Filtration

[TheRealAngeleno]

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So I'm having trouble visualizing why constriction of the afferent arteriole would cause a decrease the hydrostatic pressure but then I thought about the Bernoulli equation and so constriction of the afferent arteriole would cause an increase the blood flow velocity and thus a decrease in pressure. Is my reasoning on this matter correct? Thanks guys.

 

[Thirteeen]

The hydrostatic pressure in the glomerulus is determined by the volume of blood in it. If the afferent arteriole is constricted, the blood flow will be reduced, hence the blood volume.1 Therefore, the hydrostatic pressure (pressure pushing towards the walls of the capillary [controls filtration]) will decrease. A lower pressure means decreased filtration and higher pressure means increased filtration. As a side note, the lower hydrostatic pressure in the peritubular capillaries permits rapid fluid reabsorption.

Hope this helps. 🙂

1. According to Poiseulle's principle, Flow rate (Q) is directly proportional to radius raised to the power 4 (R^4) . So, if you reduce the radius, the flow decreases as well.

 

[Charles_Carmichael]

Here's an old post of mine regarding pressure changes with respect to vasoconstriction/dilation:

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.

Hope this helps.

It's a good idea, IMO, to think of these sort of questions in a very simple conceptual way. For pressure changes with constriction, I always think of water flowing through a garden hose and what happens if you squeeze a section of the hose (ie. constrict a section): the pressure of the water flowing out decreases because energy (pressure) is being dissipated in the constricted part while upstream of the constriction, there's an increase in pressure because there's a back-up of water (since the constriction creates a bottleneck).

Hope this helps.