The human heart is an organ that pumps blood throughout the body via the circulatory system, supplying oxygen and nutrients to the tissues and removing carbon dioxide and other wastes.

"The tissues of the body need a constant supply of nutrition in order to be active.

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In humans, mammals and birds the heart is divided into four chambers. They are upper left and right atria and lower left and right ventricles. Commonly the right atrium and ventricles are referred together as the right heart and their left counter parts as the left heart. Fish in contrast have two chambers on atrium and a ventricle, while the reptiles have three chambers. In  active  blood flows one way through the heart due to heart valves, which prevent the back flow.

The heart is enclosed in a protective sack, the pericardium, which also contains a small amount of fluid. The valve of the heart is made up of three layers they are epicardium, myocardium and endocardium. The heart pumps blood with a rhythm determined by a group of pace making cells in the Sino atrial node. These generate a current that causes contraction of the heart, travelling through the atrio ventricular node and along the conduction system of the heart

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Arun (askIITians forum expert)