Your heart is a four-chambered muscular organ that pumps blood through the blood vessels of your circulatory system. It is the centre of your circulatory system that carries blood to and from all areas of the body. Your heart is roughly about the size of your closed fist and made up of multiple layers of tissue. It sits between the lungs, slightly towards the left in the middle compartment of your chest. The main function of your heart is supplying oxygen and nutrients to the tissues and removing carbon dioxide and other wastes.