The circulatory system has three main components:

  • Blood
  • Blood vessels
  • The heart

It has two main functions:

  • Transportation of substances
  • Protection against disease

Blood components


The main blood components are:

Red blood cellCarry oxygen.Contain haemoglobin (rich in iron) to carry oxygen. Biconcave shape to provide a large surface area for the diffusion of oxygen. No nucleus to provide more space for haemoglobin.
White blood cell Defend the body against disease.Large cells that contain a nucleus. There are 2 types: Lymphocytes – make antibodies. Phagocytes – engulf and digest microorganisms.
PlateletsConvert fibrinogen to fibrin. Fibrin forms a mesh that traps blood. Important in blood clotting and scab formation. Cell fragments (very small).
Plasma Transports blood cells, digested food molecules (e.g. glucose), carbon dioxide, urea and hormones.Straw coloured liquid.

Cell lysis

If red blood cells are placed in a high concentration of water, osmosis will cause water to move into the cell and, without a cell wall, they will lyse (burst).

Salts and other chemicals are present in the blood plasma to keep its concentration similar to the concentration of the blood cells.