# PharmPK Discussion - Volumes of distribution

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• On 18 Nov 1999 at 19:53:24, shobha rani hiremath (shobha24.at.yahoo.com) sent the message
`dear all,i understand that there are more than one Vd for a twocompartment model.1.volume of distributon of centralcompartment,2.V of peripheral compartment,3.Vd(area)or Vd (beta)=CL/beta .theoretically speaking,what doesVd (area)mean? can anyone be kind enough to explain?thank you in advance.shobha`
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• On 19 Nov 1999 at 19:32:36, David_Bourne (david.aaa.boomer.org) sent the message
`[Two replies - db]X-Originating-IP: [132.204.55.237]From: "samia ezzine" To: PharmPK.aaa.boomer.orgSubject: Re: PharmPK Volumes of distributionDate: Fri, 19 Nov 1999 08:17:56 PSTHi,The volume of distribution by area (Vd)area is also known as Vz or Vd(beta). After a drug is distributed, the total amount of the drug inthe body during the elimination phaseis calculated by using vd(beta).In fact,  Vd (beta) is obtained at the psudo-distribution phase.I wish that you have your answer.---From: "Melethil, Srikumaran K." To: "'PharmPK.-a-.boomer.org'" Subject: RE:  Volumes of distributionDate: Fri, 19 Nov 1999 16:32:27 -0600This is in response to Shoba Rani Hiremath's question on the meaningof Vd(beta)in a 2 comp. model. It is apparent volume of distribution in the beta phase, orthe volume after the drug has achieved distribution equilibrium. Or, theVd(beta) * Cp = the amount of drug in the body in the beta phase.Hope this clarifies the concept.Sri Melethil, Ph.D.Professor of Pharmaceutics and MedicineUniversity of Missouri-KCSchool of PharmacyRoom 203-B, 5005 Rockhilll RoadKansas City, MO 64110816-235-1794 (fax: 816-235-5190)`
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• On 21 Nov 1999 at 16:14:40, Nick Holford (n.holford.aaa.auckland.ac.nz) sent the message
`My 2c:Vbeta is an utterless worthless parameter.Why do you want to know any apparent PK volume?First of all you might want to use it to describe (or predict)concentrations using a PK model. But Vbeta is not a parameter of a 2cpt model. It is derived from Mickey Mouse PK i.e. using AUC and"terminal" half-life. You cannot describe a 2 cpt model with 2parameters. You need 4 (plus parameters for drug input).Second, you might be interested in understanding something about thephysicochemical and anatomical basis of what determines therelationship between amount of drug in the body and itsconcentration. Vbeta is poor for this also because it is notindependent of the other processes involved in drug dispositionespecially the drug distribution phase. Vss is a much betterparameter for this purpose.So my advice is to consider Vbeta as a historical quirk that wasinvented before people figured out how to estimate volumes that wereindependent of other parameters such as Vss. It has no role at all incontemporary or future pharmacokinetic science.--Nick Holford, Dept Pharmacology & Clinical PharmacologyUniversity of Auckland, Private Bag 92019, Auckland, New Zealandemail:n.holford.-a-.auckland.ac.nz tel:+64(9)373-7599x6730 fax:373-7556http://www.phm.auckland.ac.nz/Staff/NHolford/nholford.html`
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