Direction of osmotic pressure

[blayne]

Can someone clarify the direction of osmotic pressure? I thought the technical definition was the pressure that was required to prevent the movement of water down its concentration gradient. This would imply that the vector representing osmotic pressure is opposite the direction of the flow of water. However, this is not how TBR represents it in chapter 7 pg 92 of the Chem review book. Furthermore, virtually all depictions of the role of osmotic pressure vs. hydrostatic pressure in fluid exchange at the arterial and venous ends of the capillary bed implies that osmotic pressure has a direction that is parallel (not antiparallel) to plasma/solvent flow.

However, I just got a TPRH SW problem wrong because I assume that the solvent moves in the same direction as the osmotic pressure vector and most chem texts showing osmotic pressure antiparallel to solvent flow. Can someone kindly clarify!

---

Members don't see this ad.

 

[pm1]

Can someone clarify the direction of osmotic pressure? I thought the technical definition was the pressure that was required to prevent the movement of water down its concentration gradient. This would imply that the vector representing osmotic pressure is opposite the direction of the flow of water. However, this is not how TBR represents it in chapter 7 pg 92 of the Chem review book. Furthermore, virtually all depictions of the role of osmotic pressure vs. hydrostatic pressure in fluid exchange at the arterial and venous ends of the capillary bed implies that osmotic pressure has a direction that is parallel (not antiparallel) to plasma/solvent flow.

However, I just got a TPRH SW problem wrong because I assume that the solvent moves in the same direction as the osmotic pressure vector and most chem texts showing osmotic pressure antiparallel to solvent flow. Can someone kindly clarify!

Osmotic pressure is the pressure that opposes hydrostatic pressure. Thus it is the pressure that keeps or tries to keep fluid from getting inside the vessels. Usually osmotic pressure is constant. For example, in arteries hydrostatic pressure is higher than osmotic pressure. As opposed to veins, where the hydrostatic pressure is smaller than osmotic pressure resulting in a next flow inward.

 

[blayne]

Osmotic pressure is the pressure that opposes hydrostatic pressure. Thus it is the pressure that keeps or tries to keep fluid from getting inside the vessels. Usually osmotic pressure is constant. For example, in arteries hydrostatic pressure is higher than osmotic pressure. As opposed to veins, where the hydrostatic pressure is smaller than osmotic pressure resulting in a next flow inward.

Sorry, but I find your response internally inconsistent and further confounding. My question is merely about the direction of osmotic pressure. You begin saying that osmotic pressure "tries to keep fluid from getting inside the vessels." This would suggest the osmotic pressure vector points from inside the vessel outwards towards the interstitial fluid. Yet, you then say that in veins the larger osmotic pressure than hydrostatic pressure results in a net inward flow of plasma. Aren't these contradictory statements? In virtually all texts the osmotic pressure is shown pointing into the vasculature. How is that consistent with your statement that osmotic pressure "tries to keep fluid from getting inside the vessels?"

I will try to make this clearer. We have a u tube open on both sides to the atmosphere with each half of the u tube separated by semipermeable membrane that is impermeable to ions. I add pure water to the right side of the u tube. I add a concentrated solution of NaCl to the left side. Obviously, water will move from the right side to the left side. In which direction does the osmotic pressure vector point along the bottom of the U tube (imagine the vector pointing through h semipermeable membrane):
1. To the right and against water flow
2. To the left and with water flow

 

[HesitantManatee]

Your first misconception is that pressure has a direction, it doesn't. Pressure is a scalar not a vector.

You second misconception is osmotic pressure is a pressure, it not. Solvents move from high osmotic pressure to low osmotic pressure. Osmotic pressure has the same meaning as osmolarity. Its just a way of comparing one area to another.

 

[blayne]

Thanks. Textbooks and review books should make this clearer. Both TBR and TPR show diagrams with osmotic pressure represented by an arrow, strongly suggesting a vector. There arrows are in opposite direction for the gedanken experiment that I posed. Based on your comment, is it fair then to interpret these arrows as arrows reflecting the direction of the osmolarity gradient or is there no standard convention here and I should dismiss these arrows as wrong and not representing anything meaningful unless further specified.

 

[blayne]

Thanks. Textbooks and review books should make this clearer. Both TBR and TPR show diagrams with osmotic pressure represented by an arrow, strongly suggesting a vector. There arrows are in opposite direction for the gedanken experiment that I posed. Based on your comment, is it fair then to interpret these arrows as arrows reflecting the direction of the osmolarity gradient or is there no standard convention here and I should dismiss these arrows as wrong and not representing anything meaningful unless further specified.

Their arrows

 

[HesitantManatee]

Thanks. Textbooks and review books should make this clearer. Both TBR and TPR show diagrams with osmotic pressure represented by an arrow, strongly suggesting a vector. There arrows are in opposite direction for the gedanken experiment that I posed. Based on your comment, is it fair then to interpret these arrows as arrows reflecting the direction of the osmolarity gradient or is there no standard convention here and I should dismiss these arrows as wrong and not representing anything meaningful unless further specified.

Their is no longer a writing section on the MCAT so you don't have to worry bout being grammatically correct...lol Their is also a edit function on your comments.

As far as the arrows, I don't think their is any standard representation. The way your describing them is, it seems that they point in the direction the solvent will flow.

 

[sazerac]

Your first misconception is that pressure has a direction, it doesn't. Pressure is a scalar not a vector.

You second misconception is osmotic pressure is a pressure, it not. Solvents move from high osmotic pressure to low osmotic pressure. Osmotic pressure has the same meaning as osmolarity. Its just a way of comparing one area to another.

Except solvent moves from low osmotic pressure to high osmotic pressure. You got it completely backwards, hence some of the confusion. Remember PI=iMRT, so the more particles, the higher the pressure and the higher the ability to attract solvent.

In fact, for the purposes of the MCAT and for being a doctor someday, just think of osmotic pressure as a "sucking" force and all will become clear.

 

[pm1]

Sorry, but I find your response internally inconsistent and further confounding. My question is merely about the direction of osmotic pressure. You begin saying that osmotic pressure "tries to keep fluid from getting inside the vessels." This would suggest the osmotic pressure vector points from inside the vessel outwards towards the interstitial fluid. Yet, you then say that in veins the larger osmotic pressure than hydrostatic pressure results in a net inward flow of plasma. Aren't these contradictory statements? In virtually all texts the osmotic pressure is shown pointing into the vasculature. How is that consistent with your statement that osmotic pressure "tries to keep fluid from getting inside the vessels?"

I will try to make this clearer. We have a u tube open on both sides to the atmosphere with each half of the u tube separated by semipermeable membrane that is impermeable to ions. I add pure water to the right side of the u tube. I add a concentrated solution of NaCl to the left side. Obviously, water will move from the right side to the left side. In which direction does the osmotic pressure vector point along the bottom of the U tube (imagine the vector pointing through h semipermeable membrane):
1. To the right and against water flow
2. To the left and with water flow

sorry for the confusion. I should stop answering questions from my phone 😳

 

[blayne]

Except solvent moves from low osmotic pressure to high osmotic pressure. You got it completely backwards, hence some of the confusion. Remember PI=iMRT, so the more particles, the higher the pressure and the higher the ability to attract solvent.

In fact, for the purposes of the MCAT and for being a doctor someday, just think of osmotic pressure as a "sucking" force and all will become clear.

While you are right about the direction of flow, your heuristic seems very confusing. Things usually move from high to low pressure environments. However solvents, as you note, move from low to high osmotic pressure. This seems to be contradictory to the general understanding of movement from a high pressure environment to a low pressure environment. I'm not contesting your statement about the direction of movement, I just don't think this notion of solvent being sucked to a higher pressure environment is intuitive given that things normally flow from high pressure to low pressure locales. Can we agree that osmotic pressure is a dumb name or is the something I am missing which would justify the use of the term pressure in describing what is essentially an entropic-driven phenomenon of simple diffusion?