Blood Pressure / Volume & GFR Regulation

[MDJerome]

Okay, so I understand that constricting Blood Vessels (ie, Arterioles) results in an increase in Blood Pressure. And I know Blood Pressure can also be increased by increasing Blood Volume (which essentially is just increasing Cardiac Output (Stroke Volume x BPM).

So if everything above was true, then why does this happen for the Kidney:

To main a steady Glomerular Filtration Rate - the arteriole will constrict when Blood Pressure is high and if it's low, the arteriole will dilate.

I thought constriction Raises Blood Pressure? Seriously, what the heckkkkk.

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[Pisiform]

I have;nt gotten to GFR yet but so far, relating to exercise & bp. this is all I got:


Blood Pressure increases when arteries, veins, arterioles, venules constrict. When they constrict, they increase Peripheral Resistance. So, blood pressure Increase.

BUT,

When we exercise, blood pressure Increase even though arterioles dilate (less Peripheral Resistance). This is because Blood Pressure is related to Cardiac Output and Peripheral Resistance both. Here, Cardiac Output (which is product of stroke volume and heart rate) increase far more than Peripheral Resistance could decrease. So, blood pressure increase.

Guys, if I am wrong, please correct me .

and as far as Principal of Continuity is concerned, the smaller the vessel the greater the flow/velocity.

This is also true for Capillaries but in this case, we take into account the entire cross-section of capillaries and not a single capillary. That's why, speed in capillary is slow.

(Don't think that Bernoullis Equation works here, only Principal of continuity)

 

[ilovemcat]

Okay, so I understand that constricting Blood Vessels (ie, Arterioles) results in an increase in Blood Pressure. And I know Blood Pressure can also be increased by increasing Blood Volume (which essentially is just increasing Cardiac Output (Stroke Volume x BPM).

So if everything above was true, then why does this happen for the Kidney:

To main a steady Glomerular Filtration Rate - the arteriole will constrict when Blood Pressure is high and if it's low, the arteriole will dilate.

I thought constriction Raises Blood Pressure? Seriously, what the heckkkkk.

The GFR rate wants to stay constant. If there's an increase in arteriole pressure, this means that Blood flowing through the Glomerulus will be extremely fast. As a result, a lot of Blood Plasma will get filtered (more than the average 20% of Blood Plasma passing through the Glomerulus). What it wants to do is slow things down and it does this by constricting the afferent arterioles directly (via Macula Densa cells). In doing so, it decreases the amount of Plasma that gets filtered (by decreasing Blood Flow) and therefore helps maintain a constant Glomerular Filtration Rate. This is one way of regulating Blood Pressure in the Kidney.

Throughout the Body, Blood Vessels can Constrict or Dilate to produce the opposite effect (of GFR). Constricting of Blood Vessels increases Blood Pressure. Dilating Blood Vessels decreases Blood pressure.

You can think of this interms of Ohm's equation: dP = Q x R which means Change in Pressure equals Blood Volume times Peripheral Resistance (or constriction of BVs). Assuming Blood Volume remains constant throughout the body, increasing one variable of the equation (Resistance/Constriction) increases the opposite side of the equation proportionally (Pressure).

 

[coyfish]

Okay, so I understand that constricting Blood Vessels (ie, Arterioles) results in an increase in Blood Pressure. And I know Blood Pressure can also be increased by increasing Blood Volume (which essentially is just increasing Cardiac Output (Stroke Volume x BPM).

So if everything above was true, then why does this happen for the Kidney:

To main a steady Glomerular Filtration Rate - the arteriole will constrict when Blood Pressure is high and if it's low, the arteriole will dilate.

I thought constriction Raises Blood Pressure? Seriously, what the heckkkkk.

The kidney constricts the arterioles to slow the filtration rate in cases of high velocity.

 

[Charles_Carmichael]

Please use the search function next time. This type of question relating blood flow to pressure changes and GFR has been answered multiple times before. Here is a repost of one of my previous answers to this questions:

The thing about vasoconstriction is that it has different effects on pressure upstream and downstream of region of the vessel that's constricted. Upstream of the vasoconstriction, there will be an increase in pressure. In the example of vasoconstriction of the efferent arterioles of the kidney, an increase in upstream pressure at the glomerulus results in increased GFR.

Downstream of the vasoconstriction, there will be a decrease in pressure. Think of the constriction as an increase in friction which dissipates energy (pressure) as the blood flows through the region. In the case of the afferent arterioles of the kidney, after vasoconstriction, there'll be a decrease in pressure at the glomerulus which results in a decrease in GFR.

Similarly, dilation of the afferent arteriole reduces the "friction" along the vessel. So, there's less energy (pressure) dissipated along it's length. This would increase pressure at the glomerulus. That's why vasodilation of the afferent arteriole increases GFR.

It's best not to think of this question in terms of math or equations but, rather, conceptually think through it using some basic physics principles. Hope this helps.