venous return

• characteristics of arteries


  small lumen, no valves, thick elastic walls, high pressure, away from heart
• characteristics of veins


  large lumen, valves, low pressure , thin walls , back to heart
• characteristics of capillaries


  microscopic lumen, very low pressure, one cell thick, no valves, delivers oxygen and glucose to tissues
• Why are mechanism needed to maintain venous return

  the pressure of blood in large veins is very low and gravity makes it difficult to get blood back up to heart
• List venous return mechanisms

  Skeletal muscle pump,Respiratory pump, Pocket valves, Heart ,Smooth muscle in veins
• Skeletal muscle pump


  When muscles contact or relax they press on veins squeezing blood back to heart
• Respiratory pump


  When muscles contact or relax during breathing, pressure changes occur in chest cavity which compress nearby veins squeezing blood back to heart
• Pocket valves


  Valves in veins keep the blood flowing in one direction and close to prevent backflow
• Smooth muscle in veins

  Squeezes blood back to the heart
• Heart


  Acts as a suction pump when it contracts and relaxes
• Impact of BP on venous return

  • when systolic BP increases - venous return increases as the BP in blood vessels is higherwhen systolic BP decreases - venous return decreases as BP in blood vessels has dropped
• Venous return is determined by pressure gradient :

  •  increasing right atrial pressure ( atrial systole ) decreases venous returndecreasing right atrial pressure ( diastole ) increases venous return
• A02 of VR

  • during exercise increased use of muscles in arms (swimming) and legs (swimming, cycling, running) compresses veins more pushing more blood back to the heart
  • increased breathing rate during exercise causes increased effect of respiratory pump returning more blood to the heart
  • suction pump of the heart increase as the heart beats harder and faster during exercise
  • overall increase in venous return during exercise.
• analysis of why VR changes need to occur (A03)

  • Starling’s law

  • This causes the heart muscle to stretch more increasing ejection fraction/stroke volume/cardiac output.

  • More blood leaving the heart means more blood sent to the lungs for greater gas exchange

  • More blood to working muscles supplying O2 for resynthesis of ATP.

  • The more O2 supplied the longer the performer can work aerobically, limiting production of fatiguing by-products such as lactate.

  • Can work at higher intensities for longer periods of time.