These are a network of tubes that transport the blood around the body. There are three types of blood vessels which all carry out different functions: Arteries Veins Capillaries
Arteries carry blood away from the heart. They carry rich oxygenated blood towards the body. This blood is under high pressure as it is being pumped along by the heart every time it beats. In order for arteries to withstand this pressure they have special design features. Arteries have thick muscular walls which contain elastic fibres that allow the artery to stretch under pressure. The muscle fibres also contract to push the blood along and keep it flowing. In addition the narrow lumen maintains a high pressure and ensures the blood flows quickly to all parts of the body. The contracting of the arteries can be felt in your body as a pulse, for example at your wrist or neck.
Veins carry blood back to the heart. They carry deoxygenated blood away from the body. The blood returning from the body is at a much lower pressure than that being pumped from the heart. Therefore veins do not have to be as strong as arteries. Veins are wider than arteries and have much thinner walls. Look at the image below to see the main difference between arteries and veins.
Capillaries are very narrow thin blood vessels. Arteries branch out many times to form capillaries, these capillaries then join up to form veins. Capillaries carry blood to and from the body’s cells. Capillaries are the site at which exchange of oxygen, carbon dioxide and nutrients takes place. The structure of capillaries makes them very well suited for this function. As capillaries are only one cell thick and have very thin permeable walls this means that substances can diffuse out of them very easily. Fluid leaks out of the capillaries and bathes the surrounding cells, this is called tissue fluid. Useful substances such as oxygen and food diffuse out of the blood in the capillaries into the tissue fluid where it is then taken to the cells. Waste products such as carbon dioxide diffuse from the body’s cells, into the tissue fluid and are reabsorbed back into blood in the capillaries.