Venous system 2

Question 1

4. Respiratory pump:

Answer 1

• Respiratory activity causes a sub-atmospheric
  pressure in the chest (~5 mmHg less)
• This creates an external pressure gradient > blood flows from the lower veins to chest veins > increases venous return

Question 2

5. Cardiac suction:

Answer 2

• During ventricular contraction, AV valves are drawn down which enlarges atrial cavities > atrial pressure transiently drops to <0mmHg > increases vein-atria ΔPà increases venous return

Question 3

5. Cardiac suction: 2

Answer 3

• During ventricular relaxation, there is expansion of ventricular space > ventricular pressure transiently drops to <0mmHg > increases atria-ventricular ΔP > increases venous return

Question 4

6. Increased blood volume:

Answer 4

• Short-term: bulk-flow from interstitial fluid to plasma at
  site of capillaries > increases plasma volume
• Long-term: salt & water retention through alterations in kidney function & thirst > increases plasma volume

Question 5

Summary

Answer 5

• Veins: volume reservoir
• Highly compliant & hold excess blood
• Low pressure but a maintained pressure gradient drives blood flow
• Factors that increase venous return to heart