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Extracellular Fluid (ECF) & Intracellular Fluid (ICF)

by Thad Wilson, PhD
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    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.


    About the Lecture

    The lecture Extracellular Fluid (ECF) & Intracellular Fluid (ICF) by Thad Wilson, PhD is from the course Membrane Physiology.


    Included Quiz Questions

    1. Potassium
    2. Sodium
    3. Chloride
    4. Calcium
    5. HCO3-
    1. Calculates the membrane potential at equilibrium between the two sides of the membrane for a particular ion
    2. Calculates the membrane potential at any given moment of cell activity
    3. Calculates the number of moles of electrons transferred across the membrane
    4. Calculates the total number of ions present in the cell
    5. Calculates the total number of ions transferred across the membrane
    1. A constant
    2. A charge
    3. The potential for sodium
    4. The potential for chloride
    5. The temperature of the cell
    1. All these molecules want to be at equilibrium between the two sides of the membrane
    2. All charges are at equilibrium between both sides of the membrane at rest
    3. All ions balance each other in the intracellular fluid
    4. There is an ion imbalance in the extracellular fluid
    5. Charges separate spontaneously between two sides of a semipermeable membrane such as the cell membrane
    1. Potassium
    2. Calcium
    3. Sodium
    4. Chloride
    5. Hydrogen
    1. A pump that creates an electrical potential across the membrane
    2. A hydrogen pump
    3. A pump that removes potassium from the cell
    4. A pump that allows glucose to enter the cell
    5. A pump that allows calcium in the cell
    1. Na/KATPase
    2. A transporter
    3. A ligand-gated pump
    4. A channel protein
    5. An antiporter
    1. …the number of charges of a particular ion…in the first denominator
    2. …the intracellular concentration of a particular ion…in the first denominator
    3. …the extracellular concentrator of particular ion…in the first numerator
    4. …the number of charges of a particular ion…in the second denominator
    5. …the constant of temperature times the universal gas constant…in the second numerator

    Author of lecture Extracellular Fluid (ECF) & Intracellular Fluid (ICF)

     Thad Wilson, PhD

    Thad Wilson, PhD


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    nice and exciting
    By rene p. on 18. September 2017 for Extracellular Fluid (ECF) & Intracellular Fluid (ICF)

    wonderful lecture by Dr Thad Wilson...........i got a glimpse into the minute details of the topic as a whole..........thank u sir..............