Blood enters the heart in the right atrium, passes into the right ventricle and is then pumped to the lungs; after leaving the lungs, the blood travels back into the left atrium, into the left ventricle and then flows into the rest of the body. This is called the mammalian double-circulatory system because it has two distinct circuits. Keeping oxygenated blood completely separate from deoxygenated blood improves efficiency.
One circuit of the circulatory system is called the pulmonary branch, and it carries blood from the heart to the lungs so that the blood receives oxygen in exchange for carbon dioxide. The systemic branch of the circulatory system transports the blood from the heart to the major components of the body. Blood travels from the heart in blood vessels called arteries, while it returns to the heart through blood vessels called veins. Capillaries are small vessels that connect arteries with veins.
The heart has several valves that help to ensure the blood flows in the correct direction at all times. The atrioventricular or tricuspid valve separates the right atrium and the right ventricle. The pulmonary semilunar valve separates the right ventricle from the pulmonary artery. The bicuspid valve separates the left atrium from the left ventricle. The aortic semilunar valve separates the left ventricle from the aorta.