Blood flows into the leg because it is pumped by the heart along the arteries. By the time it emerges from the capillaries it is at a low pressure (about 20 mmHg), but this is enough for the blood to return to the heart. Blood from the muscles of the leg returns through the deep veins. Blood from the skin and superficial tissues, external to the deep fascia, drains via the long and short saphenous veins — SFJ and SPJ — and communicating veins into the deep veins. Valves prevent the flow of blood from the deep to the superficial system.
The venous pressure in the foot vein on standing is equivalent to the height of a column of blood, extending from the heart to the foot. However, the same is true of the arterial system so that on standing the arterial blood pressure at the ankle rises by 80—100 mmHg, depending on the height of the person. So the blood continues to circulate, even in the absence of muscle activity. However, we also have a sophisticated series of muscle pumps that act as peripheral hearts in the venous system. These are made up of the deep veins of the calf and thigh which are surrounded by muscle. In addition, there is a foot pump which ejects blood from the plantar veins as pressure is placed on the foot during walking. On exercise the calf and thigh muscles contract compressing the veins and ejecting blood towards the heart. The direction of venous blood flow is controlled by the venous valves. The pressure within the calf compartment rises to 200—3 00 mmHg during walking and this is more than enough to propel the blood in the direction of the heart. During the muscle relaxation phase, the pressure within the calf falls to a low level and blood from the superficial veins flows through the perforating veins into the deep veins. The consequence of this is that the pressure in the superficial veins falls during walking. This can be monitored by a cannula placed in a superficial vein of the foot and connected to a pressure transducer. Normally the pressure in the superficial veins of the foot and ankle falls from a resting level of 80—100 mmHg to about 20 mmHg.
This ability to reduce the pressure in the superficial venous system is crucial to the health of the lower limb. Patients with damage to the veins in whom the superficial venous pressure does not fall during exercise may develop varicose eczema, skin damage and, eventually, leg ulceration.
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