 The purpose of this study is to better understand the role of the abdominal veins splanchnic capacitance and the sympathetic nervous system in human hypertension. The investigators will test the hypothesis that constriction of abdominal veins due to sympathetic activation contributes to human hypertension. Splanchnic capacitance will be assessed in normotensive and hypertensive subjects at baseline and during acute blockade of the autonomic nervous system. 