Blood Vessels: Structure and Functions

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

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

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

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)

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