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>Are you sure about that? I have always read that O2 is a vasoconstrictor >- not an inhibitor of vasodialation. People don't lose arms & legs in > >Again: Yes? No? Maybe? I'm not trying to portray myself as a medical >expert here - I am most certainly NOT such an expert. I just want to make >sure I understand.... There is a poorly understood reflex (in that the exact driving mechanism and chemical mediators are not understood) that allows at least some precapillary sphincters to control blood flow to the downstream tissue based on either the O2 pressure or O2 content in that tissue. In muscle, for instance, it appears that there are some vessels more effected than others (thus guaranteeing a basal blood flow but shuuting down some cappilaries when blood flow is apparently not needed. There are a couple of ways of looking at this. As tissue oxygen drops, the sphincters dilate to increase blood flow. As oxygenation becomes adequate or above adequate, blood flow decreases but does not cease (exactly as you state). The reflex indeed and quite prominently produces vasodilation at times of increasing oxygen demand. The effect in the hyperoxic direction seems less pronounced. I can only find studies (in a fairly quick and dirty search) of skeletal muscle, coronary artery, umbilical artery and retinal arteriolar reflexes so I don't know that it is better studied than that. Rat suggests that there is some data on resultant localized hypoxia from the hyperoxic control end, I need to examine this since nothing I have suggests that it occurs. There is also a large body of literature including a number of papers at the upcoming American Thoracic Society meeting in Boston which I will be attending on this reflex in pulmonary vessels. In the pulmonary circulation, everything works almost the opposite of the systemic side with hypoxia causing quite marked vasoconstriction (to the point of right heart failure in some of my patients). It is an interesting area that I need to continue to pursue and maybe understand. I am so far unaware of any situation where there is sufficient reduction in blood flow due to hyperoxia to result in impaired CO2 removal or the like. Maybe it does occur.?.? Tom ========================================================================== Tom Stibolt, M.D. <stibolt@oh*.ed*> Portland, OR, USA I'm not even sure if the opinions expressed are mine!
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