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Heart

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