osmotic pressure direction??

[MrNeuro]

osmotic pressure is responsible for forcing solvent molecules across a semipermeable membrane into the solution with greater solute concentration

Why is that wrong?

From the internet

Diffusion of water across a membrane generates a pressure called osmotic pressure. If the pressure in the compartment into which water is flowing is raised to the equivalent of the osmotic pressure, movement of water will stop. This pressure is often called hydrostatic ('water-stopping') pressure.

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[MedPR]

osmotic pressure is responsible for forcing solvent molecules across a semipermeable membrane into the solution with greater solute concentration

Why is that wrong?

From the internet

Osmotic pressure is the responsible force for preventing solvent from moving across a membrane. Osmotic pressure opposes the movement of water that you associate with osmosis.

In other words, high osmotic pressure = water really wants to go in, low osmotic pressure = water is happy where its at.

So, if you have an intracellular concentration that is hypertonic to the surroundings, the osmotic pressure will be higher than if you have a hypotonic intracellular concentration.

 

[MrNeuro]

Osmotic pressure is the responsible force for preventing solvent from moving across a membrane. Osmotic pressure opposes the movement of water that you associate with osmosis.

In other words, high osmotic pressure = water really wants to go in, low osmotic pressure = water is happy where its at.

So, if you have an intracellular concentration that is hypertonic to the surroundings, the osmotic pressure will be higher than if you have a hypotonic intracellular concentration.

so in a red blood cell if its hypertonic to its surroundings does the hydrostatic pressure cause an influx of water into the cell??? and then when the hydrostatic pressure is equal to the osmotic pressure its in equilibrium

im confused as to what causes solvent flow only hydrostatic pressure or both osmotic pressure and hydrostatic.....?

helppp

 

[MedPR]

so in a red blood cell if its hypertonic to its surroundings does the hydrostatic pressure cause an influx of water into the cell??? and then when the hydrostatic pressure is equal to the osmotic pressure its in equilibrium

im confused as to what causes solvent flow only hydrostatic pressure or both osmotic pressure and hydrostatic.....?

helppp

The concentration gradient causes water to flow.. I think it's actually called "water pressure," but don't quote me on that.

If the RBC is hypertonic to its surroundings, water is going to want to flow into the RBC. So osmotic pressure (the pressure required to prevent water from flowing in) is going to be high.

They both contribute to the direction solvent flows.

Hydrostatic pressure is just the pressure exerted by a liquid. A familiar example is blood pressure, which is a case of hydrostatic pressure that someone just decided to call blood pressure since the fluid exerting the pressure is blood. You can think of hydrostatic pressure as the pressure pushing outward, even though it doesn't really push at all.

If you're having a hard time with it, it does work (at least for MCAT) to think of hydrostatic pressure as the pressure pushing out and the osmotic pressure as the pressure pulling in.

 

[chiddler]

Osmotic pressure is the responsible force for preventing solvent from moving across a membrane. Osmotic pressure opposes the movement of water that you associate with osmosis.

In other words, high osmotic pressure = water really wants to go in, low osmotic pressure = water is happy where its at.

So, if you have an intracellular concentration that is hypertonic to the surroundings, the osmotic pressure will be higher than if you have a hypotonic intracellular concentration.

Pressure can't be negative right? So what if:

1. A blood vessel is hypertonic to its surrounding

and

2. A blood vessel is hypotonic to its surrounding

And the relative concentrations are about the same. So osmotic pressure is the same in magnitude in both cases but in opposite direction. So both of these will have a positive pressure value? Or can a negative pressure exist in such a case?

 

[MedPR]

Pressure can't be negative right? So what if:

1. A blood vessel is hypertonic to its surrounding

and

2. A blood vessel is hypotonic to its surrounding

And the relative concentrations are about the same. So osmotic pressure is the same in magnitude in both cases but in opposite direction. So both of these will have a positive pressure value? Or can a negative pressure exist in such a case?

At first glance I'm confused by w hat you are asking. Here goes, anyway.

Blood vessel lumen hypertonic to interstitium = water going to flow into vessel lumen = high osmotic pressure.

blood vessel lumen hypotonic = edema = low osmolarity.

I don't really know what you're talking about in terms of negative pressure though.. Osmotic pressure is always the pressure preventing fluid flowing into the lumen of a vessel/cell/organ. hydrostatic is in the opposite direction.

 

[SLC]

osmotic pressure is responsible for forcing solvent molecules across a semipermeable membrane into the solution with greater solute concentration

Why is that wrong?

The bolded part is what makes this an untrue statement (as far as I can tell and according to my understanding). It's not that the bolded is incorrect, just that high hydrostatic (osmotic) pressure pushes water in either direction. Which ever side of the membrane has lower osmotic pressure is the side that the water will be pushed toward, regardless of solute concentration.

Fluid movement as a result of solute concentration is what we call Oncotic Pressure, and in the body, mostly caused by the concentration of albumin in the blood.

 

[typicalindian]

Honestly the most simple way to think about it is like this:
Osmotic pressure pulls water
Hydrostatic pressure pushes water

So look at this example,
A red blood cell is placed in a hypotonic solution, what happens? Obviously the RBC will swell with water and burst. In this situation, the osmotic pressure from inside the cell pulls the water in, and the hydrostatic pressure from the solution pushes the water in. It's the same thing but different references.

Now what about an RBC in isotonic solution? well, nothing would happen, right? This is because the osmotic pressure inside the cell is balanced by the osmotic pressure of the outside of the cell. Same goes for the hydrostatic pressure, the cell is pushing water out at an equal rate that the solution is pushing water in.

Or heres another example! When you have blood flow through the capillaries, when the blood flows from the aterial side of the capillary, the hydrostatic pressure of the capillary, causes the blood to filter out, then when you get towards the venous side of the capillary, the osmotic pressure in the capillary is now greater than the hydrostatic pressure, which allows the filtrate to come back into the capillary.

Here's a handy picture for you!

*Key
Ph = hydrostatic pressure
cap = capillary
pi = osmotic pressure
cell = cell

Hope my tired 2am self helped you!

 

[MedPR]

Yea, just memorize as this:

Hydrostatic = blood pressure = water "pushing out."
Osmotic/Oncotic = water "pushing in"

 

[MrNeuro]

Yea, just memorize as this:

Hydrostatic = blood pressure = water "pushing out."
Osmotic/Oncotic = water "pushing in"

yeah but that seems to contradict what the answer says

the answer is wrong according to TPRH as osmotic pressure apparently has nothing to do w/ the flow of solvent....

 

[MedPR]

yeah but that seems to contradict what the answer says

the answer is wrong according to TPRH as osmotic pressure apparently has nothing to do w/ the flow of solvent....

What's the question?

 

[SLC]

Yea, just memorize as this:

Hydrostatic = blood pressure = water "pushing out."
Osmotic/Oncotic = water "pushing in"

This is both right and wrong.

Hydrostatic pressure occurs on both sides of a membrane, whichever side has the highest pressure is the side the water will push away from. So unless you mean pushing out solely in the context of the space with the higher hydrostatic pressure, it's incorrect to state that the hydrostatic always pushes out. High interstitial (relative to capillary) hydrostatic pressure can push fluid into the capillaries, does that make sense?

And oncotic pressure is similar. It can either push fluid out or pull fluid in. If the soilutes in a capillary are very dilute, then water will be pushed out of the capillary, while if the solutes are very concentrated, fluid will be "pulled in". As with hydrostatic pressure, high vs. low and the direction of fluid flow depend entirely on the space you're looking at.

One can get turned around quickly trying to keep this all straight. I find it easiest to just think about a balance between fluid pressure and osmotic gradient and try and decide which direction the fluid would flow.

 

[MedPR]

This is both right and wrong.

Hydrostatic pressure occurs on both sides of a membrane, whichever side has the highest pressure is the side the water will push away from. So unless you mean pushing out solely in the context of the space with the higher hydrostatic pressure, it's incorrect to state that the hydrostatic always pushes out. High interstitial (relative to capillary) hydrostatic pressure can push fluid into the capillaries, does that make sense?

And oncotic pressure is similar. It can either push fluid out or pull fluid in. If the soilutes in a capillary are very dilute, then water will be pushed out of the capillary, while if the solutes are very concentrated, fluid will be "pulled in". As with hydrostatic pressure, high vs. low and the direction of fluid flow depend entirely on the space you're looking at.

One can get turned around quickly trying to keep this all straight. I find it easiest to just think about a balance between fluid pressure and osmotic gradient and try and decide which direction the fluid would flow.

I know it's wrong, but it works for the MCAT. If you are talking about hydrostatic and oncotic from the point of view of the same lumen, say a vessel, then it works just fine. Hydrostatic is the pressure pushing outward inside the lumen, and oncotic is the pressure pulling into the lumen. Osmotic/Oncotic pressure typically pulls fluid into a vessel, say a capillary. But in situations of low lumenal solute, as you say, the gradient will reverse and oncotic pressure will be reduced = edema.

The easiest way to think about both of these is to consider them from the same side of whatever membrane you want to think about.

 

[SLC]

I know it's wrong, but it works for the MCAT.

Ok then, here's the type of discrete question you might see on the MCAT that relates to this topic:

In a patient with end stage liver disease, the capillaries have a lower hydrostatic and oncotic pressure than the interstitial space. The oncotic pressure difference is greater than the hydrostatic pressure difference. Which of the following is the net result:

A. Net movement of fluid into the vascular space.
B. Net movement of fluid into the interstitial space. ****
C. Balance of fluid movement between the vascular and interstitial spaces.
D. Osmotic diuresis and polyuria.

Answer it without google, if you can jump right on the correct answer, you probably have a handle on this concept.

 

[MedPR]

So the interstitial space has a great oncotic pressure? Insterstitium is going to pull fluid. B

If the hydrostatic pressure difference was greater (with the interstitial hydrostatic still being greater) then it would be A. Hydrostatic is the traditional "push from high to low pressure" pressure.

Not C, because the question tells you there is an imbalance in pressure.

Not D because then A would be true also. Higher vascular = higher GFR = more urine.

Also, I kind of already knew that advanced liver disease causes edema so I went straight to B regardless.

 

[SLC]

It seems like you get it just fine. I was worried about the liver disease clue, but took the chance anyway. Probably should have talked about a patient with a nephrosis instead, might not have been quite as much of a freebie.

 

[MedPR]

It seems like you get it just fine. I was worried about the liver disease clue, but took the chance anyway. Probably should have talked about a patient with a nephrosis instead, might not have been quite as much of a freebie.

I think it is a really easy topic/concept to create some really hard questions on, but I don't think MCAT writers really care about it that much. I know it's physio and so it is likely to show up, but I just don't see it being the source of one of the "hard" questions on the MCAT.

Personally I just remember that blood pressure is hydrostatic pressure, and anything opposing it is oncotic. I know it probably isn't true, but the way I have it situated in my head it makes sense and works conceptually for me.

 

[typicalindian]

no love for my expert paint drawing?! Just wait until you get to med school and see my beautiful drawing published in your physiology book, YOU'LL ALL BE SORRY THEN!!@!

 

[MedPR]

no love for my expert paint drawing?! Just wait until you get to med school and see my beautiful drawing published in your physiology book, YOU'LL ALL BE SORRY THEN!!@!

Considering how long it takes for things to get published, I hope I'm done with med school by the time your wonderful drawing shows up in any books 🙂

 

[SaintJude]

Ok then, here's the type of discrete question you might see on the MCAT that relates to this topic:

In a patient with end stage liver disease, the capillaries have a lower hydrostatic and oncotic pressure than the interstitial space. The oncotic pressure difference is greater than the hydrostatic pressure difference. Which of the following is the net result:

A. Net movement of fluid into the vascular space.
B. Net movement of fluid into the interstitial space. ****
C. Balance of fluid movement between the vascular and interstitial spaces.
D. Osmotic diuresis and polyuria.

Answer it without google, if you can jump right on the correct answer, you probably have a handle on this concept.

Noooooo--- I'm getting this wrong. I thought oncotic pressure pushes fluid in and hydrostatic pushes fluid out. Why is it B? and not A??


edit: oh, only now do I understand this is the point of the thead. I'm late. the question stem says that the capillaries have a lower "force" than interstitial space. So, it's actually considering the pressure difference from the perspective of the interstitial space and not the "usual" perspective from capillaries. So when its says the oncotic pressure is greater it's suggesting that the net fluid pushes into the interstitial space is greater than what's leaving it. So net movement is into the interstitial space.

 

[chiddler]

Noooooo--- I'm getting this wrong. I thought oncotic pressure pushes fluid in and hydrostatic pushes fluid out. Why is it B? and not A??


edit: oh, only now do I understand this is the point of the thead. I'm late. the question stem says that the capillaries have a lower "force" than interstitial space. So, it's actually considering the pressure difference from the perspective of the interstitial space and not the "usual" perspective from capillaries. So when its says the oncotic pressure is greater it's suggesting that the net fluid pushes into the interstitial space is greater than what's leaving it. So net movement is into the interstitial space.

thanks i was scratching my head for a while with this.

 

[sklee90]

This really helped me alot. thanks!

 

[sciencebooks]

Ok then, here's the type of discrete question you might see on the MCAT that relates to this topic:

In a patient with end stage liver disease, the capillaries have a lower hydrostatic and oncotic pressure than the interstitial space. The oncotic pressure difference is greater than the hydrostatic pressure difference. Which of the following is the net result:

A. Net movement of fluid into the vascular space.
B. Net movement of fluid into the interstitial space. ****
C. Balance of fluid movement between the vascular and interstitial spaces.
D. Osmotic diuresis and polyuria.

Answer it without google, if you can jump right on the correct answer, you probably have a handle on this concept.

Hey guys, sorry to harp on this old thread, but I was just wondering how often we'll see options like this and be expected to know the symptoms on the MCAT? Like I know diuretics increased urine, but I'm not exactly sure of what polyuria is.

 

[karafox]

poly = many
uria = urine

excessive passage of urine

 

[nickelbackfan]

Ok then, here's the type of discrete question you might see on the MCAT that relates to this topic:

In a patient with end stage liver disease, the capillaries have a lower hydrostatic and oncotic pressure than the interstitial space. The oncotic pressure difference is greater than the hydrostatic pressure difference. Which of the following is the net result:

A. Net movement of fluid into the vascular space.
B. Net movement of fluid into the interstitial space. ****
C. Balance of fluid movement between the vascular and interstitial spaces.
D. Osmotic diuresis and polyuria.

Answer it without google, if you can jump right on the correct answer, you probably have a handle on this concept.

I'm very confused by this. I know that if the interstium has a higher solute concentration in regards to the vessel, then liquid will flow from the vessel to the interstium. I'm having a hard time trying to apply what I know to a question like this.


Can somebody help?

EDIT: I think i'm starting to understand it. The solute concentration is higher in the intersitum, thus it has the highest oncotic pressure. Water will flow there. What I don't understand is how to incorporate the hydrostatic part.

 

[sciencebooks]

poly = many
uria = urine

excessive passage of urine

Lol, wooooooow. Thank you, 😛.