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This might be a silly question but i'm not not clear

I'm always told that blood pressure is homeostatic parameter and can't not be changed but isn't what important is the flow rate to the organs? why not maintain flow rate

Doesn't having enough blood flow to the organ already satisfy our bodies to function properly or do pressure play any role? what happens if blood flow is constant but blood pressure drop below normal

there's some case when one have blood lost , pressure drop and our bodies respond by constricting artery and that results in decreased cardiac output.Isn't that worst? decrease cardiac output just to maintain blood pressure

Forgive my little understanding out this I know my thought is kind of logically wrong but please help me understand. thank you

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A constant blood-flow would not be advantageous to organisms with a varying activity level. The blood supplies (mainly) oxygen to the tissues and the supply of course needs to match the requirements. Organisms rely on a number of mechanisms to ensure this match, one of these is to increase the blood-flow (heart-beat). But flow through a (blood) vessel relates directly to the pressure and inversely to the cross-sectional area of the vessel, so if the flow is increased, the organism must accept either a higher blood pressure or expand the blood-vessels. Of course, it is in reality a compromise of both mechanisms.

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  • $\begingroup$ Hi FrankS. Welcome to Biology. Your answer is good but if you could expand a bit on the activity part, it would be great. $\endgroup$
    – WYSIWYG
    Feb 5, 2019 at 19:45
  • $\begingroup$ What I mean by "activity" is primarily muscle work. If you exercise the muscles work harder, which requires more energy and oxygen. Also for homeothermic organisms, producing more heat to keep warm in a cold environment means higher metabolism and thus higher oxygen consumption. Constantly keeping the blood/oxygen supply at a high level is not an economical solution in a competitive nature where food is scarce. $\endgroup$
    – FrankS
    Feb 7, 2019 at 5:25
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I'm always told that blood pressure is homeostatic parameter and can't not be changed

Homeostatic doesn't mean it is constant. It means if it goes high or low it tries to revert back to equilibrium.If you check your blood pressure it doesn't show 120/80. What you call normal is the statistical average of all.

Other questions I do not know the answer.

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By regulating the pressure the body is regulating the flow rate. Pressure is the driving force for flow and as shown in this equation below (called the Hagen–Poiseuille equation) pressure drop and flow rate are related to each other for laminar flow in a cylindrical tube. So its impossible in your scenario to keep the flow rate constant but reduce pressure drop for the same size blood vessel.

{\displaystyle \Delta P={\frac {8\mu LQ}{\pi R^{4}}}}

You are right that flow rate is the key parameter because it's important to deliver adequate oxygen to all the tissues in the body or they'll die.

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  • $\begingroup$ But if your pressure drop.flow rate drop yes but doesn’t further constrictinf the vessel with drop the flow rate even more as the flow is restricted because of more resstance? Why do our body constrict the blood vessel when pressure drop just to increase pressure but reduce flow rate? $\endgroup$
    – DarkMaMon
    Feb 7, 2019 at 16:59

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