Your heart lies beneath your breastbone. It is about the size of your fist. The heart is a muscular organ that works hard 24 hours per day, pumping blood through a 60,000 mile network of vessels in the body. Your heart is actually a double pump. There is a right side of the heart that is made up of the right atrium (upper chamber) and the right ventricle (lower chamber). The right side pumps blood through the lungs. This side also receives venous blood from all veins in the body. There is a left side of the heart as well, that consists of the left atrium and left ventricle . While blood is circulating through the body, it delivers oxygen and nutrients to tissue through the arteries and picks up carbon dioxide through the veins.