Human circulatory system

• Arteries

  • high pressure away from heart
  • thick walls
  • collagen rich outer layer, smooth muscle layer and elastic layer that recoils when pulses occur causing blood to always flow forwards
  • narrow lumen lined with endothelium cells
• Arterioles

  • connect arteries to capillaries
  • thinner collagen and elastic layer but thicker smooth muscle layer
• capillaries

  • thin walls- single layer of endothelial cells
  • small diameter- causes red blood cell to press against walls
  • red blood cells travel one by one causing an increase in time available for gas exchange
  • small gaps between endothelial cells- allow tissue fluid to exit and white blood cells
• What does tissue fluid do?

  It bathes cells providing essential molecules like glucose and amino acids
• Veins

  • low pressure
  • thinner walls
  • relatively large collagen layer and really thin elastic and smooth muscle layer
  • large lumen
  • valves which prevent backflow