Compliance is the ability of a hollow organ (vessel) to distend and increase volume with increasing transmural pressure or the tendency of a hollow organ to resist recoil toward its original dimensions on application of a distending or compressing force. It is the reciprocal of "elastance", hence elastance is a measure of the tendency of a hollow organ to recoil toward its original dimensions upon removal of a distending or compressing force. Blood vessels The terms elastance and compliance are of particular significance in cardiovascular physiology and respiratory physiology. In compliance, an increase in volume occurs in a vessel when the pressure in that vessel is increased. The tendency of the arteries and veins to stretch in response to pressure has a large effect on perfusion and blood pressure. This physically means that blood vessels with a higher compliance deform easier than lower compliance blood vessels under the same pressure and volume conditions. Venous compliance is approximately 30 times larger than arterial compliance. Compliance is calculated using the following equation, where ΔV is the change in volume (mL), and ΔP is the change in pressure (mmHg): Physiologic compliance is generally in agreement with the above and adds dP/dt as a common academic physiologic measurement of both pulmonary and cardiac tissues. Adaptation of equations initially applied to rubber and latex allow modeling of the dynamics of pulmonary and cardiac tissue compliance. Veins have a much higher compliance than arteries (largely due to their thinner walls.) Veins which are abnormally compliant can be associated with edema. Pressure stockings are sometimes used to externally reduce compliance, and thus keep blood from pooling in the legs. Vasodilation and vasoconstriction are complex phenomena; they are functions not merely of the fluid mechanics of pressure and tissue elasticity but also of active homeostatic regulation with hormones and cell signaling, in which the body produces endogenous vasodilators and vasoconstrictors to modify its vessels' compliance. For example, the muscle tone of the smooth muscle tissue of the tunica media can be adjusted by the renin–angiotensin system. In patients whose endogenous homeostatic regulation is not working well, dozens of pharmaceutical drugs that are also vasoactive can be added. The response of vessels to such vasoactive substances is called vasoactivity (or sometimes vasoreactivity). Vasoactivity can vary between persons because of genetic and epigenetic differences, and it can be impaired by pathosis and by age. This makes the topic of haemodynamic