Mind the Vein
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See related article, pp 418–426
Veins lack luster when it comes to hypertension research. Compared with the scientist working on arterial branches, heart, or kidney, the vein researcher commonly feels like being at the bottom of the cardiovascular food chain. Convincing peers that project proposals or articles dealing with venous regulation are any good can be challenging. The study by Okamoto et al1 is a strong reminder that veins are more than blood sampling outlets, sources of blood clots, or commodities for cardiovascular surgeons. Indeed, in patients with severe autonomic failure, venous capacitance vessels, particularly those in the splanchnic area, take over command of blood pressure control. Moreover, veins can serve as a treatment target to improve blood pressure control in such patients.
Veins contain ≈75% of total body blood volume such that small changes in venous capacitance or compliance substantially affect cardiac loading conditions. Venous function is governed by various feedback mechanisms and reflexes, including arterial baroreflexes. Furthermore, increased intravenous volume and associated venous distension elicit local arteriolar constriction. This so-called venoarteriolar reflex suggests that veins can truly be in charge of local hemodynamic control. With standing, ≈500 to 1000 mL of blood are rapidly redistributed from the upper body to venous capacitance vessels below the diaphragm. Human venous compliance is particularly high in splanchnic and cutaneous areas. Yet, a larger proportion of the venous pooling with standing takes place in splanchnic compared with cutaneous veins. In healthy individuals, baroreflex mechanisms adjust efferent sympathetic and …