Heart The heart is a muscular organ in humans and other animals, which pumps blood through the blood vessels of the circulatory system . [ 1] Blood provides the body with oxygen and nutrients , and also assists in the removal of metabolic wastes . The heart is located in the middle compartment of the mediastinum in the chest . In humans, other mammals, and birds, the heart is divided into four chambers : upper left and right atria ; and lower left and right ventricles . Commonly the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart .Fish in contrast have two chambers, an atrium and a ventricle, while reptiles have three chambers. [ 5] In a healthy heart blood flows one way through the heart due to heart valves , which prevent backflow . 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 layer
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