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:

  • oxygen diffuses into the blood from the alveoli – the blood becomes oxygenated
  • carbon dioxide diffuses from the blood into the lungs
Diagram showing how blood is transported around the body

The systemic circulation transports:

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

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

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 pulmonary circulation transports blood to the lungs

The heart

Blood is pumped around the body by the heart.

Labelled diagram showing the left and right sides of the heart

The heart is made of cardiac muscle that is able to contract to pump blood around the two circulatory circuits of the body.

It 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.

The atria

  • Top chambers which receive blood under low pressure from veins.
  • Thin walls as little pressure is required during contraction to push blood into the ventricles.

The ventricles

  • Bottom chambers of the heart. They receive blood from the atria above.
  • Thicker walls which contract to pump blood out of the heart to areas of the body.
  • The left ventricle has a thicker muscular wall than the right ventricle as it requires more pressure to be created to pump blood to the whole body.
  • The right ventricle wall is thinner as it requires only enough pressure to pump blood to the lungs.

The valves

  • The bicuspid (left) and tricuspid (right) valves prevent the backflow of blood from the ventricles into the atria.
  • The semi-lunar valves prevent the backflow of blood from the pulmonary artery (right) and aorta (left) into the ventricles.

Circulating blood

Blood passes through the heart twice in 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.