Explain the relationship between the structure and function of arteries, capillaries and veins.

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Arteries have a thick outer layer of longitudinal collagen and elastic fibers to avoid leaks and bulges. They have a thick wall, which is essential to withstand the high pressures.

They also have thick layers of circular elastic fibres and muscle fibres to help pump the blood through after each contraction of the heart.

In addition the narrow lumen maintains the high pressure inside the arteries.

 

Veins are made up of thin layers with a few circular elastic fibres and muscle fibres. This is because blood does not flow in pulses and so the vein walls cannot help pump the blood on.

Veins also have thin walls which allows the near by muscles to press against them so that they become flat. This helps the blood to be pushed forwards towards the heart. There is only a thin outer layer of longitudinal collagen and elastic fibres as there is low pressure inside the vein and so little chance of bursting. Finally, a wide lumen is needed to accommodate the slow flowing blood due to the low pressure. They have valves to prevent blood pooling at extremities (arteries do not have valves).

 

Capillaries are made up of a wall that is only one cell layer thick and results in the distance for diffusion in and out of the capillary being very small so that diffusion can occur rapidly. Capillaries are involved with material and gas exchange with the surrounding body tissue

They also contain pores within the their wall which allow some plasma to leak out and form tissue fluid à aid the transport of material. Phagocytes can also pass through these pores to help fight infections.

In addition, the lumen of the capillaries is very narrow. This means that many capillaries can fit in a small space, increasing the surface area for diffusion.

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