Blood Vessels

There are three different types of blood vessels in the body:

  • Arteries
  • Veins
  • Capillaries

An illustrated diagram depicting blood flow in the circulatory system. On the left, a red artery labelled 'BLOOD FLOW FROM HEART' carries oxygen and nutrients 'TO CELLS'. It leads to smaller arterioles which further lead to a network of capillaries. On the right, a blue vein labelled 'BLOOD FLOW TO HEART' carries waste and carbon dioxide 'FROM CELLS'. The vein originates from smaller venules connected to the capillaries. Arrows indicate the direction of blood flow.

Structure of Blood Vessels

An illustrative comparison between an artery and a vein. On the left, a cross-section of an artery shows a 'Thick outer wall', 'Small lumen', and a 'Thick layer of muscles and elastic fibres'. On the right, a cross-section of a vein showcases a 'Thick outer wall', 'Large lumen', 'Valve', and a 'Thin layer of muscle and elastic fibres'. In the centre, a network labelled 'Bonus' depicts an arteriole, capillary, and venule connecting the two vessels. Annotations and dotted lines point to the respective features of each vessel.


Arteries are blood vessels that carry high-pressure blood away from the heart, to other parts of the body. The pulmonary arteries are the only arteries that do not carry oxygenated blood.

The aorta, which carries blood from the left ventricle, has the highest pressure.

Arteries have thick outer walls

  • This helps the arteries cope with the high pressure when blood surges through them, preventing them from bursting.

Relative to their wall thickness, arteries have a narrow lumen

  • This is the central region that blood flows through.

Arteries have a thick layer of muscle and elastic fibres

  • The elasticity allows the walls to recoil after the pulse of blood has flowed through the vessel. This helps with evening out pulses and maintaining the blood pressure.


Veins are blood vessels that carry blood back to the heart. All veins, except the pulmonary veins, carry deoxygenated blood.

Blood flows through the veins at low pressure, so they have thin walls with less muscular tissue than arteries. However, they have larger lumens than arteries.

Veins also have valves to prevent blood from flowing backwards. The valves ensure that blood only moves in one direction.

A detailed illustration of a vein cross-section, displaying how blood flow is managed. The top half showcases 'Normal blood flow' with the 'Valve open', and surrounding 'Muscles contracted', allowing blood to move upwards. In contrast, the lower half displays a 'closed valve' situation, as blood is flowing in the reverse direction. Arrows and annotations help visualise the direction and state of blood flow in both scenarios.

Faulty valves

If a valve becomes faulty, it may not be able to open wide enough, so less blood will flow to the heart. Also, it may not be able to close completely, which causes blood to flow in both directions.

A cross-sectional illustration of a vein, highlighting the abnormal blood flow due to a damaged valve. At the top, with the 'Valve open' and surrounding 'Muscles contracted', blood is depicted flowing in both directions, which is abnormal. The bottom showcases a 'Valve not closed (damaged)' with reversed blood flow, despite the 'Muscles contracted'. Arrows, accompanied by annotations, help visualise the direction and condition of blood flow in the vein.


Capillaries are tiny blood vessels that facilitate the exchange of substances between the blood and the body’s cells. For example, glucose and carbon dioxide diffuse from the blood to the cells. At the same time, carbon dioxide diffuses from the cells into the blood.

The capillary walls are very thin (just one cell thick). This decreases the diffusion distance for oxygen which increases the rate of diffusion.

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