Blood vessels

• Arteries


  • Transport blood away from the heart (usually at high pressure) to tissues
  • Arterioles: arteries branch into narrower blood vessels called arterioles which transport blood into capillaries
• Veins


  • Transport blood to the heart (usually at low pressure)
  • Venules: these narrower blood vessels transport blood from the capillaries to the veins
• Blood flow


  Through the lumen of a blood vessel; the size of the lumen varies depending on the type of blood vessel (with arteries having a narrow lumen, and the veins a wider one)
• Structure of arteries


  1. Tunica adventitia/externa

  2. Tunica media

  3. Tunica intima
• Tunica intima


  • Endothelial layer
  • Layer of connective tissue
  • Layer of elastic fibres
• Tunica media


  • Smooth muscle cells
  • Thick layer of elastic tissue
• Tunica adventitia


  • Mostly made up of collagen
• Arteries have a narrow lumen which helps to maintain a high blood pressure
• A pulse is present in arteries
• Arterioles


  • Possess a muscular layer that means they can contract and partially cut off blood flow to specific organs
  • Have a lower proportion of elastic fibres and a large number of muscle cells
• Veins


  • Tunica media is much thinner
  • Lumen is much larger than that of an artery
• Veins contain valves to prevent the backflow of blood, helping return blood to the heart
• A pulse is absent in veins
• Venules


  • Have few or no elastic fibres and a large lumen
  • As the blood is at low pressure after passing through the capillaries there is no need for a muscular layer
• Capillaries


  • Have thin walls which are "leaky", allowing substances to leave the blood to reach the body's tissues
  • Can form networks called capillary beds which are very important exchange surfaces within the circulatory system
• Structure and function of capillaries

  • Have a very small diameter (lumen) which forces the blood to travel slowly, providing more opportunity for diffusion to occur
  • Wall is made solely from a single layer of endothelial cells, only one cell thick to reduce the diffusion distance for oxygen and carbon dioxide
  • Cells of the wall have gaps called pores which allow blood plasma to leak out and form tissue fluid
  • White blood cells can combat infection in affected tissues by squeezing through the intercellular junctions in the capillary walls