Blood flow to skin and muscle fluctuates in response to local, hormonal, and reflex stimuli. Local venous pressure varies with lying, sitting, and standing. The calf muscle remains an effective pump for returning venous blood. The normal vein is pliable and can constrict or dilate over a wide range. However, high hydrostatic pressure stretches the vein, and it becomes stiff over a period of time. Prolonged muscle inactivity, as in prolonged air travel, brings about an increase in venous pressure. This pressure is transmitted to the capillaries, and some fluid may migrate outside the capillaries into the extracellular fluid. This leads to transient (temporary) edema (accumulation of fluid). Prolonged elevation of venous pressure can lead to skin ulceration and other complications.

Figure 1

To counteract gravity, a system of muscle pumps ejects the blood, while a system of valves keeps the blood from flowing backward (Figure 1). There are more valves in the infrapopliteal segment of the leg (below the knee) than in other parts of the leg. This suggests that valves at this location are very important for maintaining normal circulation.

Figure 1. Illustration of the action of the muscle pump. (a) At rest; (b) during muscle contraction; note how the valve is closed below the muscle pump to prevent blood from refluxing downward but is wide open above to let the blood be propelled toward the heart (c) during muscle relaxation. Pressure in the lower leg is indicated by the length of the column on the left of each drawing. Note how the valves close after muscle contraction (b) to prevent blood from refluxing into the leg.

(Awaiting permission from Sumner DS. Hemodynamics and pathophysiology of venous disease. In: Reutherford RD, ed. Vascular Surgery, 4th ed. Philadelphia: WB Saunders, 1995:1679.)