Effective compliance of the circulation in the upright sitting posture.

The effective compliance is defined as the relation of change in blood volume to change in central venous pressure. It was measured in 8 upright sitting male subjects and amounted to 3.3 ml/(mm Hg X kg BW). It is, therefore, by about 50% greater than the effective compliance in the supine subject which amounts to 2.3 ml/(mm Hg X kg BW). . This difference is probably due to the posture dependent blood volume distribution in the low pressure system whose "upper" and "lower" sections have nonlinear pressure-volume characteristics. Immersion to the neck reduces the effective complicance to about half the control value (1.9 ml/(mm Hg X kg BW) which probably constitutes the effective compliance of the intrathoracic circulatory compartment.