Before we get into the nitty gritty of how the kidney works, I want to review some concepts surrounding osmosis and diffusion. So to start, let's go over a little terminology. A solute is a substance that's dissolved in a solution, and an electrolyte is a specific type of solute that when it dissolves will actually dissociate into ions. For example, we have salt here, this is just like table salt, and it will dissociate into a sodium ion and a chloride ion in water. Now, when solutes dissolve in water, if they don't spread out evenly, they will form a concentration gradient, which is basically a difference in concentration of a solute over some area.
So here you can see we have a high concentration on this side and a low concentration on this side. There are blue dots representing the solutes. This means that we have a concentration gradient across this area. Now what's going to happen if we have a concentration gradient, and nothing's blocking the movement of these solutes, is we're going to have diffusion, which is the movement of molecules or atoms from an area of high concentration to an area of low concentration. And you can see that happening here, this is diffusion, and you can see we have a high concentration here, but these solutes are going to diffuse and spread out evenly throughout the solution.
Now, if we have something blocking those solutes from moving, there won't be any diffusion of solutes. As you can see here, we have a u-shaped tube, and there is a higher concentration of solutes on this particular side, and a lower concentration of solutes on this side. But, there's a membrane separating the two sides of the tube. So those solutes won't be able to pass through on either side. So they can't diffuse to spread out evenly.
But what's going to happen is we're going to have movement of water across that membrane or osmosis. And the water is going to move from the area of low solute concentration to the area of high solute concentration. And the result is going to be that the water will balance the solute concentrations on each side. So here, we actually have a higher volume on this side now, but as you can see the concentrations of the two sides here and here is the same, and that is due to osmosis. And that membrane is displaying selective permeability.
Right? That is, the ability of solutes to cross or the prevention of solutes from crossing due to the presence or absence of transport proteins. So here, there are no transport proteins on this membrane for those solutes, so they are not going to be able to cross. It's impermeable to them. Now, there are terms to describe the concentrations of solutes of two solutions.
We use the term osmolarity to talk about the concentration of a solute and its measurement of moles of dissolved solute per liter. You don't really need to worry about units for this. I mean, this is biology. You know, we just want to kind of think about it in terms of qualitative terms, like something having a higher osmolarity than something else. And there are specific terms to describe that.
So if a solution has a higher osmolarity than another solution, we say that it's hyperosmotic. So, if the solution that this cell here is sitting in is hyperosmotic, I'm just going to write hyper, water is going to leave the cell because this area outside the cell has a higher osmolarity, meaning that water is going to want to move out of the cell into that area of higher solute concentration in order to try to balance the solute concentration between the two environments, and it's going to cause the cell to shrivel. So again, the term for that type of solution is hyperosmotic. Here we have an example of an isoosmotic solution, where the solution outside the cell and the cytosol inside the cell are of the same osmolarity. So the water is going to flow in and out, at the same rate, so there's going to be no net change in the amount of liquid in the cell.
Lastly, the hypoosmotic situation, which you can see here, is going to be when the solute concentration outside, or sorry, the osmolarity is lower outside the cell than inside the cell. And so water is going to enter the cell to try to balance those solute concentrations. So again, you know, hyper is higher osmolarity, hypo is lower osmolarity, and iso is the same osmolarity. Now, there are kind of like two osmoregulatory strategies that you'll see organisms have. There are osmoconformers, which tend to be marine organisms that are mostly isosmotic with their environment.
So these guys aren't going to actively regulate their internal osmolarity. Instead, they're just going to let it be isosmotic with their environment. And that's okay because these are marine organisms, you know, they live in salty water that has a very high solute concentration, and is high enough that it's similar to the concentration inside cells, which is fairly high actually. Now, osmoregulators take a more active approach. These are guys who are going to actively regulate their osmolarity, the osmolarity of their internal environment.
Lastly, I want to mention one really kind of weird strategy that some organisms show called anhydrobiosis, which is a type of cryptobiosis. It's basically an adaptation that allows organisms to survive without any water. These organisms will basically dry out or desiccate, and can still survive for quite some time like that. And an example of that is this little guy right here, its technical name is tardigrade, though I like the common name for it which is a water bear. And this water bear is all nice and happy and plump with water, but these guys can dry out and shrivel into basically like nothing, and still live like that for quite some time.
So pretty wild adaptation, lots of organisms have, you know, developed different strategies to deal with water balance. So with that let's flip the page.
