Venous flow depends on the following factors EXCEPT? (AIIMS MAY 2008)
|A||Calf muscle contraction|
|B||Valves in perforators|
Venous return of flow depends on
I. Respiratory pump
II. Muscle pump
III. Sympathetic tone
IV. Cardiac output
VI. Venous pressure gradient that is VR = Psf – RAF/ RVR in which VR is venous return, Psf is mean systemic filling pressure, PRA is right atrial pressure, and RVR is resistance to venous return. Psf in turn depends on arterial pressure.
a. In the limbs, the veins are surrounded by skeletal muscles, and contraction of these muscles during activity compresses the veins.
b. Pulsations of nearby arteries may also compress veins. Since the venous valves prevent reverse flow, the blood moves toward the heart.
c. During quiet standing, when the full effect of gravity is manifest, venous pressure at the ankle is 85–90 mm Hg.
d. Pooling of blood in the leg veins reduces venous return, with the result that cardiac output is reduced, sometimes to the point where fainting occurs. Rhythmic contractions of the leg muscles while the person is standing serve to lower the venous pressure in the legs to less than 30 mm Hg by propelling blood toward the heart.
e. This heart ward movement of the blood is decreased in patients with varicose veinsbecause their valves are incompetent.
f. These patients may develop stasis and ankle edema.
g. However, even when the valves are incompetent, muscle contractions continue to produce a basic heart ward movement of the blood because the resistance of the larger veins in the direction of the heart is less than the resistance of the small vessels away from the heart.