26 Section 1/ General Principles of Pharmacologythrough natural membranes easily as
compared to those having lower value. The
natural substances like amino acids, bile
salts, glucose readily pass through body
membranes even if their molecules are too
large.
Site of Absorption
The site of absorption is mainly localised
in mouth (for buccal administration),
stomach, intestines or colon.
Certain vasodilators (like nitroglycerine)
and hormones which can penetrate the
buccal mucosal wall will only be kept in
buccal cavity or under the tongue. It
provides rapid onset of action and
prevention of gastrointestinal interactions.
The absorption in stomach depends
upon the gastric emptying, gastrointestinal
motility and its pH.
- Codeine, which is absorbed in the
intestine fails to give quick relief if its
emptying from the stomach is delayed. - Benzylpenicillin if allowed to stay
longer in the stomach, much of its
activity is lost.
The other important factors controlling
the rate of gastric emptying by influencing
the gastric motility are the volume of the
meal, its temperature, its viscosity and
physical position.
The gastrointestinal pH has its own
effect on the drug absorption. The range of
pH in stomach is 1 to 3.6, in duodenum is 5
to 7 and 7 to 8 in ileum and colon.
Presence of Food and Other Salts
Normally, the presence of food in the
GIT reduces the rate of absorption of drugs.
Tetracycline, when given in full stomach,
the blood level are reduced by 55 to 80% as
compared with fasting individuals. The
griseofulvin absorption is enhanced by
giving it with fatty meals.
Bile salts have favourable effect on the
absorption of drugs mainly due to their
surface activity.Routes of Administration
The drugs are mostly given by mouth,
which is considered already in previous
sections, the other routes of administration
of drugs in relation to absorption are being
discussed here.
Parenteral administration: This route is
applicable for drugs which are inactivated by
gastrointestinal tract or absorption is poor
when given orally or there is a urgency for
fast response in small dose. Intramuscular,
intravenous, or subcutaneous routes are
commonly used. The intravenous injection (in
aqueous solution) is introduced directly into
the vein by which a rapid response is
produced. The subcutaneous injection are
given through the layer of skin, while
intramuscular injection, introduced through
the skin layer deep into the muscle. The
nature of intramuscular injection may be in
aqueous or oily solution/suspension form.
The aqueous solution will be rapidly
absorbed as compared to oily solution or
suspension. So, the rate of absorption is
dependent on the nature of the preparation.
Inhalational administration: The
absorption of drug takes places through
lungs and the absorbing membrane is very
thin and surface area is quite large. The lipid
soluble drugs are readily absorbed from the