When the individual stands up, that is, undergoes orthostasis, the pressures are altered everywhere except at the heart. At the arterial side in the feet, for example, the pressure has increased to 183 mmHg and on the venous side to 93 mmHg. The increase is of course due to gravity. It can be calculated using the expression rho x G x h, where rho is the density of blood, G is the gravitational constant, and h is the vertical distance (e.g. 120 cm). The answer must be divided by a conversion factor to obtain pressure in mmHg. A gravitational component acting through the hydrostatic columns from the heart down to the feet is added to both the venous and arterial pressures. The increased transmural pressures in both vessels has the capability of stretching the blood vessels, and thus causing increased blood flow.
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