The cells in our bodies are in constant flux through the processes of osmosis and diffusion. Learn about how saturation levels force change, and why we're lucky they do.
Scuba Dives and Plane Flights
Let's say that you and a friend went away to a tropical island to go SCUBA diving. You spent your last day sitting on the beach and enjoying the sunshine, but your friend loved SCUBA diving so much that she went SCUBA diving on her very last day that the two of you had on the island. Then you got on a plane to go back home. Shortly after the plane took off, the plane had to land again because your friend was in a lot of pain. What happened?
The human body is over 60% water
Solutions and Solubility
It turns out that our bodies are largely made of water. Over 60% of our bodies are made of water, in fact. So our blood, tissues, organs and cells are essentially solutions, or mixtures of solvent and solute, where solvent, the substance that dissolves other substances in it, is water. And there are many different solutes, or substances that are dissolved in solvent. In the cases of our bodies, these are sugars, salts, proteins and dissolved gasses.
Now, what affected your friend in this case is the solubility of nitrogen in water. Solubility is the maximum amount of a solute that will dissolve in a particular solvent at a specific temperature and pressure. Nitrogen
So if the pressure changes slowly over time, like divers normally do when they come up from a dive, the nitrogen dissolved in the blood will undergo diffusion over time and everything will be totally fine.
Now diffusion in this case means the spread of molecules from areas of high concentration to areas of low concentration. So in this case, as your friend comes up from her SCUBA dive, the concentration of nitrogen molecules that are dissolved in her body is higher than that in the surrounding ocean and in the surrounding air. So over time, this nitrogen will leave her body to move toward areas that have lower concentration.
Nitrogen diffuses to the surrounding areas of lower concentration as divers come up
So something similar to this is let's say you put a little bit of salt in water and you just let it sit there. If you come back later, you'll find that the salt crystals are no longer there and the water tastes salty. This is because water dissolves salt into a solution and as the water dissolves the salt, the area immediately surrounding the salt crystals becomes very salty. This causes the spread of sodium and chloride ions away from the salt that's dissolving so that the solution can become more homogeneous.
Now provided your friend did everything right, she would end up with an unsaturated solution of nitrogen in her blood, just like we all have just from breathing because nitrogen makes up a significant portion of the air we breathe. An unsaturated solution is a solution that has less than the maximum amount of solute.
Now in the case of your friend, she probably followed the rules of SCUBA diving and came up slowly, but her blood was still saturated with nitrogen when you guys got on that airplane.
Now a saturated solution is a solution where the solvent has dissolved all of a particular solute that it can. So an example of this is let's say you're making salt water; you can pour salt in, and put salt in and keep putting salt in, but at some point, no matter how much you stir, that salt will not dissolve and that is because you have a saturated solution of sodium chloride and water.
So in the case of your friend, she has a saturated solution of dissolved nitrogen in her blood and in her tissues. As soon as you took off, you went to a higher altitude - about 5,000 feet or 1,500 meters of cabin pressure. All of a sudden, the solution had more dissolved nitrogen than it should have and it became supersaturated. A supersaturated solution is one where there's more dissolved solute than than there would be at an equilibrium for a saturated solution. Now supersaturated solutions, as you might imagine, are really unstable. The solute will precipitate out at the slightest provocation.
After a certain point, salt no longer dissolves in water because the solution has become saturated
In this case, you went to a higher altitude so the nitrogen started coming out of your friend's tissues way too fast and started to form bubbles in her veins. This is incredibly painful, and is actually called the bends because people bend over from pain.
So fortunately, you had a really good cabin crew and pilot and they were able to land the plane and your friend was able to go to the hospital. Now your friend isn't feeling great, so they want to hydrate your friend, so they give your friend a solution of saline, or sodium chloride in water.
Now let's think about this for a second. You're concerned about your friend and want to make sure it's the right kind of solution. Now if we take a step back, we can think about our cells. Our cells have what are called semipermeable membranes. Semipermeable membranes are thin slices of material that contain tiny holes and can let solvent molecules, like water, through. An example of this that you might be familiar with in your day to day life is Saran wrap; Saran wrap can let air and water vapor travel in and out so that your food stays fresh longer.
The process of solvent molecules, like water, moving across a semipermeable membrane is called osmosis.
Now as we learned from diffusion, if there's high solute concentration in one place and low solute concentration in another place that the molecules can get to, the tendency is for these things to be equalized. So if you have a whole lot of solute in one place (for example, inside a cell), and a lesser concentration of solute outside the cell, your solvent will start to flow into the cell to try to equalize the concentration of solute in the two solutions. The amount of external pressure that is needed to equalize these two solutions is called osmotic pressure.
Now in this case, this makes us think, 'okay, we want our friend to stay pretty much the same. We want her to get well hydrated without having any problems where a whole lot of water flows into her cells or a lot of water flows out of her cells due to osmotic pressure.' So we want a solution of sodium chloride that is isotonic, or has the same concentration of solute as the solution in her body (which is typically her blood) so that she can become hydrated with no issues.
Solvent will flow across semipermeable membranes to equalize the solutions
Giving our friend an isotonic sodium chloride solution is much better than giving her one that's hypotonic, or a solution that has a lower concentration of solute than a given solution, such as the blood. This would cause the influx of water into the body's tissues and could potentially cause some cell damage or cause cells to explode because there's just too much water going in.
It's also better than giving your friend a solution that's hypertonic, a solution that has a higher concentration of solute than a particular solution. The difference in concentration of the solute there could cause water to flow out of the cells, leaving them all shriveled up. Ideally, we want a solution that has the exact same concentration so that water can flow in and out without causing any disruptions.
So to recap what we've learned today, we've learned that solutions consist of solute dissolved in solvent; that solubility is the amount of a substance that can be dissolved in a particular solvent at a particular temperature and pressure.
We've learned that solutions can be saturated, that is, they can have the maximum possible amount of solute dissolved in their solvent. Or they can be unsaturated, which is where have less than the maximum possible amount of solute dissolved in them.
We've also learned that solutions can be supersaturated, which is where they have more than the maximum amount that would be dissolved at saturation under normal conditions, but they're very unstable.
We've learned that diffusion is the passive movement of substances from areas of high concentration to areas of low concentration. Osmosis is the diffusion of solvent across a semipermeable membrane, such as our cell membranes.
And we also learned that we should not fly within 24 hours of going SCUBA diving!