View Full Version : Circulatory regulatory subsystem based on body orientation?
04-25-2008, 06:59 AM
I was watching a show that partly documented some of the problems with extended periods of weightlessness. They mentioned that there was a regulatory system that changed the bloodflow based on whether the body was in a horizontal/supine position or standing. I don't have more context than that.
Is that true? If so, what is the mechanism responsible for this called and how does it work?
04-25-2008, 12:41 PM
I have only a partial answer for you. If that system does not work well, as mine does not, the malfunction is called orthostatic hypotension. The blood pressure should increase as you stand up. Mine doesn't, so I get light-headed upon standing suddenly. If I'm not careful, I can pass out. It's a bloody nuisance.
04-25-2008, 04:03 PM
There's no cool name for the mechanism, it's just a complex set of interactions throughout the cardiovascular system. In essence, when you go from a supine to a standing position there is a decrease in blood pressure in the vessels above the diaphragm as all the blood pools into the vessels in your legs. This cause a decrease in your central blood volume (the blood in your heart and great vessels) which decreases ventricular filling and ultimately decreases cardiac output and blood pressure. There's also a decrease in atrial stretch receptor firing, and a decrease in baroreceptor firing in the carotid arteries. This causes there to be an increase in sympathetic tone throughout the body resulting in increased heart rate and force of contraction, an increase in peripheral vascular resistance and muscle tone which helps force the blood pooling in your legs back into the central blood pool.
With that said, the problem with astronauts isn't a loss of this regulatory mechanism as much as it's a decrease in blood volume. Similar things happen to patients on prolonged bed rest. The reflexes work fine, they just have an upper limit in terms of how much they can compensate and that, as AskNott said, is called Orthostatic Hypotension.
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