Gravitational and/or accelerational forces cause hydrostatic pressure gradients within a body's fluid-filled compartments. The hydrostatic pressure difference (Dp) between two given points within such a compartment depends on the fluid's density (D), the magnitude of the force imposed onto the system (f), the distance (d) between the points of interest, and the direction of the gravitational field / accelerational force relative to the line connecting those points. If we take a as the angle between the latter two, than the pressure effect is proportional to cos a, and Dp = D x f x d x cos a.

This is relevant for the cardiovascular system because there is a direct link to orthostatic resilience and the syncope problem: With alteration of body posture, the heart has to deal with changed preload and afterload. Unfortunately, the hydrostatic indifference concept (Gauer & Thron, Handbook of Physiology, Sect.2, Vol. III, 1965) all but disappeared from modern textbooks of physiology - despite its obvious significance in terms of basic science (cardiovascular physiology) as well as clinical application (cardiology).

Upon reorientation of a body which is subject to gravitational attraction (on Earth, 1G) or when accelerational forces act upon this body, hydrostatic pressures within fluid filled compartments change, except one location that is 'hydrostatically indifferent', i.e. it does not experience pressure change. This is, per definitionem, the Hydrostatic Indifference Point (HIP).

Various experiments have shown that (1) for any postural change there is a certain specific HIP, (2) the referring HIP has a different location for any fluid-filled compartment, (3) the HIP location depends on the physiological state of the referring compartment, and is influenced by the actual filling volume, vessel compliance etc. For instance, the venous HIP lies below the diaphragm when a movement from supine to upright is considered, but in the atrial region for a movement from supine to head down; the arterial and venous HIPs are different; and with increasing blood volume, the venous HIP moves footward.

Typically, the right heart has to cope with a significant drop in diastolic filling pressure (preload) when a person stands up. This causes stroke volume to decrease by up to 40% and cardiac output by about 30%. The carotid baroceptor is much more influenced by this postural change than the aortic receptor area, and is in a privileged position to regulate blood pressure and cerebral perfusion with orthostatic stress.