This document describes the one compartment open model for pharmacokinetics. It states that the body is treated as a single, homogeneous unit where drugs distribute rapidly throughout and move dynamically in and out. Elimination follows first-order kinetics. The model can describe intravenous bolus administration, continuous intravenous infusion, or extravascular administration where absorption is either zero-order or first-order. Distinction between these absorption processes can be seen when plotting amount of drug remaining to be absorbed versus time.

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3. One Compartment Open Model
 The body is considered as a single, kinetically homogeneous unit.
 This model applies only to those drugs that distributes rapidly
throughout the body.
 Drugs move dynamically in an out of this compartment.
 Elimination is first order ( monoexponential ) process with first order
rate constant.
 Rate of input (absorption) > rate of output (elimination).
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4. Types
Depending on rate of input, several one compartment open
models are :
1. One compartment open model, i.v. bolus administration.
2. One compartment open model , continuous i.v. infusion .
3. One compartment open model, e.v. administration, zero order
absorption.
4. One compartment open model, e.v. administration, first order
absorption .
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5. Intravenous Bolus Administration
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6. Intravenous Infusion
 Rapid i.v. injection is unsuitable when the drug has potential to
precipitate toxicity or when maintenance of a stable
concentration or amount of drug in the body is desired. In such a
situation, the drug (for example, several antibiotics, theophylline,
procainamide, etc.) is administered at a constant rate (zero-order)
by i.v. infusion.
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7. Intravenous Infusion
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8. Extravascular Administration
 When a drug is administered by extra vascular route (e.g.
oral, im., rectal, etc.), absorption is a prerequisite for its
therapeutic activity.
 The rate of absorption may be described mathematically
as a zero-order or first-order process. A large number of
plasma concentration- time profiles can be described by a
one- compartment model with first-order absorption and
elimination.
 However, under certain conditions, the absorption of some
drugs may be better described by assuming zero-order
(constant rate) kinetics.
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9. Extravascular Administration
 Distinction between zero-order and first-order absorption processes.
Figure a is regular plot, and Figure b a semi log plot of amount of drug
remaining to be absorbed (ARA) versus time t.
Time
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10. Extravascular Administration
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Figure : The absorption and elimination phase of the
plasma concentration – time profile obtained after
extravascular administration of a single dose of a drug.
dXa

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