Splenic contraction associated with sudden, transient hypotension and sympathoexcitation in humans.

The spleen is traditionally recognised for its roles in immunity and blood filtration, and it is known to contract and release stored erythrocytes in hypoxic conditions. Recent findings suggest that the spleen may also contribute to cardiovascular regulation. This study investigated whether the spleen contracts during conditions that activate the sympathetic nervous system, including transient hypotension and psychophysiological stress responses. Eighteen healthy volunteers underwent four interventions known to be associated with either unloading of the arterial baroreceptors or sympathoexcitation: head-up tilt, cold-pressor test, thigh-cuff release and maximal-duration apnoea. Splenic volume was assessed repeatedly using ultrasound imaging, and cardiovascular variables were recorded continuously. All interventions elicited significant reductions in splenic volume (head-up tilt -5.3%, P = 0.0386; cold-pressor test -6.6%, P < 0.001; thigh-cuff release -11.9%, P = 0.00118; apnoea -9.3%, P = 0.00159; Wilcoxon's signed rank test vs. baseline), indicating that splenic contraction occurs not only during hypoxic conditions (apnoea) but also in response to baroreceptor unloading (head-up tilt, thigh-cuff release) and general sympathetic activation (cold-pressor test). These findings suggest that the spleen may act as a constitutive effector of the sympathetic nervous system, possibly contributing to short-term blood pressure regulation. The results highlight a previously underappreciated role of the spleen in cardiovascular homeostasis and raise new questions about its contribution to systemic vascular control and stress physiology.