Venous return

Question 1

Factors that control venous return (VR) = in terms of systemic circulation

Answer 1

Pressure gradient = VPperipheral - VPcentral

• Therefore to increase venous return you need to increase VPperipheral or reduce VPcentral (normally b/w 0-5mmHg)
• There are three main factors controlling venous return:
  - skeletal muscle pump (increases VPperipheral)
  - respiratory pump (decreases VPcentral)
  - sympathetic venoconstriction (increases VPperipheral)

Question 2

Skeletal muscle pump

Answer 2

• enhances venous return
  Two factors promote venous return:
  
  • Alternate contraction and relaxation of skeletal muscles
  • One-way valves prevent backflow
  • This is a significant factor increasing venous return during dynamic exercise (e.g. running, cycling)

Question 3

(ii) respiratory pump

Answer 3

• During inspiration the chest expands, reducing intrathoracic pressure
  
  • Boyle’s Law, P∝1/V
  • Plungs < Patmosphere → air drawn into lungs
• However, VPcentral also reduces
• ∆P increases, so venous return increases
• During expiration, the reverse happens and VR decreases.
• During exercise, hyperventilation produces a net increase in venous return

Question 4

(iii) venoconstriction

Answer 4

• Normally, about 65% of your blood sits in your veins.
• The veins are highly compliant and can expand to accommodate more blood volume.
• When activated by the sympathetic nervous system, the smooth muscle in the walls of the veins contracts, constricting the veins and reducing compliance.
• Blood is pushed back towards the heart, increasing venous return.