Blood Vessels: Structure and Functions

AQA Biology AS

Discusses the structure of blood vessels (arteries, arterioles, capillaries and veins) and how these structures are related to the function each of these vessels have

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Types of Blood Vessels and Blood Vessel Layers

Types of Blood Vessels:

  • arteries - carry blood from heart into the arterioles and to the tissues
  • arterioles - smaller arteries controlling blood flow from arteries to capillaries
  • capillaries - tiny vessels linking arterioles to veins that exchange metabolic materials between blood and body cells
  • veins - carry blood from capillaries in the tissues, back to the heart

Layers of Blood Vessels:

  • tough outer layer - resists pressure changes
  • muscle layer - contracts and relaxes to control blood flow
  • elastic layer - stretches and recoils to maintain blood pressure
  • endothelium (thin inner lining) - smooth and thin to prevent friction and speed up diffusion
  • lumen - not really a layer; it is the cavity in the centre of a vessel in which blood flows through
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Artery's Structure and Function

Main function of an artery: transport blood rapidly under high pressures from heart to arterioles, and eventually to the tissues.

An artery's structure is adapted to do this as follows:

  • Thick muscle layer (compared to veins)
    • smaller arteries can be constricted and dilated to control volume of blood passing through
  • Thick elastic layer (compared to veins)
    • stretches at systole (contraction) of the heart
    • recoils at diastole (relaxation of heart)
    • stretching and recoiling helps to maintain high pressure (high pressure needed for blood to reach extremities of the body)
    • it also smoothes pressure surges created by heart beating
  • Thick wall
    • resists the vessel bursting under pressure
  • No valves
    • blood does not tend to flow back as it is at constant high pressure
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Arteriole's Structure and Function

Main function of an arteriole: carry blood, under lower pressure than arteries, from arteries to capillaries. Also control blood flow between the two.

An arteriole's structure is adapted to do this as follows:

  • Thick muscle layer (thicker than arteries)
    • contraction = lumen constriction = restricted blood flow = control of blood movement into capillaries
  • Thin elastic layer (thinner than arteries)
    • blood pressure is lower
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Vein's Structure and Function

Main function of a vein: transport blood slowly, under low pressure, from tissues to the heart.

A vein's structure is adapted to do this as follows:

  • Thin muscle layer (compared to arteries)
    • veins carry blood away from tissues - means there is no need to constrict or dilate (as in arterioles), as this is to control flow of blood to tissues
  • Thin elastic layer (compared to arteries)
    • low blood pressure - means there is no danger of bursting and so no need to stretch
  • Thin wall
    • low blood pressure - means there is no danger of bursting and so no need for a thick wall
  • Valves throughout
    • ensures blood does not flow backwards (which it may do due to low pressure)
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Capillary's Structure and Function

Main function of a capillary: to exchange metabolic materials (such as oxygen, carbon dioxide and glucose) between the blood and the cells of the body.

A capillary's structure is adapted to do this as follows:

  • Very thin: only consist of endothelium
    • diffusion pathway is short
  • Numerous and highly branched
    • large surface area for diffusion
  • Narrow diameter
    • able to permeate tissues - as a result, no cell is far from a capillary
  • Narrow lumen
    • red blood cells squeezed against capillary side = brings them even closer to cells = reduces diffusion distance
  • Spaces between the endothelial cells
    • allow white blood cells to escape in order to deal with infections within tissues
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