# volume of distribution

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Concept of Volume of Distribution
Distribution volumes:
Body water divided into : vascular, interstitial and
intracellular fluid.

Concept of Volume of Distribution
The real distribution volume of a drug is related to
body water and can never exceed body water.

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Concept of Volume of Distribution
Fluid
Volume (L)
Volumes
Plasma
3 - 4
measured by
Interstitial
10 - 13
various test
Intracellular

materials
25 - 28
Total body water
40 - 46
It is possible for Vd to be close to a recognizable
volume. This would happen only if the drug is
uniformly

distributed
in
one
of
these
"compartments", but this is rare.

Concept of Volume of Distribution
In beaker 1, the conc.
Dose = 10 mg
throughout the beaker is the
same and the Volume is the
same as the size of the
beaker.

Conc. = 10 mg/L
Conc. = 2 mg/L
In beaker 2, after a rapid
equilibrium, distribution b/n
the solution and the charcoal
may be complete. However,
drug conc. within the beaker
is not uniform. Much of the
drug

is
held
with
the
charcoal
leaving
smaller
Solution
Charcoal
conc. in the solution. After
(Plasma)
(tissues of the body) measuring the drug conc. in
The volume of solution is the same in both
the solution, the Volume is
beakers = 1 L
much larger.

Concept of Volume of Distribution
Volume of distribution :
The apparent volume of distribution is the
theoretical volume of fluid (not only plasma) into
which the total drug administered would have to
be diluted to produce the concentration in
plasma.

Volume of distribution has nothing to do with
the actual volume of the body or its fluid
compartments

but
rather
involves
the
distribution of the drug within the body.

Concept of Volume of Distribution
For drugs that are highly tissue-bound,
comparatively little of a dose remains in the
circulation to be measured; thus, plasma
concentration is low and volume of distribution
is high. Drugs that remain in the circulation tend
to have a low volume of distribution.

Volume of distribution provides a reference for
the plasma concentration expected for a given
dose. Each drug is uniquely distributed in the
body. Some drugs distribute mostly into fat,
others remain in ECF, and others are bound
extensively to specific tissues.

Concept of Volume of Distribution
Acidic drugs (eg, warfarin, aspirin) are highly
protein-bound and thus have a small volume of
distribution.

Basic drugs (eg, amphetamine, meperidine) are
extensively taken up by tissues and thus have a
volume of distribution larger than the volume of
the entire body.

Volume of distribution is a volume into which a drug
appears to distribute with a concentration equal to its
plasma concentration. A parameter used to define the
process of distribution and called "the apparent volume
of distribution (V )".

d

Concept of Volume of Distribution
How Vd is measured ?

100
dose
1000 mg

g/L)
Extrapolate to
Vd find
= C = 50 mg/L
=

0 C
50 mg/L
0
(m
50
c.
Dose
= = 1 g
20 L
40
30
Plasma Con
20
Log
10 0 1 2 3 4 5 6 Time
20 L
=
x 100 = 28.6%
70 kg

Concept of Volume of Distribution
How Vd is measured ?

100
dose
1000 mg

g/L)
Extrapolate to
Vd find
= C = 50 mg/L
=

0 C
50 mg/L
0
(m
50
c.
Dose
= = 1 g
20 L
40
30
Plasma Con
20
Log
10
for a 70 kg subject, the Vd
0
1
2
3
4
5
6 Time
in percentage of body wt is:
C = 50 mg/L
0

20 L
=
x 100 = 28.6%
70 kg

Concept of Volume of Distribution
Schematic representation of the
one-compartment model
Volume = Vd
i.v.
Elimination
dose
Plasma Concentration = C