Effective compliance of the circulation in the upright sitting posture

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Abstract
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×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×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 compliance to about half the control value (1.9 ml/(mm Hg×kg BW) which probably constitutes the effective compliance of the intrathoracic circulatory compartment.