Capillaries

• Capillaries


  Extensive networks of thin-walled blood vessels where substances are exchanged between the blood and body cells
• Capillaries


  • Extremely thin wall consisting of a single layer of endothelial cells
  • Very short diffusion distance between blood and surrounding cells
  • Diameter slightly greater than red blood cell, causing them to be pressed against the wall
  • Red blood cells travel through in single file at a slower speed
• Diffusion of molecules in capillaries

  1. Oxygen and glucose diffuse from blood to body cells

  2. Waste products like carbon dioxide diffuse from cells back to blood
• Tissue fluid


  Fluid that bathes the cells, providing essential molecules like glucose and amino acids
• Capillary wall


  • Contains small gaps between endothelial cells
  • Allows tissue fluid to pass out of the blood
  • Allows white blood cells to leave the bloodstream
• Extensive branching of capillaries provides a massive surface area for exchange of materials
• No body cell is very far from a capillary
• A network of capillaries is called a capillary bed
• Capillaries


  • Have walls which are one cell thick (endothelium) so walls are permeable
  • Have small diameter (lumen)
  • Friction with the walls slow down the blood flow
  • Many capillaries in a capillary bed, providing a large total cross-sectional area which further reduces blood flow
  • Low velocity in these very thin walled vessels enhances their ability to exchange materials with the surrounding tissue
  • Have gaps / fenestrations that allow exchange of materials
• Fenestrations


  • Windows which increase transport in certain
• A capillary is composed of simple squamous cells which roll to produce a tube