TBR Bio Ch2 Passage XI / Q69

[EZR]

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So the questions states:

A reduction in the diameter of a PRECAPILLARY vessel results in...

A reduction of hydrostatic pressure. That makes complete sense!

What I don't understand is why this wouldn't also imply an increase in resistance.
The answer explains that there is actually a decrease in resistance --

How can this be?

 

[Dave12314]

I actually thought that a reduction in the diameter of a precapillary vessel resulted in greater hydrostatic pressure?

And that being the case by Q = P/R.... an increase in pressure would result in an increase in resistance.

 

[MedPR]

I actually thought that a reduction in the diameter of a precapillary vessel resulted in greater hydrostatic pressure?

And that being the case by Q = P/R.... an increase in pressure would result in an increase in resistance.

That's what I thought too.

 

[Ssina]

wait so I thought when you vasoconstrict (reduction in precap diameter) essentially the pressure of the blood that enters the capillaries are reduced, so the hydrostatic pressure is reduced... basically less pressure to force stuff out. I think it is because by vasoconstriction, the flow rate into the capillary is reduced vs. than the case we assume flow rate is constant so the pressure has to increase.

and resistance increases since you reduced the diameter (vasoconstriction) and can even use the resistance formula from physics R=Ru (L/A)

 

[MT Headed]

The original post is ambiguous. The pressure is reduced where? The resistance is reduced where?

If we are talking about at the capillary, the presure would go down, and the (turbulent) resistance would go down because the velocity would be lower.

If you want to talk about conditions at the constriction or above the constriction, the answers would be different.

 

[ljc]

Reducing diameter at point A will reduce hydrostatic pressure at point B, assuming B follows A. (A=precapillary, B=capillary). At point A, though, both pressure and resistance will be increased. As MT Headed said, the full question would be useful here as I'm sure the book isn't as ambiguous as the original post - the location is very important.

 

[EZR]

You're right, I was totally ambiguous. My paraphrasing failed me.
For future ref, am I allowed to quote directly from the book? I didn't want TBR lawyers comin after my ass.

So now I think I've got it. But let's just clarify everything that was mentioned above:

PRECAPILLARY --> CAPILLARY --> “VENOUS CIRCULATION”


1. Reducing diameter of PRECAPILLARY vessel
2. Increases resistance of PRECAPILLARY vessel
3. Reduces hydrostat pressure of CAPILLARY
4. Reduces "turbulent" resistance of CAPILLARY
5. Reduces hydrostat pressure in VENOUS circulation
6. Reduces “turbulent” resistance in VENOUS circulation

Is all of this right?

 

[MT Headed]

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