Editing Volume of distribution (section)

==Examples==
{{main|Table of volume of distribution for drugs}}
{{See also|Pharmacokinetics#Metrics}}{{Tone|date=November 2024}}
If you administer a dose D of a drug intravenously in one go (IV-bolus), you would naturally expect it to have an immediate blood concentration <math>C_0</math> which directly corresponds to the amount of blood contained in the body <math>V_{blood}</math>. Mathematically this would be:

<math>C_0 = D/V_{blood}</math>

But this is generally not what happens. Instead you observe that the drug has distributed out into some other volume (read organs/tissue). So probably the first question you want to ask is: how much of the drug is no longer in the blood stream?
The volume of distribution <math>V_D</math> quantifies just that by specifying how big a volume you would need in order to observe the blood concentration actually measured.

An example for a simple case (mono-compartmental) would be to administer D=8&nbsp;mg/kg to a human. A human has a blood volume of around <math>V_{blood}=</math>0.08 L/kg 
.<ref name="alberts_table">{{Cite book | chapter-url=https://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Search&db=books&doptcmdl=GenBookHL&rid=mboc4.table.4143| chapter = Leukocyte functions and percentage breakdown|access-date=2007-04-14|via=NCBI Bookshelf|year=2005| vauthors = Alberts B | title = Molecular Biology of the Cell}}</ref>
This gives a <math>C_0=</math>100&nbsp;μg/mL if the drug stays in the blood stream only, and thus its volume of distribution is the same as <math>V_{blood}</math> that is <math>V_D=</math> 0.08 L/kg. If the drug distributes into all body water the volume of distribution would increase to approximately <math>V_D=</math>0.57 L/kg
<ref name="Guyton_1976">{{cite book|title=Textbook of Medical Physiology| vauthors = Guyton AC |year=1976|edition=5th|publisher=W.B. Saunders|location=Philadelphia|page=[https://archive.org/details/textbookofmedica0005guyt/page/424 424]|isbn=0-7216-4393-0|url=https://archive.org/details/textbookofmedica0005guyt/page/424}}</ref>

If the drug readily diffuses into the body fat the volume of distribution may increase dramatically, an example is [[chloroquine]] which has a <math>V_D=</math>250-302 L/kg
<ref>{{cite journal | vauthors = Wetsteyn JC, De Vries PJ, Oosterhuis B, Van Boxtel CJ | title = The pharmacokinetics of three multiple dose regimens of chloroquine: implications for malaria chemoprophylaxis | journal = British Journal of Clinical Pharmacology | volume = 39 | issue = 6 | pages = 696–699 | date = June 1995 | pmid = 7654492 | pmc = 1365086 | doi = 10.1111/j.1365-2125.1995.tb05731.x }}</ref>

In the simple mono-compartmental case the volume of distribution is defined as: <math>V_D=D/C_0</math>, where the <math>C_0</math> in practice is an extrapolated concentration at time = 0 from the first early plasma concentrations after an IV-bolus administration (generally taken around 5 min - 30 min after giving the drug).

{| class="wikitable"
|+Compounds with different V<sub>D</sub> for a 70 kg man<ref>{{Cite web |url= https://www.cambridgemedchemconsulting.com/resources/ADME/distribution.html |title=Distribution and plasma protein binding| vauthors = Swain C |website=Cambridge MedChem Consulting |language=en |access-date=2020-04-02 }}</ref>
| '''Drug''' || '''V<sub>D</sub>''' || '''Comments''' 
 |- 
 | [[Warfarin]] || 8 L  || Reflects a high degree of plasma protein binding. 
 |- 
 | [[Theophylline]], [[Ethanol]]|| 30 L  || Represents distribution in total body water.
 |- 
 | [[Chloroquine]] || 15000 L || Shows highly [[lipophilic]] molecules which sequester into total [[body fat]].
 |-
 | [[NXY-059]] ||  8 L  || Highly charged [[hydrophilic]] molecule.
|}