I'm reading the Kaplan Physiology book and it states weightlessness decreases sympathetic thus decrease heart rate. Does this make sense to anyone? Thanks for your help.
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Step I Weightlessness and Baroreceptors
Started by Kluver_Bucy
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Kluver_Bucy said:
Yes, that makes sense to me. When you stand up, you have to increase sympathetic tone to keep blood going to your brain... people who are weightless are "lying down" all the time, as they do not experience the force of gravity.
Also, would you please consider using one Q&A thread? You have no less than 10 on the front page here. We could call it Kluver's Kwestions or something.
Wrigleyville said:
I second that suggestion - I don't mind the questions but I just dont like seeing the front page of the message board plastered with them.
ICC only a week away - you ready to start back up, Wrigs?
-Bobby Bee
bobbybee22 said:
I leave for my cruise tomorrow... 4 days of caribbean sunshine. I should be tanned, rested and ready when I get back.
Wrigleyville said:
What's with the cruises. Dr. Jam is also taking a cruise with LB...
Kluver_Bucy said:
here's my take on it...
As you stand (under normal gravity)--> blood pools into the venous capacitance vessels. It's the contraction of the gastrocneimus and other leg muscles that aid in increasing the return of blood to the heart so that you don't develop orthostatic hypotension.
the venous system is a huge pool of blood, that serves as a reservoir under normal conditions for use during exercise, in hypovolemic shock, freaking out on a date, etc.
Without the effect of gravity, the blood in those vessels is part of the active circulation. Effectively, the circulating volume is now increased because of this "extra blood". You have now shifted you're venous return curve up. So here's what eventually happens with regards to the SNS system
1)With more blood circulating--> increased distension of the atria--> increased release of ANF--> oppose actions of RAS (AT-II mediated effects) and Increase GFR.
2) Increased volume--> increased stretch on the baroreceptors (AO Arch (Vagus is afferent) and Carotid sinus (glosspharyngeal is afferent); AO responds to increased pressure only). --> increased firing back to medulla--> decreased SNS activity and increased PSNS activity--> decreased HR (no direct effect at vessels from PSNS).
3) Increased perfusion of the kidneys so decreased production of renin by the JGA cells and thereby reduced AT-II production. Since we want to ensure our GFR at all costs, the increasing ANF mentioned earlier serves to dilate the afferent and constrict the efferent to maintain the appropriate GFR.
hope this helps and is understandable! G'luck
Ucb
"3 days till sheer chaos"
