Filtration fraction and oncotic pressure

[MudPhud20XX]

So Kaplan physio explains that filtration fraction (GFR/RPF) directly impacts the oncotic pressure in the peritubular capillaries. And it also says increased filtration fraction will lead to increased oncotic pressure in the peritubular capillaries. I thought I got this first, but now, I can't seem to get the reason why.

Shouldn't the concentration/oncotic pressure of the fluids in all three compartments: Bowman space, glomerulus capillaries, and peritubular capillaries still be the same?

Since the fluid entering Bowman's space is an ultrafiltrate of plasma which means that the filtrate has the same concentration/osmolarity/oncotic pressure of dissolved substances as plasma right? So why the heck does the change of FF affect the concentration/oncotic pressure of the peritubular capillaries?

I would really appreciate if anyone can explain why increased FF leads to increased oncotic pressure in the peritubular capillaries?

---

Members don't see this ad.

 

[Bancrofti]

Proteins are what govern the oncotic pressure. Under normal circumstances, proteins are not filtered through the glomerular capillaries. Therefore, when you have an increase in filtration fraction, you have more water/ions/whatever normally gets filtered leaving the glomerular capillaries, while protein stays behind to move along to the peritubular capillaries as it normally does.

So the more water that goes out during filtration, the greater the effective protein (oncotic) pressure is after that. The greater the oncotic pressure, the greater reabsorption is favored.

 

[MudPhud20XX]

Proteins are what govern the oncotic pressure. Under normal circumstances, proteins are not filtered through the glomerular capillaries. Therefore, when you have an increase in filtration fraction, you have more water/ions/whatever normally gets filtered leaving the glomerular capillaries, while protein stays behind to move along to the peritubular capillaries as it normally does.

So the more water that goes out during filtration, the greater the effective protein (oncotic) pressure is after that. The greater the oncotic pressure, the greater reabsorption is favored.

Thanks for the explanation. That's exactly what I thought, but then I was thrown off when I read this from Kaplan:

- The fluid entering Bowman's space is an ultrafiltrate of plasma; that is the filtrate has the same concentration of dissolved substances as plasma, except proteins.
- The osmolarity of the filtrate is 300 mOsm/L (which is basically the same as the plasma osmolarity).

If proteins govern the oncotic pressure then the osmolarity of the filtrate and the plasma should NOT be the same, right?

I've already asked this question before here and my apology for the redundancy, but I still don't seem to get it. Many thanks in advance.

 

[Bancrofti]

Proteins (albumin) are large, but don't really affect the overall osmolarity of the plasma. Remember that osmolarity is a function of the # of particles in solution, not the size. Those things that are able to diffuse through the membrane are the things that contribute the most to osmolarity (Na+, Cl-, glucose, amino acids, etc.) because there are a ton of those things in solution. Proteins are relatively scant compared to the number of all those things that are diffusing across the glomerular capillary.

 

[sazerac]

Sounds like the OP is trying to equate oncotic pressure and osmotic pressure, and that is the source of the confusion.