A four-chambered pump
The human heart has four chambers: two upper atria that receive blood and two lower ventricles that pump it out. The right side handles deoxygenated blood; the left side handles oxygenated blood. A thick muscular wall (the septum) keeps the two sides apart so the two blood streams never mix — the basis of double circulation.
The path of blood
Deoxygenated blood returns from the body through the vena cava into the right atrium, drops into the right ventricle, and is pumped through the pulmonary artery to the lungs (the pulmonary circulation). In the lungs it loses carbon dioxide and gains oxygen. Oxygenated blood returns through the pulmonary vein to the left atrium, drops into the thick-walled left ventricle, and is pumped through the aorta to the whole body (the systemic circulation). The heart muscle does not absorb oxygen from the blood inside its chambers; instead it is fed by its own coronary arteries, which branch off the aorta so the busy muscle never runs short of oxygen while it works.
Example
The left ventricle has the thickest muscular wall because it must generate enough pressure to push blood all the way to the toes and brain, while the right ventricle only pushes blood to the nearby lungs.
Comparing blood vessels
Arteries carry blood away from the heart. They have thick, elastic, muscular walls to withstand high pressure and carry oxygenated blood — except the pulmonary artery. Veins carry blood towards the heart under low pressure; they have thinner walls, a wider lumen and valves that prevent backflow. Capillaries are one-cell thick, allowing exchange of oxygen, nutrients and wastes between blood and tissues.
| Vessel | Direction | Feature |
|---|---|---|
| Artery | Away from heart | Thick elastic wall |
| Vein | Towards heart | Valves, thin wall |
| Capillary | Between them | One-cell thick wall |
Remember
"A" for Artery and Away. The pulmonary artery is the exception that carries deoxygenated blood, and the pulmonary vein is the exception that carries oxygenated blood.