VENOUS PHYSIOLOGY AND PATHOPHYSIOLOGY
In order to treat venous problems properly, it is essential to understand the normal pattern of venous blood flow and the effects of venous valves. For practical purposes, the venous system of the leg can be considered to have two major parts, a deep component and a superficial component. The superficial veins serve as a primary collecting system and are connected through perforating veins into the deep veins, which normally carry about 90 percent of the blood from the leg back to the central circulation.
As arterial blood flows into the leg, distal superficial veins and venous reservoirs are constantly filling and must be emptied into the deep veins regularly and then somehow returned to the right side of the heart. Venous return from the leg must flow uphill against gravity, against fluctuating thoraco-abdominal pressures, and sometimes in the face of additional backpressures such as the elevated right atrial pressures of congestive heart failure. This process depends on the patency of the flow circuit and on the correct functioning of a complex series of valves and pumps.
When muscles of the calf and leg contract, they compress deep veins within the muscles, forcing the blood in those veins to flow upward past venous valves. When the muscles relax, the deep veins expand and draw in more blood from superficial collecting veins (Figure 4-1). This system is known as the “calf muscle pump” and sometimes is referred to as a “peripheral heart.” Veins and muscles of the calf and of the foot both play an important role in the normal functioning of this system, which can achieve pumping pressures of several hundred torr before valve failure occurs.1 Particularly important venous capacitance reservoirs are found within the soleus muscle and within the deep muscles of the foot.
A. The calf muscle pump with muscle contracted. A pressure of 2 atm forces the vein segment to empty. B. The calf muscle pump with muscle relaxed. Zero or negative pressure permits the vein segment to refill.
Normal deep and superficial veins contain one-way valves that permit flow from the superficial system into the deep system and from distal to proximal. The calf muscle pump cannot function without competent valves both above and below. Venous valves are bicuspid and are located every few centimeters in all normal veins below the level of the common femoral vein.2
Recent studies of venous valves indicate that there may be several different types of valves including some that offer some resistance but do not shut completely.3 Normal venous valves can withstand pressures of up to 3 atm, but congenitally weak ...