Extracellular Fluid (ECF) & Intracellular Fluid (ICF)

00:01 So now let's take a couple of examples from cells that's representative of what's in our body.

00:09 So we have ECF which is extracellular fluid, that's the fluid around the cell.

00:14 And ICF, which is the intracellular fluid.

00:18 So if we take first, one particular ion, and we'll do this one at a time.

00:25 Sodium has a concentration outside the cell of around 145 mmol. Inside the cell, it's very low, about 12.

00:35 So there's a gradient across the membrane set up here.

00:41 If we look at potassium, it is high within the cell, about 120 mmol, versus 4 on the outside.

00:51 Calcium is what looks like to be low on both side, but in actuality, the extracellular fluid has more calcium than the intracellular fluid.

01:04 Intracellular fluid has almost none free calcium.

01:09 And finally we have chloride, which is high on the outside of the cell and low on the inside.

01:16 One of the things you should notice about these various molecules is their charge.

01:22 Sodium and potassium have a positive charge, and they have a +1. They're considered monovalent.

01:30 Calcium is a +2. It's divalent. And then chloride is an anion or negatively charged molecule.

01:40 The other thing to keep in mind is which direction the arrows are pointing.

01:45 You'll notice that they always point from the base of the highest number to the area of the lowest number.

01:51 Why? Because all these molecules want to get into equilibrium. Meaning that they'd be the same on both sides of the membrane.

02:00 That is their goal. They want to live in harmony and be the same on both sides of the membrane.

02:07 The membrane though, is going to be somewhat selective about which ones it allows to pass.

02:18 So we can calculate the exact voltage difference based upon those concentrations.

02:26 Now it's not important to be able to calculate this on a minute by minute basis.

02:31 There are only going to be numbers in which you have to be familiar with.

02:36 So for example, you see that this potential for sodium is around +61 mV.

02:46 So that's based upon the concentration in the extracellular fluid versus the intracellular fluid.

02:52 And you're gonna ask how in the world did you figure out that that was 61 mV.

02:58 You could have done it in two ways.

03:00 One of them, we could have put a reference electrode on one side of the membrane and then add electrode on the other side.

03:07 Or we can use a formulated calculator, which is called the Nernst equation.

03:13 At the Nernst equation, you take 61.5, divide it by the valence, times the log of the concentration inside the cell, over the concentration outside the cell.

03:26 So we either directly measuring it or using the Nernst equation, you can calculate the particular voltage that the concentration of those two ions inside and outside the membrane would cost.

03:43 If we look at potassium, you'll notice that it also has a voltage.

03:49 But the potassium potential comes out to be -90. Why? Because potassium was high inside the cell and low outside.

03:59 Calcium's membrane potential, very high. 120 in this particular example.

04:05 And again, that's based upon the ion concentration in either the measured value or the Nernst value.

04:12 Remember for the Nernst value, this one is going to be different because it's divalent.

04:17 So you're gonna have to take 61.5 divide it by now 2, because it has a +2 charge, times the log of the concentration on the inside over the concentration in the outside of the cell.

04:31 The final ion that we have here is chloride. And chloride is a -52 mV.

04:39 And remember that it has a higher concentration on the extracellular fluid than the intracellular fluid.

04:45 So the arrows become very important and their particular Nernst equation value becomes important.

04:53 So we have +61, +120, -90, and minus about 52. All from the ions that we had, or we directly measure them from the cell.

05:07 So that looks like there's four numbers here, what is the cell really at? Do we simply just add up all four of those and divide by four, so we get a mean? That would be too easy, wouldn't it? No. What we need to do is add another factor in play.

05:30 That other factor is which channels are opened, and which channels are closed.

05:35 Because again if you don't let the ions travel across, they don't count.

05:41 So we have to make sure which ions move becomes more important.

05:46 So let's start with calcium.

05:48 At rest, calcium is closed. Therefore we don't actualize that potential of 120.

05:59 Sodium channels also close at rest. Therefore that +61, non factor.

06:09 What is a factor? Potassium. Potassium channels are open at rest. Therefore, an excitable cell will have a negative membrane potential.

06:23 Because there's a conductant for potassium, it will be more likely close to the potassium equilibrium, which is -90.

06:33 So there will be some cells in the body that are at a -90 if these cells are opened all the time.

06:40 Most cells that were not at -90, they are a little bit above that, and that's because of one other factor, and that's the sodium-potassium pump, or the sodium-potassium ATPase which is constantly cycling and cycling and cycling, moving what? Potassium back into the cell and sodium out.

07:01 That is called electrogenic pump because it creates some electrical potential across the membrane.

07:09 Calcium pump is also there and that can also be part of the issue but it's primarily the sodium-potassium ATPase.

07:16 What we have is -70. You also notice that I didn't talk about chloride.

07:22 Chloride channels are also closed at this condition so they are not part of the membrane potential, just like calcium and sodium.