The pressure in the venous system is largely determined by gravity since the system can be visualized as a simple manometer (Figure CR.19). In the erect position, a hydrostatic gradient is produced from head to toe. The hydrostatic pressure difference between any two points in the venous system can be calculated by taking the product of the difference in height between the points (h), the density of blood (p = 1.050 g/cm3), and the acceleration due to gravity (g = 980 cm/s). These figures give a pressure difference equivalent to 0.77 mmHg for every centimetre difference in height.
The venous pressure in the internal jugular vein is normally atmospheric at neck level if the vein is collapsed. Venous pressure above the neck may be less than atmospheric and can be estimated, e.g. 30 cm above the neck (sagittal sinus) venous pressure is less than atmospheric by 30 x 0.77 = 23 mmHg.
Similarly in the foot, 150 cm below neck level, venous pressure is greater than atmospheric by 150 x 0.77 = 115 mmHg.
Such hydrostatic pressure differences are reduced significantly by the primary and secondary mechanisms outlined above and the presence of valves or venous obstruction. Thus, in the foot venous pressures may only be 80 mmHg when standing and decrease further to < 30 mmHg when walking.
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