Blood Vessels

• Blood Vessels


  How are they adapted their functions?
• Blood Vessel Types


  • Arteries (including aorta)
  • Arterioles
  • Capillaries
  • Veins
• Arteries


  • Transport blood away from the heart
  • High pressure in the walls, so they are thick to withstand the pressure
  • Lumen (hole that blood travels through) is small
  • Lots of elastic tissue in the walls to stretch when ventricles contract and recoil when they relax, smoothing out surges in blood pressure
  • Wall is lined with smooth endothelial tissue to reduce resistance to flow
  • Aorta/pulmonary artery has a valve to prevent backflow into ventricles
  • Muscle in the wall can contract to reduce lumen diameter and increase blood pressure
• Arterioles


  • Transport blood away from heart, connect to capillaries and control blood flow into them
  • Muscle tissue in the wall to control blood flow
  • Muscle contraction constricts the vessel (vasoconstriction) to reduce blood flow to capillaries
  • Muscle relaxation dilates the vessel (vasodilation) to increase blood flow to capillaries
• Capillaries


  • Where exchange takes place
  • Walls are one cell thick to give short diffusion pathway for efficient exchange
  • Walls made of endothelial cells, flattened to give short diffusion pathway
  • Many branches give large total cross-sectional area, lowering velocity and giving more time for exchange
  • Narrow diameter gives greater resistance and slows blood flow to allow more time for exchange
  • Gaps between endothelial cells (fenestrations) allow fluid to move out with less resistance
• Veins


  • Carry blood back to the heart
  • Low pressure so walls don't need to be thick
  • Small amount of elastic and muscle tissue in walls
  • Lumen is large, giving less resistance to blood flow
  • Decreasing total cross-sectional area as smaller veins join, increasing velocity to return blood to heart
  • Valves to prevent backflow
  • Surrounding skeletal muscle contraction compresses veins to move blood upwards