# osmotic pressure vs hydrostatic pressure?

Discussion in 'MCAT Discussions' started by aye, 03.16.05.

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1. ### ayeSenior Member 7+ Year Member

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can anyone tell me the difference? specifically in the case of liquid exchange in the capillaries. Thanks!

3. ### Prophecies...I bleed orange... 5+ Year Member

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Osmotic pressure describes the tendency of water to move into a solution via osmosis; hydrostatic pressure describes the tendency of water to stay where it is and not move out of solution (hydro = water, static = not moving). They are opposites...to help you remember, EK says:

"Students often think about osmotic pressure as the pressure pulling into a solution, and hydrostatic pressure as the pressure pulling out of a solution. Although this is technically incorrect because pressure is a scalar and has no direction, thinking about osmotic pressure in this way may give you some intuition about it."

As far as membrane transport/liquid exchange goes, here is how to think of it (from one of my physiology textbooks):

Imagine you have a container divided into into two compartments by a membrane permeable to water, but impermeable to some solutes. When there is only water in the container, the water moves back and forth across the membrane in equal amounts, so that the net water movement is zero.

If a solute to which the membrane is impermeable is added to one compartment of the container, the activity of the water molecules on the side containing the solute is reduced, so that the flow of water from that side is reduced. The flow of water into the solute-containing side continues as before, so that there is a net flow of water into the solute-containing side, which creates a hydrostatic pressure difference (the pressure required to keep the water static has decreased on one side). The osmotic pressure of the solution could be measured by determining the pressure required to move the water from one side to the other.

This principle is the same for the capillaries of the body. Hope this helps!

4. ### ayeSenior Member 7+ Year Member

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let me see if i understand this from your scenario. lets say compartment A contains solutes while compartment B just contains water. since there is less activity of water on A, there will be a net flow into A. So the hydrostatic pressure gets lower on A and increases in B, since water from B will flow to A?

sorry, i'm just confused... all i learned was osmosis and how water goes from low to high concentration. that's it

5. ### Prophecies...I bleed orange... 5+ Year Member

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I really wouldn't worry about the hydrostatic pressure part--it isn't very important for the MCAT. All you need to know about the topic is that osmotic pressure is used to move water into a solution, and you need to know the formula for osmotic pressure. Don't stress out about hydrostatic pressure

6. ### crys20make it happen! 5+ Year Member

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I wouldnt say dont worry about it rather i would understand it as its a crucial topic in all of physio but dont worry it is easy.
just picture you have 2 containers seperated by a semipermeable membrane that only allows water across. Now you have a Left and Right side ( one on each sideo fthe membrane. Now say your goal is to see how much water you can get into the Right box so there are 2 ways you can get water into that Right sided compartment: So before taht you start by adding water into the left sided compartment and your goal is to get as much water from the left side to the right side through the membrane so first,
1. All that water you added to the left compartment is sitting there, lets say there are 10 gallons and you ahve 0 in the right side- so bam its like a flood gate, water will flow fast through the membrane from the left to right side since things flow from high to low( think of a water dam and what would happen if you took down the dam all of a sudden-the water would rush from one side across to the other side because all that water was building up on one side) ok so this is HYDROSTATIC pressure- when there is a difference in quantiity that forces flow from high to low.

now soon water will finish flowing when there are 5 gallons on each side, no more flow but yous till want more water on the right side so hwo do you do taht?
2. dump a ****load of salt into the right sidea nd make it mroe concentrated which will create a hypertonic solution on the right compared to pure water on the left so what will water due to equal things out? the salt sucks some water from left to right to make each side an equal tonicity or concentration---this "suckign" force is what is called Osmotic pressure.

So combine 1 and 2 and you get both a hydrostatic(driven by a High to low gradient force) and a osmotic(driven by a sucking action of solute driven by equalization of tonicity) and there you ahve it.

7. ### crys20make it happen! 5+ Year Member

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No offense dude but i hope you are not going into academia cause that is the most confusing explanation of a extremely simple subject taht i have ever seen.

8. ### stoleyerscrubzRegistered User and Stuff 5+ Year Member

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That's good you see there are 2 pressures you will have to deal with to figure out what is going on. These 2 forces work against each other or with each other and adding them will tell you which way fluid will move. fluid moves from high pressure to low pressure. remember that systems with high energy tends to gravitate to a state with lower energy(Thermodynamics).

The hydrostatic pressure I'm talking about is from the heart pumping blood. The osmotic pressure is from the difference in solute concentration.

At the arterial end of the capillary bed the the hydrostatic pressure is greater than the osmotic pressure so net flow is out of the capillary. At the venule end the hydrostatic pressure is lower than the osmotic pressure so net flow is into the capillary bed. Hydrostatic pressure(blood pressure) drops off as it move into and through the capillary bed(know how that chart looks for blood pressure during certain points in the blood system). Net flow is from high pressure to low pressure.

I think net fluid exhange is about 10% from capillary to interstitial fluid(contains no blood) so osmotic pressue does not change much if at all.The net fluid flow is dependent on hydrostatic pressure.

EK bio has a good picture that illustrates this in the Cardiovascular system chapter and Campbell's has something similar picture with explanation in the chapter 42 circulation and gas exchange and the water potential explanation in chapter 36 Transport in Plants will help as well.

in brief:
If two conatiners are seperated by a selectively permeable membrane and one side has pure water and the other has some solute it is possible to overcome osmotic pressure if there is a force pushing on the solute greater than osmotic pressure.

9. ### stoleyerscrubzRegistered User and Stuff 5+ Year Member

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looking at a picture of this makes it extremely easy to understand. This is why I love the EK series. I actually am visualizing charts, pictures and graphs when answering questions.

10. ### medicomelPurveyor of short posts. 10+ Year Member

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yay i get it!!!! thanks!

11. ### Nutmeg#NeverTrump 10+ Year Member

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You're KIDDING me. I saw several questions on hydrostatic pressure on the MCAT. My sincere advice is DISREGARD THIS ADVICE ENTIRELY. The questions on hydrostatic pressure made me glad I studied ChemE.

The concept is really simple if you go with Crys20's example. Hydrostatic is pressure resulting from the weight of a fluid, osmotic pressure is due to a gradient across a semipermeable membrane.

12. ### MattDCurmudgeon 7+ Year Member

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It looks like your question has been answered as well as needed to get an MCAT question right, but I just have to nitpick one comment that was made. Hydrostatic pressure isn't the water's 'tendency not to move out of solution', the only force that would cause any mass to tend not to change would be inertia. Hydrostatic pressure is called what it is called to differentiate it from something called dynamic pressure. H. pressure is the pressure exerted by a fluid WHEN IT IS NOT MOVING, it has nothing to do with KEEPING it from moving. This is different from dynamic pressure, which is additional pressure the fluid exerts on it's container(pipe, valve, blood vessel, whatever) due to it's velocity. H. pressure of a fluid in a tank can be calculated by simply multiplying the specific weight of the fluid by the height of the fluid column above it. In regards to blood vessels, the H. pressure is caused by heart contractions and vessel elasticity, and refers to the height that pressure could lift a column if it were attached to a manometer. This is why you see B.P. expressed as 120 or whatever mm Hg, it means the pressure in the vessel could lift a column of mercury 120 mm. In capillaries, on the arterial side there is a higher H. pressure in the blood vessel than in the intersticial space, which causes the water to move toward the intersticial space. This is opposed by osmotic pressure, which is a mechanism I just trust cuz I couldn't tell you HOW it works, which tries to push the water back into the capillary. At the arterial side the H. pressure is higher than the osmotic pressure, so water flows out. At the venous side, osmotic pressure is higher (due to the blood dilution) so the water flows back in. And that's how it works. And somehow I explained the entire topic over again, even though it was already explained above.... oops!

13. ### cfdavidBannedBanned 7+ Year Member

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I just think of the blood as having lots of solute (albumin, and electrolytes etc.) The heart creates the hydraulic pressure which exerts on the inside diameter of the blood vessells. So, water would want to cross the blood vessell to the interstitial space. BUT, the high solute concentration of the blood, creates 'desire' of water from the interstitial space to go into the blood vessel to balance out the concentration gradient. So, a sort of equillibrium exists.

The reason that we lose a net 10% of "water"/fluid from the blood volume to the interstitial environment is because in order for flow to occur, there must be a hydrostatic pressure drop across a section of blood vessel. If you tied off a blood vessel, the pressure would quickly equalize throughout the entire length of the vessel. Thus, no pressure drop = no flow.

Like someone else elaborated, with the hydrostatic pressure being higher on the arterial side of the vessel, there is a flow out of the vessel on the arterial side. Somehow (I don't know exactly how), osmostic pressure is maintained constant throughout the blood vessel. Therefore, on the venous side of the blood vessel, where the hydrostatic pressure (pushing out) is a bit less than just "upstream", the osmotic pressure 'wins' in this case, with fluid coming back into the blood vessel on the veinous end. However, it turns out that there is still a net loss of 10% of blood volume to the interstitial space, and the lymphatic system takes this up.

I know the OP said he/she got it. But maybe this helps someone else.

14. ### Prophecies...I bleed orange... 5+ Year Member

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Three things:

1.) To the person who said I shouldn't go into academia--like I said (but you obviously didn't read), that explanation did not come from me personally, because I don't know how to explain it accurately. All I did was provide sources (from ExamKrackers and from my physiology text book) to help describe the pressures. The explanation from the textbook includes a diagram which I could not post up. Sorry about that...no need to be so harsh.

2.) To the person who said nitpicked about hydrostatic pressure...I believe EK mentions that thinking of osmotic pressure as "pulling out" and hydrostatic as "pushing in" is not entirely accurate, but the best way to think of it for the MCAT. I also put that in my post...I did not mean to be misleading...only help people remember stuff for the MCAT.

3.) Hydrostatic pressure is not an important concept for the MCAT...osmotic pressure is. I think most people would agree that, instead of being confused about a subject that likely won't appear on the MCAT, one should focus on the topics that most certainly WILL be on the MCAT. To me it just seems like a waste of all that time to memorize something that won't be on the test. Yes, hydrostatic and osmotic pressures are VERY important concepts in physiology, but the poster is referring to the MCAT, not physiology.

Anyway, the OP understands the concept now and that is all that matters. Good luck to you!

15. ### MattDCurmudgeon 7+ Year Member

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Hey, sorry if I came across wrong, I wasn't trying to imply that you were being misleading, I just wanted to point out the actual definition in case it DID wind up on the test. While it's surely unimportant for the BS section, I'm not confident that you can discount any questions at all about hydrostatic pressure on the PS section. After all, it's a fluids problem just like bouyancy problems are, and bouyancy shows up all the time on the AAMC practice tests. I agree with you (and EK), however, that the simplified idea is going to be more convenient for the BS section. Again, I wasn't trying to crack on you, just pointing something out in the interest of accuracy, after all, we're all scientists, don't we LIKE to know how stuff works???

16. ### superduper12 2+ Year Member

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ok so EK says that

there is higher concentration on side B....water flows from A to B.

osmotic pressure at b = Pb - Pa = pghb - pgha ... a and b are subscripts...why this is equation useful?

also when water is flowing into side b, does the osmotic pressure stay the same or change because the water changes the molarity of the solution. I guess as soon as the water starts flowing, the osmotic pressure decreases till it hits 0....and equilibrium is established.

17. ### Premo

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I think Kaplan's answer is the best here:

Hydrostatic Pressure pushes fluid out of the vessel (blood vessel) while osmotic pressure pulls the fluid back into the vessels. Hydro-static pressure is dependent on the blood pressure driven by the heart; Osmotic pressure is dependent on the number of particles dissolved in the plasma

18. ### muhali3 7+ Year Member

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I think of osmotic pressure as the influence of a concentration gradient on water flow. Hydrostatic pressure is the influence of arterial contractions/physical forces on water flow.

19. ### homycraz2

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So are Hydrostatic and osmotic pressures inversely related?