PHARMACOKINETICS
Pharmacokinetics is the study of the drug concentration during absorption, dis-
tribution, and elimination of a drug in the patient. About 80% of all drugs are
administered orally and flow through the gastrointestinal tract (GI) into the small
intestine where the membrane of the intestine absorbs drug particles passing
them into the bloodstream, where plasma circulates the particles, throughout the
body. Drug molecules move to the intended site of action in the plasma but
sometimes this journey can be limited because they have to get into the interior
of a cell or body compartment through cell membranes. These membranes could
be in the skin, the intestinal tract, or the intended site of action. Drug particles
then attach themselves to receptor sites resulting in its therapeutic effect.
There are three ways in which drug particles are absorbed. These are:Passive Diffusion
Passive diffusion is the flow of drug particles from a high concentration to a low
concentration—similar to how water flows downstream. There is a higher concen-
tration of water upstream than there is downstream. There is no energy expended
in passive diffusion because drug particles are moving along the natural flow.Active Diffusion
Active diffusion is how drug particles swim upstream against the natural flow
when there is a higher concentration of plasma than there is of drug particles.
Drug particles don’t have enough energy to go against the natural flow without
help. Help comes from an enzyme or protein carrier that transports drug parti-
cles upstream across the membrane and into the plasma. The enzyme or protein
carrier expends energy to move drug particles.Pinocytosis
Pinocytosis is the process of engulfing the drug particle and pulling it across the
membrane. This is similar to how you eat an ice pop by engulfing a piece of it
in with your mouth and swallowing it.ABSORPTION RATE
Absorption begins where the drug is administered. This can be by mouth, injec-
tion, through the skin, and many other sites. How quickly the drug becomes ther-(^26) CHAPTER 2 Drug Action and Drug Interactions