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BLOOD FLOW AND ITS APPEARANCE ON COLOR FLOW IMAGING
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for example, there is a relatively small pressure dif- ference between the venous pressures at the ankle and right atrium. However, when an individual is standing, there is a column of blood between the right atrium and the veins at the ankle. If the hydro- static pressure is assumed to be zero in the right atrium, the hydrostatic pressure at the ankle will be equal to the distance between the two, which is obviously dependent on the person’s height, but is usually between 80 and 100 mmHg. Therefore, in a standing position, there is a significant pressure gradient to overcome in order for blood to be returned to the heart; this is achieved by the calf muscle pump mechanism assisted by the presence of the venous valves.
The muscle compartments in the calf contain the deep veins and venous sinuses, which act as blood reservoirs. Regular small contractions occur in the deep muscles of the calf, causing compression of the veins, thereby propelling blood flow out of the leg, with the venous valves preventing the blood refluxing back down. This also generates a pressure gradient between the superficial and deep veins in the calf, and blood drains through the per- forating veins and major junctions from the super- ficial to the deep venous system. The valves in the perforators prevent blood flowing from the deep to the superficial veins. During more active exercise, such as walking or running, the calf muscle pump mechanism is able to produce a significant pressure reduction in the deep and superficial venous systems to approximately 30mmHg. The pressure change that occurs during exercise is called the ambulatory venous pressure. At rest, because the hydrostatic pressure is the same on both the arterial and venous sides, the pressure drop across the capillary bed is the same whether the person is standing or lying down. However, after exercise the pressure on the venous side of the capillary bed will drop, but the pressure on the arterial side will remain the same, creating a pressure drop across the capillary bed and aiding the return of blood to the heart. Once the muscle contraction stops, the venous pressure in the lower leg will begin to rise due to filling of the venous system from the arterial system via the capillaries.
It is possible to measure the ambulatory venous pressure by inserting a small cannula into a dorsal foot vein, which is then connected to a pressure
Figure 5.22 Doppler waveform demonstrating the effect of respiration on the blood flow in the common femoral vein. The large arrow indicates the cessation of flow during inspiration and the small arrows show small changes in flow due to the cardiac cycle, which may not always be seen in the common femoral vein.
the thorax brought about by movement of the diaphragm and ribs. Inspiration during calm breath- ing expands the thorax, leading to an increase in the volume of the veins in the chest, which in turn causes a reduction in the pressure in the intra-thoracic veins. This creates a pressure gradient between the veins in the upper limb and head and those in the thorax, producing an increase in flow into the chest. Flow is decreased during expiration as the volume of the thorax decreases, leading to an increase in central pressure.
The reverse situation is seen in the abdomen as the diaphragm descends during inspiration, increas- ing intra-abdominal pressure. This leads to a decrease in the pressure gradient between the peripheral veins and the abdominal veins, thus reducing flow. During expiration the diaphragm rises, producing a reduc- tion in intra-abdominal pressure, and the pressure gradient between the abdominal veins and peripheral veins increases, causing increased blood flow back to the heart. The effects of respiration are observed as phasic changes in flow in proximal deep peripheral veins (Fig. 5.22). Breathing maneuvers are often used to augment flow when investigating venous disorders (see Ch. 12).
Changes in venous blood pressure due to
posture and the calf muscle pump
Large pressure changes occur in the venous system, due to the effects of hydrostatic pressure generated by posture (Fig. 5.1). If an individual is lying supine,