Glomerular Filtration Rate

[sunshine02]

Why does constriction of the efferent arterial lead to an increase in glomerular pressure, followed first by an increase and then by a decrease in glomerular filtration rate?

I thought the glomerular pressure would increase, which would cause the filtration rate to increase?

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[Volvulus10]

I believe it's more of a reflex drop in GFR (someone correct me if I'm wrong). Vasoconstriction of the efferent arteriole via angiotensin II occurs when there is a decrease in perfusion to the kidney. The macula densa of the DCT senses low Na+ and the JG smooth muscle cells sense a low perfusion pressure leading to activation of the renin-angiotensin-aldosterone system. In the setting of normal physiology (no hemorrhage, no heart failure), the increase in GFR (+ decrease RPF = elevated FF) will eventually lead to an elevation of Na+ reaching the DCT (the opposite of earlier). Adenosine (normally a vasodilator) causes vasoconstriction of the AFFERENT arteriole to decrease GFR/RPF/FF.

Hope that helps.

 

[Lunasly]

I'm also having trouble with this question. I didn't really understand Volvulus' explanation.

 

[IslandStyle808]

You have to think about the problem more systematically. When the efferent arteriole is constricted, this will initially cause an increase filtration through the glomerulus (due to higher pressure). This will initially increase the filtrate going through the nephrons. However, the nephrons will function under the impression that blood pressure in systematic circulation had increased. The nephrons will adjust accordingly (less ADH, less aldosterone, etc.) and this will cause the blood pressure in systematic blood pressure to go down because their is less water being reabsorbed into blood. Thus there will be less blood coming from the afferent arteriole and this leads to the eventual decrease in blood filtration through the glomerulus.

Another adjustment that can be done, as volvulus has mentioned, is to get a vasoconstrictor such as adenosine (not sure of mech of action on this one) to constrict the afferent arteriole. This will also decrease the amount of blood going through the afferent arteriole.

 

[Lunasly]

Ah, ok. I initially thought that the question was asking for an immediate response. Essentially, the body responds to reduce GFR (and thus maintain homeostasis) after it has been increased via constriction of the efferent arteriole.

Thank you!

 

[sunshine02]

You have to think about the problem more systematically. When the efferent arteriole is constricted, this will initially cause an increase filtration through the glomerulus (due to higher pressure). This will initially increase the filtrate going through the nephrons. However, the nephrons will function under the impression that blood pressure in systematic circulation had increased. The nephrons will adjust accordingly (less ADH, less aldosterone, etc.) and this will cause the blood pressure in systematic blood pressure to go down because their is less water being reabsorbed into blood. Thus there will be less blood coming from the afferent arteriole and this leads to the eventual decrease in blood filtration through the glomerulus.

Another adjustment that can be done, as volvulus has mentioned, is to get a vasoconstrictor such as adenosine (not sure of mech of action on this one) to constrict the afferent arteriole. This will also decrease the amount of blood going through the afferent arteriole.

Thanks! That makes a lot of sense.

Could you also think of it as pressure difference? If there is higher pressure in the glomerulus then blood is less likely to flow there since it always goes from high-->low pressure?

 

[IslandStyle808]

Thanks! That makes a lot of sense.

Could you also think of it as pressure difference? If there is higher pressure in the glomerulus then blood is less likely to flow there since it always goes from high-->low pressure?

Yes, you could think of it like that. My suggestion is to read up on hydrostatic (osmotic pressure) and oncotic pressure, this will tell you how glomerulus filtration works more or less (same way it works for blood to tissue). The hydrostatic pressure is usually greater in blood and this is what causes a flow of fluids into the glomerulus. As blood flows by, it starts to increase in oncotic pressure (due to the proteins in the blood acting as solutes). This is what causes fluids to be sucked back in from the glomerulus. If the hydrostatic flow of fluids exceeds the oncotic flow of fluids then you have a "net filtration of fluids" into the glomerulus and into the nephrons.