Blood is confined to a closed system of blood vessels and to the four chambers of the heart (essentially dilated vessels). Blood travels away from the heart through arteries, which branch into smaller vessels, the arterioles. Arterioles branch further into the smallest vessels, the capillaries. Gas, nutrient, and waste exchange occurs across the capillary walls. The blood returns to the heart as capillaries merge to form venules, which further merge to form large veins, which connect to the heart. Blood circulates through the following two separate circuits:
In the pulmonary circulation, deoxygenated blood travels from the right side of the heart to each of the two lungs. Within the lungs, O 2 enters and CO 2 leaves the capillaries by diffusion. Oxygenated blood returns from the lungs to the left side of the heart.
In the systemic circulation, oxygenated blood travels from the left side of the heart to the various areas of the body. Gas, nutrient, and waste exchange occurs across the capillary walls into the interstitial fluids outside the capillaries and then into the surrounding cells. The deoxygenated blood returns to the right side of the heart.