The heart is an organ that is comprised mostly of muscle. It is a small organ that is located behind the sternum and situated on the left side of the human chest.
The main function of the heart is to pump blood through a network of veins and arteries throughout the body. The human body uses double circulation, which means that at all times the heart pumps both oxygenated and deoxygenated blood. The deoxygenated blood is carried to the lungs to pick up oxygen, while the oxygenated blood is carried to the rest of the body to provide its cells and tissues with oxygen and nutrients.