The heart and blood circulation The heart is a four-chambered muscular pump, about the size of a fist. It beats about 100,000 times a day pumping about 2,000 liters of blood through about 100,000 miles of organic tubes. The heart is a large muscle with four valves connected together to form a two-stage pump. The heart gets its energy by oxidizing sugars in the blood. This energy releases, contracts the numerous muscle cells of the heart and the four chambers compress the blood in the arteries. Arteries are thick-walled muscular tubes that carry blood away from the heart. The heart has two parts: the left ventricle and the right ventricle. The artery of the right ventricle pumps blood to the lungs, it is called the pulmonary artery and is the only artery that carries deoxygenated blood. The artery from the left ventricle pumps blood to the rest of the body and is called the aorta. Veins are thin-walled tubes that return blood to the heart, have a large diameter and valves. The veins return...