Pressure of Fluids

[plzNOCarribbean]

Ok, So I feel really stupid asking this question because I should know this but, I've always had a hard time grasping the concept of osmolarity, especially when everyone in here refers to it as either blood osmolarity, and some say blood osMOLALITY.

When they say blood osmolarity, do the mean the concentration of solute molecules inside the blood cell compared to the extracellular environment?? I understand that osmosis is the movement of water to dilute a more concentrated solution, and that this is important so cells don't lyse if they take up to much water but what is blood osmolarity???

and how does a fluid exert a pressure?? fluids exert pressure? basically, does the cell know when to allow water in, and when to not let water in based on the concentration of solutes in the cell and outside of the cell?? also, what would happen if say, a blood cell was taken from a 1% sodium solution and dumped into a 10% sodium solution?? would it shrivel or burst?? Thanks guys I know this sounds dumb but i need to get this down before the mcat i have been seeing it a lot.

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[fizzgig]

fluids exert pressure because they move to equalize solute concentration. books usually show this by a ushaped pipe with a semipermeable membrane at the bottom of the Ucurve. if you put salt on one side, water will tend to move to the salty side because it will want to equalize concentrations on both sides of that membrane. as it moves, though, the water height on the salty side of the membrane gets taller, and that water is exerting a different kind of pressure (hydrostatic, rho*g*h). at some point the extra weight of water on that side (wants to push water OUT of the salty side) will equal the osmotic pressure (wants to draw water INTO the salty side), and movement stops.

cells in animals have no cell wall so they really can't take a big difference in solute concentration, so we need about the same number of solute molecules outside as inside cells. put a cell in a supersalty solution, water will exit, cell shrivels. put a cell in pure water, it will burst.

blood has proteins in it that can't leave the vessels. as fluid goes down a capillary, the hydrostatic pressure from the arterial side is pushing fluid out of the capillaries. as fluid leaves and proteins can't, the blood gets more concentrated, and the osmotic pressure builds. when osmotic P equals the hydrostatic P of the blood, fluid stops flowing out. as blood continues down the capillary, it loses even more hydrostatic P, and so near the end of the cap (i think) the osmotic P actually pulls back some fluid into the vessel. the net fluid flow is still out, and that fluid goes to the lymphatic system.

hope i answered something in that pile...

 

[the humming sea]

As far as I know the MCAT never asks specific questions about Blood Osmolarity vs molality. I wouldn't be to worried about the distinction and would venture to guess that people meant to say molarity and mistyped...

You have it exactly right when you say that blood osmolarity is the concentration of the solutes inside a cell compared to the extracellular environment. The regular blood osmolarity is 300 mosm (or 300/1000 or a mole).

The typical cell is completely permeable to water and thus the water can move into and out of the cell freely to keep its osmolarity around 300 mosm.

In your hypothetical example the cell would quickly shrivel because the water would rush out of the cell to extracellular fluid where the osmolarity is much higher than inside the cell. In this case the cell osmolarity would change markedly too, because as the water rushes out the osmolarity increases until it is equivalent to the surrounding fluid.

 

[plzNOCarribbean]

Thanks guys that cleared things up a a lot. However, so this rushing of fluid into and out of the cell is because of the pressure?

In fizzgigs example, he mentioned that as water rushed to the more concentrated side on the Utube, the level of water would go up on that side. then, there would be a Hydrostatic (I think) pressure exerted against the membrane to prevent any more water from coming into the more concentrated side once it has adequately been diluted enough.Is this right? did that make sense

 

[fizzgig]

yep, i think you got what i was saying. the water starts level on both sides, then water goes to the salty side to even out the concentration difference, so the level goes up until the hydrostatic pressure of the extra water on the saltyside equals the osmotic pressure pulling water into the saltyside.

and yes the same osmotic P that pulls water from the pure to saltyside in the Utube is responsible for the rush of water in or out of the cell if you put it in a really salty or pure water solution. if you put it in pure water, for example, the inside of the cell is the saltyside of the Utube - water wants those concentrations equal, so it flows into the salty intracellular compartment. pressure inside the cell goes up as the membrane stretches, but in this case the membrane can't take that kind of pressure, and the cell lyses.

also i'm a she. not that it's relevant to chemistry but anyway