The cardiovascular system is a closed circulatory system seen in humans and other vertebrates. Its components include the heart, blood vessels, and blood. The heart has chambers such as the atrium (pl. atria) and the ventricle(s). In humans, there are four heart chambers, i.e. two atria and two ventricles. The right atrium is the chamber of the heart that receives returning deoxygenated blood, which next enters the right ventricle for re-oxygenation and removal of carbon dioxide (i.e. pulmonary circuit). The left atrium receives oxygenated blood (from the pulmonary circuit). It then passes blood to the left ventricle, which in turn pumps blood out of the heart to reach the various parts of the body via the blood vessels (i.e. systemic circuit). The blood vessels transport blood to different parts of the body and are of three major types: (1) arteries, (2) veins, (3) capillaries. The arteries are blood vessels that carry blood away from the heart. The veins are those that carry blood toward the heart. The capillaries are the vessels with thin walls where chemicals are exchanged between blood and the interstitial fluid.