Overview
This lecture explains the mechanisms that increase venous return—the amount of blood returning to the heart—and the importance of this process for maintaining stroke volume, especially during exercise.
Starling's Law and Venous Return
- Starling's Law states that stroke volume (blood ejected per heartbeat) is equal to venous return (VR).
- Venous return is the amount of blood returning to the right atrium and determines the blood available to be pumped out.
Importance of Venous Return During Exercise
- Increased venous return during exercise ensures adequate oxygen and nutrient delivery to working muscles.
Mechanisms Increasing Venous Return
Gravity
- Gravity assists venous return from the upper body via the superior vena cava when upright.
- Elevating legs or lying with legs raised can help blood return from lower limbs by taking advantage of gravity.
Smooth Muscle Pulse in Veins
- Veins contain smooth muscle that contracts (vasomotor tone) to rhythmically squeeze and help push blood toward the heart.
- Contraction narrows the lumen, increasing pressure and aiding venous return.
Pocket Valves
- Pocket valves in veins prevent backflow of blood, ensuring one-way movement toward the heart.
- More valves are found further from the heart where blood pressure is lower.
Skeletal Muscle Pump
- Veins run through skeletal muscles, which contract and squeeze the veins, pushing blood back to the heart.
- Movement or active recovery keeps these muscles working, enhancing venous return during and after exercise.
Respiratory Pump
- Breathing changes thoracic cavity pressure, aiding venous return by squeezing blood in veins toward the heart.
- Increased depth and frequency of breathing during exercise amplifies this effect.
Key Terms & Definitions
- Starling's Law — Stroke volume equals venous return.
- Venous Return (VR) — Blood volume returning to the right atrium per minute.
- Smooth Muscle — Involuntary muscle in vessel walls that can contract to influence blood flow.
- Pocket Valve — Flap-like structures in veins preventing backflow of blood.
- Skeletal Muscle Pump — Action where contracting skeletal muscles compress veins to aid blood return.
- Respiratory Pump — Pressure changes from breathing that assist venous blood flow to the heart.
Action Items / Next Steps
- Review mechanisms of venous return and be able to explain each.
- Consider how these mechanisms relate to exercise performance and recovery.
- Prepare for potential questions regarding Starling’s Law and venous return on exams.