The Heart and Its Functions The heart is defined as "the cardiac muscle that maintains the circulation of the blood".It is divided into four cavities; two atria and two ventricles. The left atrium receives oxygenated blood from the lungs. From there the blood passes to the left ventricle, which forces it via the aorta, through the arteries to supply the tissues of the body. The right atrium receives the blood after it has passed through the tissues and has given up much of its oxygen. The blood then passes through the right ventricle into the lungs where it gets oxygenated. There are four major valves in the heart; the lef