 The investigators hypothesize that the decrease in BP induced by autonomic blockade with trimethaphan results from an increase in splanchnic capacitance leading to a reduction in venous return. To test this hypothesis, the investigators will compare the effect of sympathetic withdrawal on splanchnic capacitance between hypertensive and normotensive subjects. Splanchnic venous capacitance will be measured by radionuclide plethysmography. Abdominal blood volumes will be measured using labeled red cells with technetium-99 while applying different levels of continuous positive airway pressure. 