In this chapter of AS/A-Level Biology, we will learn about the structure and the adaptations of different parts in the circulatory System.

• Artery walls are thick to withstand high pressure
• The lumen is small to maintain the high pressure
• The inner layer consists of a thin layer of elastic tissue that allows the walls to stretch and recoil
• The middle layer consists of a thick layer of smooth muscle
• The outer layer is a thick layer of collagen and elastic tissue for strength and to support the recoil for maintaining pressure.

• Small blood vessels that distribute the blood from arteries to the capillaries 
• They have a layer of smooth muscle which contracts to increase resistance to flow and reduces the rate of blood flow
• Constriction of arteriole walls is used to divert blood to regions of the body that are demanding oxygen

• Very thin walls consisting of a single layer of endothelium
• Narrow lumen to squeeze red blood cells against the wall to aid in the transfer of oxygen and reduce the diffusion distance
• Leaky walls to allow blood plasma and dissolved substances to leave the blood (e.g. in lymph)

• These collect blood from the capillary bed and lead into the veins 
• Consists of thin layers of muscle and elastic tissue as the pressure is relatively low

• Carry blood back to the heart
• Large lumen in order to ease the flow of blood
• Walls are thin as they do not need to stretch and recoil and are not actively constricted in order to reduce the blood flow
• They contain valves to prevent back flow of blood and help the blood flow into the heart properly 

This is the end of the topic

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