The relationship between active constriction and passive recoil of the veins at various distending pressures.

Abstract

AbstractThe vasoconstrictor fibre influence on veins, exposed to varying distending pressures, was analysed with respect to the relative contribution of active venoconstruction and passive venous recoil, respectively, to total venous blood mobilization. The results indicate that active venoconstriction can produce only a minor reduction of venous capacity when venous transmural pressure is low but a considerable diminution of the capacity at higher venous pressures, provided the pressure is not so high as to “overstretch” the muscle elements. — The passive emptying of blood at vasoconstrictor fibre stimulation is very pronounced in the low pressure range, but becomes small at higher pressures, in accordance with the pressure‐volume characteristics of the veins, demonstrating an apparently very high distensibility at low pressures. This high “distensibility” is mainly due to changes of vessel geometry, associated with the collapse, and does not reflect true wall distensibility. … Consequently, when venous pressure is low, venous function is primarily governed by passive factors, while active shortening of venous smooth muscles will play a dominant role first when the vessels are kept well distended. — The implications of these findings for venous control is discussed, with special respect to man, where venous pressure varies greatly with body position. Some methods earlier used for assessing venous tone changes are critically analysed.