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osmotic pressure/oncotic pressure question

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by GingerSue GingerSue (Member)

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now this should be simple to understand,

and I thought that I did,

but here's a question in the manual:

The drawing force for water excreted [I assume that's a typo] by

solute particles is ______ pressure.

The explanation in my textbook for osmotic pressure is

"the amount of pressure required to stop the osmotic flow of water"

"water will move to the more concentrated side of the vessel until the pressure generated by the height of the higher column of water will oppose further movement"

"osmotic pressure is determined by the concentration of solutes in solution

"Oncotic pressure is colloidal osmotic pressure and it is osmotic pressure exerted by colloids in solution... protein molecules attract water, pulling fluid from the tissue space to the vascular space"

would the correct answer by osmotic or oncotic?

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250 Posts; 3,803 Profile Views

now this should be simple to understand,

and I thought that I did,

but here's a question in the manual:

The drawing force for water excreted [I assume that's a typo] by

solute particles is ______ pressure.

The explanation in my textbook for osmotic pressure is

"the amount of pressure required to stop the osmotic flow of water"

"water will move to the more concentrated side of the vessel until the pressure generated by the height of the higher column of water will oppose further movement"

"osmotic pressure is determined by the concentration of solutes in solution

"Oncotic pressure is colloidal osmotic pressure and it is osmotic pressure exerted by colloids in solution... protein molecules attract water, pulling fluid from the tissue space to the vascular space"

would the correct answer by osmotic or oncotic?

This is a tough question. I do not think the word "excreted" is a typo. My GUESS is that the answer is Osmotic, because it says solute. If it had said colloid, then the answer would be Oncotic.

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1,842 Posts; 28,337 Profile Views

and so osmotic pressure that "stops" and "opposes"

is also considered to be a "drawing force"?

as I had also thought - because the question asks about "solutes"

instead of "colloid" - this is maybe the reason that it isn't oncotic

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Myxel67 specializes in Diabetes ED, (CDE), CCU, Pulmonary/HIV.

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1,842 Posts; 28,337 Profile Views

actually, the answer provided is "osmotic"

so, I am trying to understand more clearly

the reason why "osmotic" is considered a

"drawing force"?

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Ill take a stab at it. Im thinking you have to think in terms of osmotic pressure on both sides of a membrane. Although it doesnt make sense that somthing exerting pressure would be "drawing" water in. However, the pressure on the other side of the membrane is lower and therefore would be drawing fluid in to balance it out. Thats just a bad quesiton.

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thanks

this is also how I was trying to think through what happens

{maybe just the explanation in the book about osmotic pressure as force that stops or opposes, isn't as complete as it could be?}

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1,842 Posts; 28,337 Profile Views

now here's more of an answer about osmotic pressure:

"hypertonic solutions raise the osmolality .... the higher osmotic pressure draws water out of the cells into the ECF."

this is the explanation for 3% NaCl and the risk for intracellular dehydration and intravascular fluid volume excess.

{and packed RBCs increase the oncotic pressure and pull fluid into the intravascular space}

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29 Posts; 1,291 Profile Views

now here's more of an answer about osmotic pressure:

"hypertonic solutions raise the osmolality .... the higher osmotic pressure draws water out of the cells into the ECF."

this is the explanation for 3% NaCl and the risk for intracellular dehydration and intravascular fluid volume excess.

{and packed RBCs increase the oncotic pressure and pull fluid into the intravascular space}

OK I made sense of it. Your body, well physics actually, wants all the solutions in your body to be the same. They all want to reach equalibrium.

If one side of a membrane has a higher concentration of lets say Na+ than the otherside of said membrane, the Na+ is going to do everything it can it get out an mingle in thoes fluids.... This is a high osmotic pressure. It wants to get out really bad,

However, since thoes membranes are semi-permiable, only fluid can pass realitivy quickly across the membrane. So that high concentration of Na+ draws water in, to balance out it's eagerness to escape. There are many applications to this which you have mentioned. This leads to cell crenation, and death basicly.

Oncotic pressure is the same thing, except there isn't a solution trying to pass through the membrane, its a large protein. Since thoes protiens are so eager to excape, they exert a high oncotic pressure. Once again water comes to the rescue and is "drawn in" to balance the equation out.

Hope that helps.

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