The wall of the heart is made up of three layers: epicardium, myocardium, and endocardium. The heart is enclosed in a protective sac, the pericardium, which also contains a small amount of fluid. 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 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. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to the lungs. The heart is a muscular organ in most animals, which pumps blood through the blood vessels of the circulatory system.

The long dark increase carry weight