The cardiovascular system, which consists primarily of the heart and the blood vessels, is the first organ system to develop in humans. It provides oxygen and nutrients to all the organs and tissues of your body.

Lungs The lungs have a double blood supply: the pulmonary circulation allows for gas exchange with the alveoli (air sacs), and the bronchial circulation supplies blood to the lung tissue. Blood from the bronchial circulation drains into the left side of your heart through the pulmonary arteries. The pulmonary circulation is a low-pressure, low-resistance system, made possible by the four-chambered heart.

Brain The human brain is very metabolically active and consumes a great deal of energy, so it requires a large and constant supply of blood. Although it comprises only 2% of total body weight, your brain utilizes 25% of total oxygen consumption and receives 20% of your body's blood supply. The brain is supplied with blood through two pairs of arteries: the internal carotids and the vertebrals.

Kidney Because the kidneys filter waste products from the blood, they are heavily vascularized and receive 20% of the heart's output. Blood is furnished to your kidneys via the largest artery in the body, the abdominal aorta, and returns from the kidneys in the ascending vena cava.

Liver The liver is the only major organ that doesn't receive its blood supply entirely from arteries branching off of the aorta. The hepatic artery, which provides about a third of the liver's blood supply, receives blood from the aorta. But the other two thirds of the liver's blood supply are brought to it by the portal vein, which brings blood containing oxygen and nutrients from the small intestine, to be processed by the liver.