A soldier stands motionless at attention for 30 minutes and reports light-headedness, whereas marching in place relieves the symptoms. The physiological explanation for relief during leg movement is:
- A Muscle contraction raises venous pressure in the legs, increasing filtration into tissues
- B Contraction of skeletal muscle compresses veins and, with competent valves, drives blood toward the heart, lowering venous and capillary pressure in the legs ✓
- C Leg movement activates carotid sinus baroreceptors directly, raising heart rate
- D Exercise abolishes the effect of gravity on the arterial circulation
Correct answer: B. Contraction of skeletal muscle compresses veins and, with competent valves, drives blood toward the heart, lowering venous and capillary pressure in the legs
Explanation
Standing still allows venous pooling; foot vein pressure can approach 90 mmHg and capillary hydrostatic pressure rises, promoting filtration and reducing effective circulating volume. Rhythmic muscle contractions compress deep veins, and unidirectional valves convert each squeeze into centripetal pumping, lowering venous and capillary pressures. Option A reverses the direction of the effect, and gravity on the arterial column persists regardless of movement, excluding option D.
Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.
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