The Splanchnic Circulation

This chapter focuses on the effect of splanchnic circulation on overall circulatory function. The splanchnic circulation powerfully influences systemic arterial pressure via two distinct mechanisms. Widespread contraction of arteries in the splanchnic bed reduces blood flow to the region. Active constriction of veins in the splanchnic organs reduces regional blood volume. This has relatively little effect on total peripheral resistance but raises cardiac output and arterial pressure by increasing central blood volume and thus cardiac preload. Splanchnic sympathetic nerve activity (splSNA) is controlled by arterial and cardiopulmonary baroreceptors, and by inputs from other somatic and visceral afferents. Some stimuli are able to alter splSNA in a selective fashion. Impaired sympathetic splanchnic vasoconstriction is an important cause of orthostatic hypotension and other orthostatic disorders such as postural tachycardia syndrome. In portal hypertension there is a very marked increase in sympathetic nervous system activity, including to the splanchnic organs. During the development of arterial hypertension in humans, increased vascular resistance is observed first in the splanchnic circulation.