The reflex-feedback system in the human body is a person’s answer to gravity. When people stand up, gravity tries to pull the blood in their systems downward. To keep circulation in equilibrium, the veins and arteries constrict momentarily, thereby maintaining blood pressure.
Alcohol dilates blood vessels and impairs this reflex, which may explain why some people faint upon standing after downing only a few drinks, Virend K. Somers of the Mayo Clinic in Rochester, Minn., and his colleagues report in the Feb. 1 Circulation.
The researchers tested blood pressure in 14 volunteers, average age 26. The participants reclined in a metal cylinder equipped with a vacuum system that reduced the pressure on their legs suddenly, to simulate the stress of standing. Tests after the volunteers drank two or three beers showed that blood pressure dropped significantly and that their vessel-constriction reflex was turned off. Sober participants were unaffected.