between the ionized and unionized form; as the unionized form is removed, the balance
shifts make up for the loss. Molecules C and F are being moved by active transport.
They must also be unionized and fat soluble; however, their transport does not rely on
differences in concentration. This transport process also accounts for the absorption of
a drug by the individual cells within the body.
3-10. DISTRIBUTION OF DRUGS
a. Once the drug is absorbed, it enters the circulation and is carried throughout
the body. The location in the body where the drug goes varies from drug to drug. The
drug may be stored in bone or fat, bound to the proteins in the blood plasma, or
circulate freely as the unbound drug. The drug will find its way into many organs.
Finally, some of the drug will reach the target tissue where it can cause the effect for
which it was administered. An equilibrium will be established between the circulating
unbound drug and each area of the body.
b. The distribution of a drug in the body happens in a very systematic manner.
Figure 3-4 demonstrates the concept of distribution.
Figure 3-4. Drug distribution within the body.
Assume that 100 micrograms (100 mcg) of a drug have been absorbed and is
distributed based on the percentages noted in Figure 3-4. Of the 100 micrograms
absorbed, only two micrograms of the drug will arrive at the target tissue to give the
desired pharmacological effect. If two micrograms is enough drugs to produce the
desired pharmacological effect, the desired effect will be obtained. However, if the
amount of drug required to produce the effect is four micrograms, the desired
pharmacological effect will not be obtained. The dose of the drug can be increased so
that 200 milligrams of the drug can be absorbed, thus providing the amount of drug
needed to give the desired pharmacological effect. However, doubling the dose may
present problems. Doubling the dose would also double the amount of drug in the other
areas of the body. Perhaps this increased dosage may produce some response by
another body organ. For example, the patient may become nauseous, vomit, lose his