The cardiovascular system is comprised of blood, blood vessels and the heart. Blood and blood vessels distribute oxygen and nutrients to the cells, carry carbon dioxide and waste products from the cells, help to regulate body temperature and prevent serious blood loss following injury through the formation of clots.

The heart is at the center of the cardiovascular system. It is approximately the size of a closed fist and is located behind the sternum, between the lungs. The heart is divided into four chambers that receive circulating blood: the two upper chambers are the right atrium and the left atrium the two lower chambers are the right and left ventricles.

When the heart contracts, blood accumulates in the right atrium. When the heart relaxes, blood flows from the right atrium into the right ventricle and continues on, into the pulmonary trunk. The pulmonary trunk consists of right and left pulmonary arteries. The pulmonary arteries transport blood to the lungs, where carbon dioxide is expelled and oxygen is brought into the body. The newly oxygenated blood returns to the heart via four pulmonary veins. These veins empty the blood into the left atrium. From the left atrium the blood passes into the left ventricle. The blood is then pumped into the ascending aorta and distributed throughout the body by several large arteries.