TeenToDoctor
Badges: 5
Rep:
?
#1
Report Thread starter 2 years ago
#1
Getting confused with Vd=Q/Cp! I've read the definition a bunch of times (The theoretical volume that would contain the total body content of the drug at a concentration equal in the plasma) but I'm finding it hard to get my head around what this actually means. Also, how is this value then proportionate to the body compartment in which a drug distributes?

I made it simpler to try and understand it - if the plasma concentration of a drug is say, 5g/L and I put in 20g of the drug into a body, what is the volume I would need to equate to the plasma concentration - 4L. That's fine, it's just how do you take this information and say the drug must, for example, distribute only within the plasma.
0
reply
macpatgh-Sheldon
Badges: 16
Rep:
?
#2
Report 2 years ago
#2
Hi,

Let me try to explain the concept to you in a different way:

The Vd is the apparent volume of fluid [apparent because it has no physical equivalent) that WOULD hold the total amount of drug in the body IF THE CONCENTRATION IN THAT APPARENT VOLUME WERE TO BE THE SAME AS THE ACTUAL CONCENTRATION IN THE PLASMA.

So, if for example, you take a drug that is highly protein bound e,g, warfarin or phenytoin, the ACTUAL concentration in the plasma would be lower [simply because the portion bound to protein is unavailable as part of the plasma fraction]; therefore, it follows that, if the concentration we are considering is lower, it will require a lesser amount of FLUID to hold the total amount of drug in the body i.e. a lower value of Vd.

Now, coming to your specific difficulty {copied: just how do you take this information and say the drug must, for example, distribute only within the plasma. ):- You know that blood volume (in the average 70kg male - before the advent of Mac and other fast food - now more like 100kg ) is about 5 l; so assuming a haematocit [the percentage of the blood volume occupied by cells, i.e. NOT plasma] of 40% odd [= 2 l], the plasma volume = 3 l. if we take your example of Vd = 4l, you can predict that, since the volume of fluid needed to hold the total drug amount in the body (Vd) is roughly the same as the plasma volume [3 l], nearly all the drug must be present in the plasma - you will know that, on aministration of a drug by most routes [including oral, IM [intramuscular] and IV [intravenous]), it will end up in the blood first, simply out of intestinal absorption (gastric absorption in the case of Scotch or a pint ).

Hope this makes it easier.

M (clinical pharmacologist)
1
reply
TeenToDoctor
Badges: 5
Rep:
?
#3
Report Thread starter 2 years ago
#3
(Original post by macpatelgh)
Hi,

Let me try to explain the concept to you in a different way:

The Vd is the apparent volume of fluid [apparent because it has no physical equivalent) that WOULD hold the total amount of drug in the body IF THE CONCENTRATION IN THAT APPARENT VOLUME WERE TO BE THE SAME AS THE ACTUAL CONCENTRATION IN THE PLASMA.

So, if for example, you take a drug that is highly protein bound e,g, warfarin or phenytoin, the ACTUAL concentration in the plasma would be lower [simply because the portion bound to protein is unavailable as part of the plasma fraction]; therefore, it follows that, if the concentration we are considering is lower, it will require a lesser amount of FLUID to hold the total amount of drug in the body i.e. a lower value of Vd.

Now, coming to your specific difficulty {copied: just how do you take this information and say the drug must, for example, distribute only within the plasma. ):- You know that blood volume (in the average 70kg male - before the advent of Mac and other fast food - now more like 100kg ) is about 5 l; so assuming a haematocit [the percentage of the blood volume occupied by cells, i.e. NOT plasma] of 40% odd [= 2 l], the plasma volume = 3 l. if we take your example of Vd = 4l, you can predict that, since the volume of fluid needed to hold the total drug amount in the body (Vd) is roughly the same as the plasma volume [3 l], nearly all the drug must be present in the plasma - you will know that, on aministration of a drug by most routes [including oral, IM [intramuscular] and IV [intravenous]), it will end up in the blood first, simply out of intestinal absorption (gastric absorption in the case of Scotch or a pint ).

Hope this makes it easier.

M (clinical pharmacologist)
Hi, that makes a lot of sense! Thank you so much for your help
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Bournemouth University
    Midwifery Open Day at Portsmouth Campus Undergraduate
    Wed, 16 Oct '19
  • Teesside University
    All faculties open Undergraduate
    Wed, 16 Oct '19
  • University of the Arts London
    London College of Fashion – Cordwainers Footwear and Bags & Accessories Undergraduate
    Wed, 16 Oct '19

How has the start of this academic year been for you?

Loving it - gonna be a great year (136)
17.82%
It's just nice to be back! (204)
26.74%
Not great so far... (274)
35.91%
I want to drop out! (149)
19.53%

Watched Threads

View All