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AS/A-Level Biology - Circulatory System

Topic3 - Circulatory System

December 28, 2021

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

  • 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.

Arterioles

  • 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

Capillaries

  • 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)

Venules

  • 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

Veins

  • 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