The circulatory system includes two circuits: the systemic circuit and pulmonary circuit. Pulmonary circulation is the exchange of blood between the heart and the lungs. Systemic circulation is the blood flow between the heart and the rest of the body. These circuits allow blood pressure to remain high enough for sufficient delivery of oxygen-rich blood to all tissues of the body. Pulmonary circulation transports blood from the right side of the heart, through the lungs, and back to the left side of the heart. Systemic circulation is blood flow from the left side of the heart, through the body, and back to the right side of the heart. Each circuit contains arteries, capillaries, and veins. As blood flows away from the heart, arteries continue to branch and decrease in diameter until they become smaller arterioles and finally capillaries. The capillary is the narrowest type of blood vessel where gas and nutrient exchange occurs between the blood and interstitial fluid, the fluids found outside of the cells. Blood flows out of capillaries into venules and then veins, which converge, becoming larger in diameter until the veins empty into the heart.
Blood flow through the body is coordinated by the double-pump system of the heart. The right atrium receives oxygen-poor blood from the body via the vena cavae. When the right atrium contracts, blood is pumped through the tricuspid valve from the right atrium to the right ventricle. Upon ventricular contraction, blood leaves the right ventricle, passing the now-open pulmonary valve in the pulmonary trunk, through the pulmonary arteries, to the lungs. While in the pulmonary capillaries, the blood is picking up oxygen from and releasing carbon dioxide to the lungs. The oxygen-rich blood enters the pulmonary veins, which empty into the left atrium as the myocardium of the heart relaxes. This oxygen-rich blood moves past the bicuspid valve to the left ventricle, and during ventricular contraction it passes the aortic valve, entering the aorta for transport to the rest of the body. The right side of the heart exclusively contains oxygen-poor blood, and the left side contains oxygen-rich blood.