The heart

The heart is a unidirectional pump.

Valves are present to prevent the backflow of blood.

The right side pumps deoxygenated blood (low in oxygen and high in carbon dioxide) to the lungs.

The left side pumps oxygenated blood (high in oxygen and low in carbon dioxide) to the organs of the body.


  1. Deoxygenated blood enters the right atrium from the vena cava.
  2. Blood moves into right ventricle.
  3. Blood is pumped into the pulmonary artery.
  4. The pulmonary artery carries deoxygenated blood to the lungs.
  5. The blood becomes oxygenated in the lungs.
  6. Oxygenated blood leaves the lung via the pulmonary vein.
  7. Blood enters the left atrium.
  8. Blood moves into the left ventricle.
  9. Blood is pumped into the aorta, which carries oxygenated blood around the body.

This unidirectional flow of blood through the heart shows that mammals have a double circulatory system.

This means blood travels through the heart twice in one circulation of the body.


Ventricular walls are thicker than atrial walls because the ventricles have to pump blood further.

The left ventricle wall is thicker than the right because it pumps blood around the body while the right pumps blood to the lungs, located close to the heart.

The following arteries and veins transport blood to and from some of the body’s organs:

Blood vesselFunction
Vena cavaCarries deoxygenated blood from the body back to the heart.
Pulmonary arteryCarries deoxygenated blood from the heart to the lungs.
Pulmonary veinCarries oxygenated blood from the lungs to the heart.
AortaCarries oxygenated blood from the heart around the body.
Hepatic arteryCarries oxygenated blood to the liver.
Hepatic veinCarries deoxygenated blood back to the heart. Carries digested food (glucose and amino acids) from the liver around the body.
Hepatic portal veinCarries digested food from the small intestine to the liver.
Renal arteryCarries oxygenated blood (also rich in urea) to the kidneys for excretion.
Renal veinCarries deoxygenated blood (also low in urea as it has been purified in the kidney) back to the heart.

The oxygen and glucose carried in oxygenated blood is used for respiration in the body’s cells.

The coronary arteries provide the heart muscle with the glucose and oxygen it needs for respiration.

These are small blood vessels that branch off the aorta and can be seen on the external surface of the heart.