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Distribution is the transport of a drug from the
bloodstream to tissue sites where it will be effective, as well
as to sites where the drug may be stored, metabolized, or eliminated
from the body. Once a drug reaches its intended destination, the
drug molecules move from blood through cellular barriers to various
tissues. These barriers include the walls of blood vessels, the
walls of the intestines, the walls of the kidneys, and the special
barrier between the brain and the bloodstream that acts as a filtration
system to protect the brain from exposure to potentially harmful
substances.
The drug molecules move from an area of high drug concentration-the
bloodstream-to an area of low drug concentration-the tissues-until
a balance between the two areas is reached. This process is known
as diffusion. When a drug reaches its highest concentration in the
tissues, the body begins to eliminate the drug and its effect on
the body begins to diminish. The time it takes for the level of
a drug to fall by 50 percent is known as the drug's half-life. Depending
on the drug, this measurement can vary from a few minutes to hours
or even days. For example, if a drug's highest concentration level
in the blood is 1 mg/ml and this level falls to 0.5 mg/ml after
five hours, the half-life of the drug is five hours. A drug's half-life
is used to determine frequency of dosage and the amount of drug
administered.
Distribution of a drug may be delayed by the binding of the drug
to proteins in the blood. Because the proteins are too large to
pass through blood vessel walls, the drug remains in the blood for
a longer period until it is eventually released from the proteins.
While this process may increase the amount of time the drug is active
in the body, it may decrease the amount of the drug available to
the tissues.
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