Venous return to hea during quiet standing is facilitated by all of the following factors, except
**Core Concept**
During quiet standing, the body must adapt to counteract the effects of gravity on blood distribution. This involves the coordinated action of various physiological mechanisms to ensure adequate venous return to the heart. Key factors facilitating venous return include muscle contraction, compression of veins by surrounding tissues, and the presence of venous valves.
**Why the Correct Answer is Right**
Gravitational increase in arterial pressure does not directly facilitate venous return during quiet standing. In fact, the increased hydrostatic pressure exerted by gravity on blood in the veins would tend to impede venous return. Instead, the body relies on other mechanisms, such as muscle contraction and compression, to counteract this effect. Additionally, the presence of venous valves prevents backflow of blood into the lower extremities.
**Why Each Wrong Option is Incorrect**
**Option A:** Calf muscle contraction during standing is actually a key factor in facilitating venous return. When calf muscles contract, they compress the veins in the lower extremities, helping to push blood back towards the heart.
**Option B:** Valves in perforators are essential for preventing backflow of blood into the lower extremities. They ensure that blood flows in one direction, towards the heart, rather than being pushed back down into the lower extremities by gravity.
**Option C:** The sleeve of deep fascia provides compression of veins, which helps to facilitate venous return. This compression, combined with muscle contraction, plays a crucial role in ensuring that blood is returned to the heart efficiently.
**Clinical Pearl / High-Yield Fact**
Remember that venous return during quiet standing is primarily facilitated by muscle contraction and compression of veins by surrounding tissues, rather than by gravitational increases in arterial pressure.
**✓ Correct Answer: D. Gravitational increase in arterial pressure**