Structure of blood and blood vessels

Blood is carried through three different types of blood vessels in the body:

  1. arteries
  2. capillaries
  3. veins

All blood vessels are specifically structured to perform their function. For example, a capillary is microscopically thin to allow gases to exchange, the arteries are tough and flexible to cope with high pressure blood flow and the veins contain valves to prevent the blood from travelling backwards when at low pressure. All vessels feature varying lumen size. The lumen is the hollow opening or the space inside the blood vessel.

Table showing the differences between an artery, vein and capillary.
FunctionCarry blood away from the heart (usually oxygenated blood, except for the pulmonary artery)Carry blood towards the heart (usually deoxygenated blood, except for the pulmonary vein)Allows diffusion of gases and nutrients from blood into the body cells
WallThick, muscularThinnerVery thin, one cell thick
LumenSmallLargeVery small, only allows blood to pass through one cell at a time
Other featuresThick muscular walls to withstand blood flowing at high pressure as it leaves the heart; the largest artery is the aortaContain valves to prevent back flow of bloodWalls are made of semi-permeable membrane to allow transport of gases and nutrients into and out of the blood


The main function of blood is to transport nutrients and oxygen to the cells of the body.

Blood is made up of four components:

  1. red blood cells – these transport oxygen around the body
  2. white blood cells - these fight infection
  3. platelets - these clot to prevent blood loss during injury
  4. plasma - this is the liquid part of blood
Red blood cells

Red blood cells are very important for sport and physical activity because they contain haemoglobin. Haemoglobin allows them to carry oxygen from the lungs to the working muscles.

Red blood cells are disc-shaped cells with no nucleus. They are very small but their flattened shape gives a relatively large surface area which allows rapid diffusion of oxygen.