The heart is a hard-working, fist-sized muscle found in people and animals. It pumps out blood through tube-like structures called blood vessels. The heart and these vessels together make up the body-wide circulatory system. Blood carries in oxygen-rich air and nutrient-packed food to the body and takes away waste-filled substances like carbon dioxide to the lungs. In people, the heart is about the size of a closed-hand and sits in between the lungs, in the chest-centered cavity.
In humans, the heart has four parts called chambers. These are the upper-left and upper-right atria, and the lower-left and lower-right ventricles. People often talk about the right-side atrium and ventricle together as the right heart, and the left-side ones as the left heart.
In a well-working heart, blood goes through the heart in one direction. Special flaps called valves keep out blood from going backward. The heart sits inside a fluid-filled protective bag called the pericardium. This bag holds in a small amount of fluid to help the heart move smoothly.
The heart wall has three layered parts: the epicardium (outer layer), the myocardium (middle layer), and the endocardium (inner layer).