Pharmacokinetics III

Time Course of Drug Concentration in
Plasma
A. Drugs are taken up into and eliminated
from the body by various routes. The
body thus represents an open system
wherein the actual drug concentration
reflects the interplay of intake (ingestion)
and egress (elimination). When an
orally administered drug is absorbed
from the stomach and intestine, speed
of uptake depends on many factors, including
the speed of drug dissolution (in
the case of solid dosage forms) and of
gastrointestinal transit; the membrane
penetrability of the drug; its concentration
gradient across the mucosa-blood
barrier; and mucosal blood flow. Absorption
from the intestine causes the
drug concentration in blood to increase.
Transport in blood conveys the drug to
different organs (distribution), into
which it is taken up to a degree compatible
with its chemical properties and
rate of blood flow through the organ.
For instance, well-perfused organs such
as the brain receive a greater proportion
than do less well-perfused ones. Uptake
into tissue causes the blood concentration
to fall. Absorption from the gut diminishes
as the mucosa-blood gradient
decreases. Plasma concentration reaches
a peak when the drug amount leaving
the blood per unit of time equals that
being absorbed.