Getting blood around the body requires a network of blood vessels. These come in different shapes and sizes depending on their location and function. In this article we will look at the structure of arteries, veins, capillaries.
Arteries carry oxygenated blood away from the heart to the tissues of the body (except the pulmonary artery which carries deoxygenated blood from the heart to the lungs). Blood leaving the heart is under high pressure, so arteries need to be strong and stretchy to cope with that. This is why their walls have a thick layer of smooth muscle and elastic tissue. The fact that arteries recoil also helps to maintain the pressure of the blood. The very inner layer, the endothelium, is folded to allow it to expand.
- Thick smooth muscle and elastic tissue layer
- Folded endothelium
- Smaller lumen than veins
Arteries branch into smaller blood vessels called arterioles, which eventually branch into capillaries. Arterioles contain smooth muscle which can contract or relax to help direct and control the flow of blood to different areas and tissues, depending on where the demand is. Arterioles also have elastic tissue, but less than arteries.
Capillaries are the very smallest blood vessels. Their walls are only one cell thick which means it is easy for substances to be exchanged by diffusion e.g oxygen, carbon dioxide and glucose. Remember that a short diffusion pathway is one of the factors which speeds up the rate of diffusion. In tissues, there are networks of capillaries called capillary beds which we will look at when we cover the formation of tissue fluid.
- One cell thick endothelium
- Tiny lumen
Capillaries first combine into smaller blood vessels called venules, which then combine to form larger veins. The blood is now deoxygenated, having unloaded oxygen in the respiring tissues, and is on its way back to the heart. Again, the exception is the pulmonary vein which carries oxygenated blood back to the heart from the lungs. The blood is under much lower pressure now so needs a little help to keep going in the right direction. Valves in the veins make sure that blood only flows in one direction, and contraction of muscles nearby also helps the blood to move along. As the pressure is lower, the layers of smooth muscle and elastic tissue are a lot thinner than in the arteries, and the lumen is wider. The endothelium is not folded because it does not need to stretch.
- Thinner smooth muscle and elastic tissue layer
- Non-folded endothelium
- Wider lumen than arteries
- Contain valves
I have already summarised the structure of the blood vessels next to the diagrams. Just remember to be able to relate their structure to their function – it’s all to do with blood pressure and exchange of substances!