Fluid flow physics in a capillary/ Osmotic Pressure...

[MCAT guy]

This is part physics part bio...

So there are capillaries and I have this equation

delta P = [P(c) + PI(i)] - [P(i) + PI(c)]

Where the change in pressure is equal to the hydrostatic pressure minus the osmotic pressure, P(c) for capillaries, Pi(i) osmotic pressure for the interstitial space.

So the pressure to move OUT of the capillaries is dependent on the first term:
[P(c) + PI(i)]

The hydrostatic pressure (capillaries) + the osmotic pressure (interstitial).

The problem had the first part (pressure move out of capillaries) equal to 26 and the second part (pressure to move in) equal to 17. Giving a difference of 9.

Anyway, this first term confuses me, the hydrostatic pressure in the capillaries obviously wants to go out of the capillaries, but the osmotic pressure of the interstitial space also leads to water wanting to exit the capillaries? As far as I understood, osmotic pressure is the resistance to the flow of water, so Pi(i) should resist flow of water into the interstitial place, right?

wikipedia:

Osmotic pressure is the pressure applied by a solution to prevent the inward flow of water across a semipermeable membrane.[1]

hyperphysics online I believe takes a slightly different approach in defining osmotic pressure. I can't seem to reconcile the equation above (osmotic pressure of the interstitial causing the need to flow out of the capillaries)

Note that in the calculation at left, the osmotic pressure is attributed to the solution, whereas the definition of osmotic pressure that has been used here treats positive osmotic pressure as the relative energy density of the pure solvent in relation to the solution. Since most current biology and chemistry texts attribute high osmotic pressure to the solution rather than the pure solvent in relation to the solution, I may have to give in and redefine osmotic pressure to be consistent with those common uses. It does seem backward, physically.

I've heard some say that highly concentrate solutions have high osmotic pressure, but then I've also seen physicist define osmotic pressure differently, as posted above...

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[MCAT guy]

So I found a few pics online

So I'm trying to figure out, what direction is the force of the osmotic pressure pointed INSIDE the capillaries?

I understand the idea of net osmotic pressure, but I'm just not understanding which direction each osmotic pressure points for the capillary osmotic pressure and the interstitial osmotic pressure...

 

[Phantastic]

For the MCAT, just know that in the beginning of the capillaries, the osmotic pressure that would force water into the capillaries is overcome by the hydrostatic pressure of water wanting to leave the capillaries. Once enough fluid has left the capillaries (i.e. near the end of the capillary bed), the osmotic pressure is now higher than the hydrostatic pressure, and water will enter the capillary. There is a net loss of fluid which is collected by the lymphatic system.

 

[MCAT guy]

For the MCAT, just know that in the beginning of the capillaries, the osmotic pressure that would force water into the capillaries is overcome by the hydrostatic pressure of water wanting to leave the capillaries. Once enough fluid has left the capillaries (i.e. near the end of the capillary bed), the osmotic pressure is now higher than the hydrostatic pressure, and water will enter the capillary. There is a net loss of fluid which is collected by the lymphatic system.

Thanks bro!

 

[EZR]

I had the same question as the OP. I don't think the question was quite answered.

The question makes a distinction between Interstitial oncotic pressure [PI(i)] and Plasma oncotic pressure [PI(p)].

Interstitial Oncotic Pressure -- has a force pushing against the capillary
Plasma Oncotic Pressure -- has a force pushing into the interstitium

So then how can the Interstitial Oncotic Pressure favor movement of fluid OUT of the vessel?

[Shouldn't it be the opposite - the Plasma Oncotic Pressure encourages fluid out of the vessel instead?]

Thanks in advanced.

 

[Ssina]

Osmotic pressure pulls vs. hydrostatic pressure that pushes
so IOC will pull fluid out of blood vessel into the interstitial
and POP will pull fluid into the blood vessel out of the interstitial

so at the beginning of capillary we have hydrostatic PUSHing out (due to the blood pressure) and Interstitial Onc Press PULLing into the tissue
and at the end we have Plasma Onc Press PULLing into the vessel with the net loss of liquid

 

[EZR]

Gotcha! I had the push/pull twisted.
Thanks, you're wonderful :]