Name: Drug Absorption Overview - Pharmacology Lect 2
Uploaded: 2013-10-25T09:09:47.000Z
Duration: 17 min 14 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Let's jump into  section two on our series on pharmacokinetics

and we're going to talk about absorption and bioavailability today.

So a quick review from the last lecture. What did we do then?

Well first, we defined pharmacokinetics and you need to know this before we move on.

And so, pharmacokinetics is really looking at the change in drug concentration as the drug moves through the different compartments of the body.

So as the drug moves through your body, how does this concentration change.

And we are going to talk about absorption today and absorption is really the process of a substance entering the systemic circulation.

And so we are going to look at how the concentration changes as a drug enters the systemic circulation

And just a little key point to remember is that absorption is highly dependent on the route of administration.

So let's get to our most important concept of the day first and then we'll get to the good stuff.

So the most important concept. Well this is something that you probably learned in high school.

Concentration is equal to mass over volume.

But in terms of pharmacology, what are we talking about?

Well, when we talk about the mass, we're really talking about how much drug we gave right?

If you look at the dosages for most drugs, it's given in milligrams.

So that's referring to mass - the amount of drugs given in milligrams.

So, if we're talking about blood volume, we want to use you know liters.

If we're talking about the volume in a syringe, we might use cc's or cubic centimeters or milliliters.

In any case, the volume depends on what you're dissolving it in.

So if a drug is dissolved in a certain volume, and that gives us our concentration.

Well, blood volume - blood volume is about give or take 5 liters

And commonly used as opposed to blood volume which is not really used when we talk about pharmacokinetics is plasma volume

And so plasma as you know relates to blood and that it's you know

you take the plasma, throw in the - you take the blood, throw in the center fuse, you get out that pale yellow substance and that is your plasma.

And so it's your blood volume - let's just say your blood volume minus the cells.

And your cells make up about 50% of your blood.

So that leaves us with about 50% left and that's about 2.5 liters.

Now on an exam, you might see this actually written in a different way.

So they might write out that your plasma volume is equal your blood volume x 1 minus your Hematocrit.

So remember, hematocrit is what? It's the volume - it's a percentage.

And it's the percentage of your blood volume that the red blood cells take up.

And so, if I was going to multiply this out, I would get blood volume x 1 minus my blood volume x my hematocrit.

So just in case you saw this a different way.

And finally, just to remind you, 1 cc is 1 cubic centimeter and that is equal to 1 milliliter.

So let's do a problem and solidify this concept.

So a 10 mg dose of drug X is 100% absorbed, what is the plasma drug concentration?

That is our mass. That is the amount of drug and we know it's 100% absorbed.

So this - the only way anything is ever 100% absorbed is if you give it IV

and when I say absorbed 100% that means it's all getting into the plasma.

So what we want to do is figure out the plasma drug concentration.

So the concentration in the plasma is equal to the mass over the volume.

Well the mass is the 10 mg of drug that I gave.

Well we just said the volume of the plasma is 2.5 liters.

So overall, I get a concentration of plasma of 4mg/L and we're going to come back to this number on the next slide.

So, here's our case and this is our "in" case

so, we're talking about drugs getting into our body.

So we're going to give a drug IV bolus which means intravenously and bolus means that it's all at once

and it goes into one compartment and that compartment is the plasma.

and we also assume here that there is no elimination.

Honestly, this is the easiest case that we can do.

So we start off by administering this drug IV bolus and we want to get it into our systemic circulation.

So let's just draw a little picture here.

Here is our syringe, we give this drug IV bolus and there's a certain volume of drug in here

and there's a certain concentration of that drug in here.

And so remember, concentration is equal to what?

Concentration is equal to mass divided by volume.

So what we want to do is draw a little graph

and on the Y axis of this graph, we're going to put the plasma drug concentration.

and on the X axis, we're going to put time.

And so if I'm going to draw a graph here, well I would say I gave a fixed amount of drug right - a certain amount

and it goes into our systemic circulation so this is let's say our blood volume or plasma volume

and it has nowhere to go. There's no exit.

So,I'm going to get a set concentration and because the mass is set (how much drug) and the volume is set

and it doesn't go anywhere. So I just get a graph that is pretty boring.

and there is no elimination so this is a straight line.

Now I really can't do it like this. I need to actually see some numbers.

So numbers help me. I think they actually help everybody.

So let's start off by figuring out - Let's figure out the plasma drug concentration.

So in order to figure that out, I need to know what was the total mass of drug I gave and the total plasma volume.

So 10 cc means that the volume total was equal to 10 ml.

Now that doesn't mean anything to me because in order to figure out how much drug I gave, I need to figure out the mass.

Let's say the concentration of drug in this syringe is let's say is 1 mg/ml.

While using this, I can now figure out the amount of drug I gave.

So the volume in the needle is 10 ml. The concentration in the needle is 1 mg/ml.

And so just by looking at this, if I multiply these two numbers together.

Milliliters will cancel out and I get the amount of drug given a.k.a. the mass.

And so I do that and I see I gave 10 mg of this drug.

Now, remember from that last slide, we had blood which is about 5 L and plasma which was the blood minus the cells which is equal to 2.5 L.

So if I want to figure out the plasma concentration - plasma concentration and the brackets indicate concentration.