The human circulatory system involves the pulmonary and systemic circulatory systems. In the pulmonary circulation system, deoxygenated blood leaves the heart through the right ventricle and is transported to the lungsvia the pulmonary artery. The pulmonary artery carries blood to the capillaries where carbon dioxide diffuses out of the blood into the alveoli (lung cells) and then into the lungs, where it is exhaled. At the same time, oxygen diffuses into the alveoli, and then enters the blood and is returned to the left atrium of the heart via the pulmonary vein.
Systemic circulation refers to the part of the circulation system that leaves the heart carrying oxygenated blood to the body's cells, and returning deoxygenated blood to the heart. Blood leaves through the left ventricle into the aorta, the body's largest artery. The aorta leads to smaller arteries that supply blood to all organs of the body. These arteries finally branch into capillaries. In the capillaries, oxygen diffuses from the blood into the cells, and waste and carbon dioxide diffuse out of cells and into blood. Deoxygenated blood in capillaries then moves into venules that merge into veins, and the blood is transported back to the heart. These veins merge into two major veins, namely the superior vena cava and the inferior vena cava. The deoxygenated blood enters the right atrium via the superior vena cava.