In a healthy heart, blood flows one way through the heart due to heart valves, which prevent backflow. Fish, in contrast, have two chambers, an atrium and a ventricle, while most reptiles have three chambers.
Commonly, the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart. In humans, other mammals, and birds, the heart is divided into four chambers: upper left and right atria and lower left and right ventricles. 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. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to the lungs. This organ pumps blood through the blood vessels of the circulatory system. The heart is a muscular organ in most animals.
