Quick question about blood pressure increase and decrease

[gimmedat]

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I'm currently reviewing the excretory system.

The book says: "The macula densa also causes a direct dilation of the afferent arteriole, increasing blood flow to (and thus blood pressure and filtration rate in) the glomerulus."

This seems counterintuitive to me. Wouldn't dilation decrease blood pressure, thus decrease the rate of filtration into the renal tubule?

 

[WSHRocks]

I'm currently reviewing the excretory system.

The book says: "The macula densa also causes a direct dilation of the afferent arteriole, increasing blood flow to (and thus blood pressure and filtration rate in) the glomerulus."

This seems counterintuitive to me. Wouldn't dilation decrease blood pressure, thus decrease the rate of filtration into the renal tubule?

It is counterintuitive. If the flow rate remains constant, an increase in cross-sectional A (via increase in radius) decreases velocity (Q=Av). A decrease in velocity results in an increase in pressure (P+pgh+1/2pv^2=k).

 

[gimmedat]

then why does vasoconstriction cause blood pressure to increase?

 

[WSHRocks]

then why does vasoconstriction cause blood pressure to increase?

When only a small cross-section of the vessels is dilated (directly, as stated), the flow rate can be considered constant. If the amount of blood in a given cross-section at a given time before the dilation is the same as in another cross-section after the dilation, the area increases while the amount (per cross-section) stays constant, so velocity must decrease.

There's also an equation that relations pressure to flow rate and resistance (P=Q*R). Since vasoconstriction causes increased resistance in the artery, when the cardiac output is pumped from the heart, pressure must increase to maintain constant flow. Here, blood pressure is not a comparison of the same tube at different points but rather a different system ("more constricted than normal") - a global effect.

 

[image187]

When only a small cross-section of the vessels is dilated (directly, as stated), the flow rate can be considered constant. If the amount of blood in a given cross-section at a given time before the dilation is the same as in another cross-section after the dilation, the area increases while the amount (per cross-section) stays constant, so velocity must decrease.

There's also an equation that relations pressure to flow rate and resistance (P=Q*R). Since vasoconstriction causes increased resistance in the artery, when the cardiac output is pumped from the heart, pressure must increase to maintain constant flow. Here, blood pressure is not a comparison of the same tube at different points but rather a different system ("more constricted than normal") - a global effect.

This bernoullis principle business holds true where flow is constant. In this case, the flow changes the second the afferent arteriolar dilates (flow increases as blood is drawn from the system). This results in higher pressure through the glomerulus.

"Vasoconstriction causes increased blood pressure" is true, but not locally where the vasoconstriction is taking place. An increase of peripheral vascular resistance causes increased pressure to occurs upstream (larger arteries of arterial circulation) due to shunting of blood.

 

[circulus vitios]

then why does vasoconstriction cause blood pressure to increase?

Blood pressure before the site of constriction increases because it's like blowing through a straw, then pinching the straw in the middle. For a constant flow rate, the flow velocity increases after that point of constriction. Pressure drops after that point of constriction. Pressure increases before that point of constriction because you have to blow harder to maintain the flow rate.

edit: At least I think this is right?

 

[gwjib04]

This discussion reminds me of a tough question in the BR Kidney section:

What happens if you constrict the efferent glomerular arteriole?

First, this increases glomerular pressure because of the constriction so fluid tends to flow out of the glomerulus due to hydrostatic pressure. As a result of plasma leaving the glomerulus for the tubule, the concentration of solutes remaining in the glomerulus will increase to the point that it will be unfavorable for any more fluid to leave and the glomerular filtration rate will then decrease.

Actually these 2 happen at the same time to regulate GFR; afferent dilates, efferent constricts. So a bottleneck of fluid occurs in the glomerulus

Originally posted by image187
"Vasoconstriction causes increased blood pressure" is true, but not locally where the vasoconstriction is taking place. An increase of peripheral vascular resistance causes increased pressure to occurs upstream (larger arteries of arterial circulation) due to shunting of blood.

Very helpful thanks

 

[pacer]

Dilation of the afferent arterioles brings more blood into the glomerulus which in turn increases the rate of filtration because more materials can not pass through into the Bowman's capsule.

Since dilation causes a rush of blood flow into the area, the increased flow causes an increase in pressure in that given area based on Q = P/R