heart

The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system.[1] The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to the lungs.[2] In humans, the heart is approximately the size of a closed fist and is located between the lungs, in the middle compartment of the chest, called the mediastinum.[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 most reptiles have three chambers.[5] In a healthy heart blood 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.[7] In all vertebrates the heart has an asymmetric orientation, almost always on the left side. According to one theory this is caused by a developmental axial twist in the early embryo.[8][9]