When two molecular species react with each other (a bimolecular reaction), but one of the substances is present in a concentration greatly in excess of the concentration of the other and/or does not change in concentration during the reaction, the velocity of the reaction at any time is really determined only by the concentration of the other substance. Such a pseudo-monomolecular reaction, because the velocity is determined by the concentration of only one of the two reactants, still follows first order kinetics.
Following administration of a drug, it may be eliminated from the body only after "reacting " with tissue components which are present in high concentrations and which are not used up to any degree during the drug's stay in the body. Such eliminative processes mimic pseudo-monomolecular reactions, and the drug is eliminated from the body according to first order kinetics,. The apparent velocity constant determined for such a process is called the elimination rate constant, kel, and the elimination half-life can be computed as 0.693/kel.
Cf.,
Half-Life,
t1/2(t1/2),
Zero-Order Kinetics.
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