heart

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The heart is a muscular organ in humans and other animals, which pumps blood through the blood vessels of the circulatory system.[1] The blood provides the body with oxygen and nutrients, as well as removing metabolic wastes.[2] The heart is located in the middle compartment of the mediastinum in the chest.[3] In humans, other mammals and birds the heart is divided into four chambers: upper left and right atria; and lower left and right ventricles.[4][5] Commonly the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart.[6] Fish in contrast have two chambers, an atrium and a ventricle, while reptiles have three chambers.[5] Blood normally flows one way through the heart due to heart valves, which prevent backflow.[3] The heart is enclosed in a protective sac, the pericardium, which also contains a small amount of fluid. The wall of the heart is made up of three layers: epicardium; myocardium; and endocardium.