The circulatory system circulates blood around the body. 

It consists of:

  • Blood
  • Heart
  • Blood vessels


The blood is made up of:

  • Plasma (liquid part) which is made up of water, salts and protein
  • Red blood cells (erythrocytes)
  • White blood cells (leukocytes)
  • Platelets

Blood acts a transport system of our body, carrying oxygen in red blood cells and other nutrients in plasma. It also plays a role in our immunity as it consists of white blood cells. 


The heart is a organ made up of cardiac muscle. It contains four chambers:

  • Right atrium 
  • Left atrium
  • Right Ventricle
  • Left Ventricle

The right side of the heart carries deoxygenated blood whilst the left side of the heart carries oxygenated blood. The right and left sides of the heart are separated by a wall called septum


The heart also has valves to prevent back flow of blood. 

  • Tricuspid valve prevents back flow of blood from right ventricle to right atrium. 
  • Mitral valve prevents back flow of blood from left ventricle to left atrium. 
  • Pulmonary valve prevents back flow of blood from pulmonary artery to right ventricle. 
  • Aortic valve prevents back flow of blood from Aorta to left ventricle. 

Blood Vessels

There are three types of blood vessels:

  • Arteries
  • Veins
  • Capillaries

Arteries carry blood away from the heart. Arteries (except pulmonary artery) always carry oxygenated blood.
Pulmonary artery is the only artery which carries deoxygenated blood. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs. 

Arteries carry blood under high pressure. To prevent bursting, arteries need to be muscular. They have thick muscular walls and a small lumen.

Arteries branch into capillaries.

Capillaries are blood vessels which are one-cell thick. This allows substances such as glucose, oxygen and other nutrients to diffuse from the capillaries into the tissues. 

Carbon dioxide and other waste products diffuse from the tissues into the capillaries. 

The pressure inside capillaries is low as they are delicate. 

They branch together to form veins.

Veins carry blood to the heart. Veins (except pulmonary vein) always carry deoxygenated blood.
Pulmonary vein is the only vein which carries oxygenated blood. The pulmonary vein carries oxygenated blood from the lungs to the left atrium. 

Veins carry blood under lower pressure. Therefore, they are less muscular than arteries. The have thinner walls but have a larger lumen than arteries. 

Learn more about blood circulation