The heart

Blood is pumped around the body by the heart.

Labelled diagram showing the left and right sides of the heart

The heart has four chambers - two atria and two ventricles.

Note that left and right refers to the side in the person's body, not how it is viewed on the page.

Blood enters the heart through the atria. Blood from two vena cavae enters the right atrium. Blood from the pulmonary veins enters the left atrium.The atria fill, followed by the ventricles. Blood is prevented from flowing back into the atria by heart valves.

Blood leaves the heart in the body's main artery - the aorta - from the left side, and the pulmonary artery, from the right.

Double circulation

Humans have a double circulatory system. The heart pumps blood through two circuits:

The pulmonary circulation transports blood to the lungs. At the lungs:

Diagram showing how pulmonary circulation transports blood to the lungs

The systemic circulation transports:

  • oxygen and nutrients to the body
  • carbon dioxide and other wastes away from cells

The systemic circulation is under high pressure - it has to deliver blood to the extremities of the body.

The walls of the left ventricle are thicker and contain more muscle than those of the right ventricle. This is to create more pressure on the blood ready for entering the systemic circulation.

The pulmonary circulation is under lower pressure, because:

  • blood is delivered to the lungs only, which are very close to the heart
  • in a healthy person, this lower pressure is optimum for the diffusion of gases
Diagram showing how blood is transported around the body

Circulating blood

Blood passes through the heart twice in one complete circulation of the body.

Following the blood through one complete circulation of the body:

Deoxygenated blood from the body enters the right atrium

Note that although this is shown as a sequence, the atria contract in unison, and when full, the ventricles also contract in unison. Part of the blood is being delivered to the body, while the remainder of the blood is being transported to the lungs. This means that oxygen can be picked up from the lungs, while at the same time, blood is also being delivered to the body.