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Osmotic pressure
Written by Tim Sheppard MBBS BSc. Created 8/11/09; last updated 11/1/10

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What is osmotic pressure?

Osmotic pressure is considered to be the pressure across a semi-permeable membrane caused by a solution. Another way of thinking of it is the pressure that you would need to apply to stop water trying to cross a membrane because of osmosis. Confusing, hey? Of course, you need to understand osmosis, but once you've done that, how do you understand what this osmotic pressure is?

I tend to think of it a bit like a pulling effect that is attracting water across the membrane. If there is a lot of solute dissolved on one side of the membrane, and not much on the other, the effect of osmosis says that the water will transfer to the side where there is lots of solute, until eventually the two solutions have the same amount dissolved in them.

So the osmotic pressure is that pressure which forces the water to move from where there is little dissolved to where there is lots dissolved - the kind of pressure which is trying to make everything even. I often think of it more like a pull - that where there is more dissolved in solution, the huge amount of solute is pulling the water (or other liquid) across to join it.

Importantly, this doesn't have to be the same solute - you might have sodium dissolved on one side, and potassium on the other. The important thing is comparing the amount dissolved on one side and the amount on the other, and the pressure that this difference creates.

Strictly, osmotic pressure should be defined in terms of the amount of pressure you'd need to apply to something in order to stop water from flowing into it by osmosis ... however, this to my mind just complicates it. The simple way of thinking about it is remembering that the more there is dissolved in a solution, the more it will attract water to dilute it up again. Whenever you put a cell, surrounded by a cell membrane, into a solution with a much lower concentration of solute (e.g. pure water), water will try to leak into the cell by osmosis to try to balance things out. The pressure which achieves this leakage is called osmotic pressure.

What is osmotically active?

A molecule is osmotically active if it contributes towards osmotic pressure - that is, a molecule is osmotically active if it is causing osmosis to occur. If a molecule is dissolved in a liquid and it cannot cross a membrane but the liquid can, then it will be osmotically active. All of the molecules in the liquid which can't cross the membrane are increasing the concentration of the solution. Because of osmosis, water (or whatever the liquid is) will want to cross the membrane to even out the concentrations on either side. Anything which contributes towards this can be called osmotically active. There are therefore two ways that something will not be osmotically active:

The first is that the substance might not dissolve in the liquid at all. If a substance doesn't dissolve in the liquid, then it's not affecting the concentration of particles in the liquid, so it's not going to make any difference to osmosis. The remaining particles, which are dissolved in the liquid, will determine the concentration of that liquid. Even if there are more particles in the first solution, if half of them are not dissolved in it, then water may leave the solution, instead of moving towards it.

The second is if the substance can cross the membrane. If it crosses the membrane, then simple diffusion says that the substance will even itself out across the two sides of the membrane. Instead of the water crossing the membrane to even out the concentrations (i.e. osmosis), the substance itself will (i.e. diffusion). Therefore the substance isn't going to make any difference to how osmosis is occuring. You may start with more particles in the first solution (so water would move towards it), but if the particles diffuse through the membrane then you end up with an even number on each side. The remaining particles, which do not cross the membrane, will determine any difference in concentration and therefore osmosis will depend on them.

What is an osmole?

An osmole is a unit of measurement, and is essentially one mole of an osmotically active substance. Since a mole is 6.022 x 1023 molecules, an osmole is 6.022 x 1023 osmotically active molecules.

Because pressure and concentration are so intimately related, another way of describing moles and molarity is to say that one osmole is the number of osmotically active particles that exert an atmospheric pressure of 1 atmospheres when dissolved in 22.4L of a solvent at a temperature of 0 degrees celsius. But of course that's a very complicated way of thinking about it!

What is osmolarity?

The osmolarity of a liquid is basically the concentration of all the things dissolved in it. It's usually written in terms of the number of osmoles per litre. More specifically, it's the concentration of all the things which are osmotically active, that is, those things which are dissolved in it which are contributing to the osmotic pressure.

A solution will have a higher osmolarity if there is more dissolved in it that contributes to this osmotic pressure. A solution will have a lower osmolarity if there is less dissolved in it that contributes to osmotic pressure. Osmolarity is very similar to molarity - but with one important difference. In molarity, you're talking about the concentration of all the particles. In osmolarity, you're only talking about the concentration of those molecules which are osmotically active.

What is osmolality?

Osmolality is essentially very similar indeed to osmolarity, but with a slightly different definition, and measured in a slightly different way. In the same way that molality is moles per kilogram of solution and molarity is moles per litre of solution, osmolality is osmoles per kilogram of solution while osmolarity is moles per litre of solution. In other words, osmolality is expressed in terms of weight, while osmolarity is expressed in terms of volume.

The strict definition says that osmolaity is the molality of a stable solution which has the same osmotic pressure as the solution we are talking about. That seems rather back-to-front to me, but it ensures that you are talking about a stable environment, rather than the slightly more complicated active environment we see in life.

What affects plasma osmolarity?

Since ions have a charge, they do not easily pass through biological membranes. This means that they are osmotically active. Other molecules that are polar or which are too big will also not fit through the membrane. These all contribute towards osmolarity.

Normal human blood is made up mainly of red blood cells and plasma (the liquid everything is dissolved in). The most important substances which contributes towards plasma osmolarity are sodium, chloride and potassium. These contribute to the biggest proportion of osmotically active substances in the blood. Human plasma normally has an osmolarity of somewhere between 275 and 300 milliosmoles per litre.

Calculating plasma osmolarity requires you to add up the concentrations of all the osmotically active particles in the plasma. Since sodium and potassium are often bound to chloride, there is roughly the same number of chloride particles as there are sodium and potassium. This means the calculation is simplified to 2 x the concentration of [sodium plus potassium]. When someone with diabetes gets a build up of glucose, this can make a big difference to the osmolarity, so calculations of osmolarity should also include this. The formula which is commonly used to calculate osmolarity is therefore:

Plasma osmolarity in mmol/l = [urea] + [glucose] + 2([Na] + [K])

Urea is included in this calculation even though it diffuses across biological membranes and therefore is not osmotically active. This is probably more of a historical reason than anything else. Osmolarity has been calculated in the past by seeing how much the freezing point of something is reduced by the amount dissolved in it - the more dissolved in it, the more the freezing point is reduced. You see this when salt is used to make ice melt by reducing the temperature of the freezing point. This works fine when you're thinking about salts like sodium and potassium, but it messes us up when it comes to things like urea - because urea dissolved in water will cause the freezing point to drop, but it is not osmotically active. Fortunately the concentration of urea isn't very high - usually less than 10, compared to around 130 for sodium - so it doesn't skew the results too much. Plasma osmolality calculations are therefore more about calculating how much is dissolved in plasma, even though there is a slight difference between that and osmotic activity.

What is oncotic pressure?

Oncotic pressure is basically a kind of osmotic pressure but is specifically related to proteins. In the same way that ions and polar molecules tend to be osmotically active because they don't pass through membranes, proteins are huge molecules and don't pass through membranes easily. As a result they cause osmosis to happen to try and even up the concentrations of chemicals on either side of the membrane.

Another name for oncotic pressure is colloid osmotic pressure, because it is the osmotic pressure caused by proteins.

What affects plasma oncotic pressure?

Plasma oncotic pressure will obviously depend on how much protein there is in the blood. Albumin, a protein made by the liver, makes up around 60% of all the protein that you'll find in the blood. It's an important protein for transporting other chemicals around, because they latch onto it in order to use it as a courier. In the case of oncotic pressure, it's incredibly important in providing the protein in the plasma that is needed to make sure that oncotic pressure is maintained.

Those things which affect the concentration of proteins, especially albumin, will affect oncotic pressure. For instance, if you have a poor diet for a long enough length of time, you will start to produce less protein to fill your blood with. Similarly, if you have leaky kidneys, you can loose protein through your kidneys and again the oncotic pressure can go down.

Why are osmolarity and oncotic pressure important?

Osmolarity and oncotic pressure sound like the kinds of things that are very interesting in theory, but in fact have very little impact on real life. In fact, they are incredibly important and make a huge difference to the way that the body functions.

The first way that they are important is in controlling how much you pass urine. Part of the reason that you wee is to give the opportunity to control the amount of stuff dissolved in your blood. If your plasma osmolarity was allowed to increase too much, then through osmosis it would suck all of the liquid out of your cells and you would die. On the other hand, if your plasma osmolarity was allowed to decrease too much, then through osmosis liquid would leak into all of the surrounding tissues and they would swell up and die. Instead, your kidneys are usually very good at controlling this to make sure that there is a healthy balance; if the osmolarity is too high, it can either get rid of osmoles or retain water to make sure the osmolarity drops. If it's too low, it can get rid of water in the urine.

So, if you drink too much plain water, you dilute the blood and the osmolarity drops. The kidneys sense this, and you get rid of water so that the osmolarity returns to the right level. If, on the other hand, you take too much salt, your kidneys might decide to hold on to water in order to dilute the blood and make sure the osmolarity isn't too high. Holding on to water too much, however, can mess up the blood pressure mechanisms - which is why having too much salt is considered bad for you.

Oncotic pressure is important because capillaries are leaky. The pressure of the heart pumping sends the blood shooting off around the body, and it's necessary in order to make sure the blood gets to where it's going. However, it also means that there's a pressure out of the blood vessels that tends to force water out of the blood vessels and into the surrounding fluid - something we call hydrostatic pressure. However, because of oncotic pressure, there is an opposite effect drawing water back into the blood. These cancel each other out, so instead of loosing lots of liquid into the surrounding areas, only a small amount is lost over time.

Imagine what happens, then, when this delicate balance is upset. You might have less protein in the blood because it leaks out of the kidneys (as in nephrotic syndrome) or because the liver isn't making enough of it. Then the oncotic pressure isn't enough to cancel out the hydrostatic pressure. In these circumstances, you get liquid leaking out of the blood vessels. If this happens in the ankles (where the help of gravity is going to make the pressure greatest), you'll get swollen ankles. However, if it happens in the abdomen you'll get something called ascites, and if it happens in the lungs you'll get pulmonary oedema.

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