blood vessels consisting of arteries, capillaries and veins.
Arteries bring oxygenated blood to the tissues (exceptpulmonary arteries), and veins bring deoxygenated blood back to the heart (except pulmonary veins). Blood passes from arteries to veins through capillaries, which are the thinnest and most numerous ofthe blood vessels and these capillaries helps to join tissue with arterioles for transportation of nutrition to the cells.
The systems of fish, amphibians, reptiles, and birds show various stages ofthe evolution of the circulatory system.
In fish, the system has only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues.This is known as single circulation. The heart of fish is therefore only a single pump (consisting of two chambers). In amphibians and most reptiles, a double circulatory system is used, but the heart...
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