Venous
pathophysiology
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 (Fig. 16.4). 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 (Fig.
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.