What do arteries veins and capillaries do?

Arteries, veins and capillaries are the three types of blood vessels in our bodies. We need blood vessels to carry our blood around the body.

The importance of blood

Blood has to be circulated around the body because it contains substances that are needed by cells around the body, including oxygen and nutrients. The circulatory system also carries waste products of metabolism away from cells, so that they can be excreted.

Arteries, veins and capillaries all have different roles within the circulatory system; because of this, they have different structures which are adapted to their specialised function.

Arteries

Arteries are blood vessels that carry blood AWAY from the heart . (Remember: Arteries carry blood Away).

Because arteries carry blood from the heart, it means that they must be able to withstand the high pressure of the blood during each heart beat.

They have thick walls containing lots of muscle and elastic fibres. The muscle makes them strong, while the elastic fibres makes them able to stretch when blood passes through them. Because their walls are thick, arteries have a small lumen.

Almost all of the ateries in the body carry oxygenated blood from the heart to the rest of the body. An exception to this is the pulmonary artery, which carries deoxygenated blood from the heart to the lungs, where it becomes oxygenated again.

Veins

Veins carry blood from tissues back to the heart . You can remember this as veins carry blood into the heart. Because the blood in veins has already travelled a long distance from the heart, it is much lower pressure blood than is carried in arteries. This means that veins don't need the same structure as arteries. Veins have thin walls with less muscle and elastic fibres than arteries, which means that they have a large lumen. Because the blood in veins is under low pressure, there is a risk that it can travel backwards, especially because of the effects of gravity. To prevent this happening, veins contain one-way valves that stop blood flowing backwards.

Alost all the veins in the body carry deoxygenated blood. However, just like witth arteries there is an exception: the pulmonary vein. This vein carries blood that has been oxygenated in the lungs back to the heart, so that it can be pumped around the systemic circulation.

Capillaries

Capillaries are very small blood vessels that connect arteries to veins. Capillaries are the site of exchange between the blood and body tissues. The function of capillaries is to allow food and oxygen to diffuse from the blood to cells, and at the same time, allow waste to diffused from cells into the blood. Capillaries have very thin walls that are only one cell thick. This means that substances only have to diffuse a short distance, allowing for maximum exchange. The blood in capillaries is under low pressure. This means that the blood travels slower than in arteries, giving more time for exchange.