DRUG ABSORPTION THROUGH GIT

1. Unit 1:
Lecture 3:
DRUG ABSORPTION THROUGH GIT
By
D2 Sir

2. Disclaimer:
All the content material provided here is only for teaching purpose.
Content is provided from reference book

3. Introduction to the gastrointestinal system
The gastrointestinal tract (GIT) consists of a hollow
muscular tube starting from the oral cavity, where food
enters the mouth, continuing through the pharynx,
oesophagus, stomach and intestines to the rectum and
anus, where food is expelled.
There are various accessory organs that assist the tract by
secreting enzymes to help break down food into its
component nutrients. Thus the salivary glands, liver,
pancreas and gall bladder have important functions in
the digestive system.
Food is propelled along the length of the GIT by peristaltic
movements of the muscular walls

5. Basic structure
The gastrointestinal tract is a muscular tube lined by a
special layer of cells, called epithelium.
The contents of the tube are considered external to the
body and are in continuity with the outside world at the
mouth and the anus.
Although each section of the tract has specialised functions,
the entire tract has a similar basic structure with regional
variations.

7. GASTROINTESTINAL ABSORPTION OF DRUGS
The oral route of drug administration is the most common
for systemically acting drugs and therefore, more
emphasis will be given to gastrointestinal (GI) absorption
of drugs.
Moreover, it covers all the aspects of variability observed
in drug absorption. Before proceeding to discuss
absorption aspects, a brief description of cell membrane
structure and physiology is necessary.
Cell Membrane: Structure and Physiology For a drug to be
absorbed and distributed into organs and tissues and
eliminated from the body, it must pass through one or
more biological membranes/barriers at various locations.
Such a movement of drug across the membrane is called
as drug transport.

9. The cellular membrane consists of a double layer of
amphiphilic phospholipid molecules arranged in such a
fashion that their hydrocarbon chains are oriented inwards
to form the hydrophobic or lipophilic phase and their polar
heads oriented to form the outer and inner hydrophilic
boundaries of the cellular membrane that face the
surrounding aqueous environment.
Globular protein molecules are associated on either side of
these hydrophilic boundaries and also interspersed within
the membrane structure.

10. In short, the membrane is a mayonnaise sandwich where a
bimolecular layer of lipids is contained between two
parallel monomolecular layers of proteins.
The hydrophobic core of the membrane is responsible for
the relative impermeability of polar molecules.
Aqueous filled pores or perforations of 4 to 10 Å in
diameter are also present in the membrane structure
through which inorganic ions and small organic water-
soluble molecules like urea can pass.
In general, the biomembrane acts like a semipermeable
barrier permitting rapid and limited passage of some
compounds while restricting that of others

11. The GI lining constituting the absorption barrier allows
most nutrients like glucose, amino acids, fatty acids,
vitamins, etc. to pass rapidly through it into the systemic
circulation but prevents the entry of certain toxins and
medicaments.
Thus, for a drug to get absorbed after oral administration, it
must first pass through this biological barrier.

12. References:
1. Brahmankar DM, Jaiswal SB, Biopharmaceutics and
Pharmacokinetics A Treatise, Vallabh Prakashan, 1995, Page no. 9-10.

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