Changes in venomotor tone during sympathoexcitation: influence of gender and female cycle phase

Abstract

Moving from the supine to the upright posture presents a significant challenge to the human body since the lower limb veins become distended due to gravitational pooling and venous return is decreased. An increase in leg venomotor tone would assist in countering pooling and help to maintain a pressure gradient for forward flow. Quantifiable proof of active venoconstriction in humans is sparse with the majority of work in this field having been done on forearms. The studies within this thesis have looked directly at lower limb responses to venoconstriction in young healthy volunteers and have compared different venous vascular beds (deep and superficial) and also the influence of female hormones. The data show that increases in venomotor tone in the calf are relatively modest and are specific to superficial rather than deep veins and that calf limb responses are attenuated in females. Venous function in women appears to be modified during oral contraceptive use compared to normally menstruating women. It is unlikely that the degree of venoconstriction observed in these experiments is sufficient on its own to maintain venous return during orthostatic challenge.