Table 1.

Description of terms used in this review.

Capacitance	The term capacitance remains poorly defined and is not synonymous with venous volume as a smaller amount of volume resides within the heart and the arterial tree (Figure 1). In general terms it relates to the total contained volume of the vasculature to a given transmural pressure over at least the physiological range of transmural pressures. In this review we will use, for simplicity, the term venous capacitance synonymous with venous volume.
Compliance	A general term describing the change in dimension following a change in stress. Translated into venous physiology this means that compliance is the ratio of the change in volume (ΔV) resulting from a change in transmural distending pressure (ΔP), or ΔV/ΔP. It is the slope of the pressure/volume relationship (PVR) at a given point of the ‘curve’ (Figure 2). Because venous compliance is very high at low pressures, the slope of the venous PVR over the physiological pressure range (10–40 mmHg) is nearly linear.
Unstressed volume	The volume of blood in a vessel at zero transmural pressure is defined as unstressed volume. It is a virtual volume established by extrapolating a linear portion of the PVR to zero transmural pressure.