Page 12, hemodynamics, Dr. D. Penney

Effects of Orthostasis:

Let us discuss further the effects of gravity on blood pressure. When an individual is lying down, neither the arterial pressure nor the venous pressure is affected by gravity. Venous pressure at the feet may be 5 mmHg, and the same at the head. Blood is flowing from the head to the heart and from the feet to the heart. Some loss of pressure occurs in the venous system because of a slight vascular resistance. The pressure at the heart on the arterial side, approximately 100 mmHg, is mainly produced by the cardiac contractions.

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.