Effects of the renin-angiotensin-aldosterone system on the cardiovascular system during 20-days bed rest.

Long term change of renin-angiotensin-aldosterone system (RAAS) induced by bed rest and its effects on cardiovascular system are still controversial. The purpose of this study was to obtain a general conclusion on these questions by analyzing our two 20-days horizontal bed rest experiments in past two years with 18 subjects. Plasma renin activity and aldosterone were consistently increased during the bed rest, but angiotensin II was increased only during the early days. Decrease in urinary sodium excretion and increase in urinary potassium excretion were observed during day 3-8 and day 7-12, respectively. Mean arterial pressure increased during day 3-8. Pulse pressure was returned to pre-bed rest level by day 10 after an initial decrease. All these results indicated an activated RAAS and its active effects on cardiovascular and overall fluid regulating systems during our horizontal bed rest studies. Direct effect of change in gravitational force on renal pressure-sensitive cells or effects related to physical inactivity may explain our results.