There are three different types of blood vessels in the body:
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
Relative to their wall thickness, arteries have a narrow lumen
Arteries have a thick layer of muscle and elastic fibres
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.
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.
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.