Question about osmotic pressure

[thechairman]

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ok, we know the formula is:

π = iMRT

where, M is the molarity of the solution. now, is M the concentration of the solute, or the solvent? Say you had a two containers seperated by a wateronly permeable membrane. One container had salt, the other one is distilled. Now, would the salty solution be the one with the higher osmotic pressure, or the other way around?

In this question:

Which of the following conditions will favor the
movement of water, ions, and glucose from the intercellular
space of the proximal tubule into the peritubular
capillaries?
A. High capillary hydrostatic and low capillary
osmotic pressures
B. High intercellular hydrostatic and low capillary
osmotic pressures
C. Low capillary hydrostatic and high capillary
osmotic pressures
D. Low intercellular hydrostatic and high intercellular
osmotic pressures

The answer is C. So that means, the capillaries have higher concentration of water, which will induce solute to flow from tubules into capillaries. So the hydrostatic pressure takes care of the water flow, while the osmotic pressure gradient takes care of the solute flow, correct?

 

[changarang]

this is a really good question. when they say "high capillary osmotic pressure," i interpret that to mean there's a lot of pressure for water to move into the capillaries. so i would say that the salty solution would have the high osmotic pressure.

you phrased the question in terms of solute flow, but for this problem, i think it's less about high vs low solute concentration and more about the net effect of hydrostatic vs osmotic pressure. because technically, even if you had a salty cell and expected water to flow into the cell, if the cell's hydrostatic pressure were higher than its osmotic pressure, then there would be a net flow of fluid (with the small solutes, not just water) out of the cell. or if the cell's hydrostatic pressure (wanting to push fluid out) equaled its osmotic pressure (wanting to push water in), then there wouldn't be a net flow of fluid. the major reason i think of it this way is that osmotic pressure isn't solute-specific - in other words, it's determined by the difference in the total concentration of all dissolved solutes between two regions (like inside and outside a cell). so, just because there's a lot of, let's say, hydrogen ions inside the cell doesn't necessarily mean that there will be a net flow of hydrogen ions out of the cell. i know...in chem, you would expect the ions to flow from high to low concentration (and normally they would), but in a cell, there's the added complication of cell membranes, which are nonpolar in the middle. so polar or charged things like ions have a tough time passing through. so it ends up that for a lot of things to flow into or out of a cell, they need to piggyback on the direction of net fluid flow.

so for this question, if you want stuff to flow into the capillaries, you want the capillary hydrostatic pressure to be low (weak push of fluid out of the capillaries) and the capillary osmotic pressure to be high (strong push of fluid into the capillaries), so that the net flow of fluid will be into the capillaries. if you wanted stuff to flow out of the capillaries, you want the reverse conditions: high capillary hydrostatic pressure (strong push out), low capillary osmotic pressure (weak push in), or, in terms of intercellular conditions, low intercellular hydrostatic pressure (weak push out of intercellular) and high intercellular osmotic pressure (strong push into intercellular).

the interaction between hydrostatic and osmotic pressure is the big reason behind the way materials flow into and out of capillaries. generally, the osmotic pressure is relatively constant, but the hydrostatic pressure (i.e., the blood pressure) decreases as you go from the the artery end to the venous end of the capillaries. since the blood pressure is higher than the osmotic pressure at the artery end, there is a net flow of fluid out, delivering the nutrients needed by the tissues. but on the venous end, the blood pressure is lower than the osmotic pressure, which is why there is a net flow of fluid into the capillaries, bringing with it the metabolic wastes.

kinda cool how it all works out, huh? 🙂

 

[thechairman]

so for this question, if you want stuff to flow into the capillaries, you want the capillary hydrostatic pressure to be low (weak push of fluid out of the capillaries) and the capillary osmotic pressure to be high (strong push of fluid into the capillaries), so that the net flow of fluid will be into the capillaries. if you wanted stuff to flow out of the capillaries, you want the reverse conditions: high capillary hydrostatic pressure (strong push out), low capillary osmotic pressure (weak push in), or, in terms of intercellular conditions, low intercellular hydrostatic pressure (weak push out of intercellular) and high intercellular osmotic pressure (strong push into intercellular).

the interaction between hydrostatic and osmotic pressure is the big reason behind the way materials flow into and out of capillaries. generally, the osmotic pressure is relatively constant, but the hydrostatic pressure (i.e., the blood pressure) decreases as you go from the the artery end to the venous end of the capillaries. since the blood pressure is higher than the osmotic pressure at the artery end, there is a net flow of fluid out, delivering the nutrients needed by the tissues. but on the venous end, the blood pressure is lower than the osmotic pressure, which is why there is a net flow of fluid into the capillaries, bringing with it the metabolic wastes.

kinda cool how it all works out, huh? 🙂

thanks for you reply, but if high osmotic pressure means that the solute concentration is higher on the capillary side, then wouldn't that make solutes flow out of the capillary, instead of back in, like the question asks?

 

[changarang]

normally, you would expect solutes to flow from high to low solute concentration. but in cells, that's not always the case because of the membrane. in particular, charged and polar compounds have trouble crossing the nonpolar section of the phospholipid bilayer. as a result, the movement of solutes is dictated more by hydrostatic vs osmotic pressure instead of high vs low solute concentration. as you said, if the capillaries have a high osmotic pressure, meaning there is a higher solute concentration inside the capillaries, you would expect the solutes to flow out of the capillaries. except that they don't really because they have trouble crossing the membrane. so instead of solutes flowing out of the capillaries, the high osmotic pressure tends to push fluid into the capillaries, bringing more dissolved solutes with it, as long as the capillaries also have a low hydrostatic pressure. if the hydrostatic pressure were too high, then it would work against the osmotic pressure, and there wouldn't be a net flow of fluid into the capillaries anymore.

 

[SBK]

Water transport is composed of 2 passive methods : hydropstatic + osmotic

Hydrostatic pressure is the pressure that arises from gravity; since membranes are very flexible; this pressure is extremely negible.

Osmotic pressure therefore is what drives nearly wall water transport


Also notice that the answer key did not give you a High and High option ; which would theoretically be even more correct.