Thirty-one (21 men, 11 women) subjects were studied during the course of this project. The purpose of this project was to examine the differences in the methods of blood pressure regulation in men and women, specifically the differences in the ways arteries, veins, and the heart work to control blood pressure. Furthermore, this study was conducted during 60 – 90 days of head down tilt bed rest, an analog for the cardiovascular deconditioning that occurs in astronauts during actual spaceflight, so differences between upper and lower body changes to bed rest were of significance.
The general results of this study indicated that men and women have some similar and some different methods of blood pressure regulation. Before bed rest, women had lower blood pressures than men and were more prone to develop orthostatic hypotension (extremely low blood pressure when standing up). However, both men and women had similar elevations of heart rate with bed rest and similar norepinephrine responses to standing. These results were expected, and the goals of this study were to examine the physiological mechanisms that might explain why women have a higher incidence of orthostatic intolerance.
Before bed rest, women had smaller veins in the feet than did men. After bed rest, women increased the size of these veins while men decreased the size of these veins. This, along with measures of strain gauge plethysmography, indicate that women increased their venous compliance while men did not, which would make women more susceptible to fainting than men. There were no differences between men and women in the response to drugs that affect the size of veins, such as phenylephrine, isoproterenol, acetylcholine or nitroglycerin.
There were no arterial function differences between men and women, either before or during bed rest. However, this is the first study to show that upper and lower body arteries respond differently to bed rest. Ultrasound-measured arterial diameter showed decreases in the leg with bed rest, but not in the arm. Intima-medial thickness, the layer of the blood vessel wall that contains the smooth muscle which is responsible for controlling blood vessel constriction (and therefore blood pressure), decreased in the arteries of the leg, but not in the arm. Furthermore, responses to nitroglycerine, a measure of how well the smooth muscles in the artery walls are able to constrict or dilate, were reduced in the leg after bed rest, but not in the arm.
Results from systemic infusions of phenylephrine and sodium nitroprusside, two agents that cause the blood vessels to constrict and relax, respectfully, were used to assess differences in baroreflex function. Women controlled their blood pressure more by utilizing changes in heart rate via the parasympathetic branch of the autonomic nervous system, while men exhibited greater sympathetic nervous system balance. Overall, baroreflex function was diminished with bed rest, but women had more of an attenuated baroreflex response than men, which may explain their predisposition to orthostatic intolerance.
The results of this study highlighted the differences in autonomic and baroreflex control of blood pressure regulation between men and women. Furthermore, changes in arterial structure and function were no different between men and women, although women developed a larger venous compliance after bed rest, while men did not.