It is defined as “the process of movement of unchanged drug from the site of administration to systemic circulation.” Absorption can also be defined as “the process of movement of unchanged drug from the site of administration to the site of measurement i.e.plasma Thus both the rate & the extent of drug absorption are important. The oral route of drug administration is the most common for systemically acting drugs & therefore more emphasis will be given to gastrointestinal (GI) absorption of drugs .

2. Absorption:
 It is defined as “the process of movement of
unchanged drug from the site of administration to
systemic circulation.”
 Absorption can also be defined as “the process of
movement of unchanged drug from the site of
administration to the site of measurement
i.e.plasma

3.  A drug that is completely but slowly absorbed may
fail to show therapeutic response as the plasma
concentration for desired effect is never achieved.
 On the contrary,a rapidly absorbed drug attains the
therapeutic level easily to elicit pharmacologic
effect.
 Thus both the rate & the extent of drug absorption
are important.

4. Gastrointestinal absorption of drugs:
 The oral route of drug administration is the most
common for systemically acting drugs & therefore more
emphasis will be given to gastrointestinal (GI)
absorption of drugs .
Drug Transport:
 For a drug to be absorbed & distributed into organs &
tissues & eliminated from the body, it must pass through
one or more biological membranes/barriers at various
location. Such a movement of drug across membrane is
called as Drug Transport.

5. Cell Membrane: structure & physiology
 Membranes are major structure in cells,
surrounding the entire cell (plasma
membrane)
 Membrane act as a boundary between the
cell & the interstitial fluid.
 In addition, the membrane enclose the
most of the cell organelles (e.g. the
mitochondrion membrane, nuclear
membrane)

6. Cell:

7.  Functionally membrane acts as a selective
barrier to the passage of molecules.
 Cell membranes acts as Semi-permeable
Membranes

8.  The Trans membrane movement of drugs is
influenced by the composition & structure of the
cell membranes.
 Cell membranes are generally thin,
approximately 70 to 100 Angstrom in thickness.
Composition of Cell membrane :
 Cell membranes are primarily composed of
phospholipids: with inter-dispersed carbohydrates
 The integral proteins that protrude all the way
through the membranes
 The peripheral proteins that are attached only to
the surface of the membrane & do not penetrate.

9.  Active transport : some of the integral proteins
acts acts carrier proteins for transporting
substances against the concentration gradient,
which is called as active transport.
Peripheral proteins:
These occurs either entirely or almost on the
inside of the cell membrane, & they are normally
attached to one of the integral proteins.
These peripheral proteins functions almost
entirely as enzymes.

10. Membrane Carbohydrates :
1.These occur almost invariably in combination
with proteins & lipids in the form of glycoprotein
& glycolipids.
2. In fact, the most of the integral proteins, &
about 1/10 th of the lipid molecule are
glycolipids.
3. The glycoportion of these molecules almost
invariably protrude to the outside of the cell,
dangling outward from the cell surface
4.Proteoglycans

12. Anatomical & Physiological considerations
of the gastrointestinal tract :

13.  The gastrointestinal tract (GI tract), also
called the digestive tract, or the alimentary
canal, is the system of organs within
multicellular animals that takes in food, digests
it to extract energy and nutrients, and expels
the remaining waste. The major functions of
the GI tract are digestion and excretion

14. Upper gastrointestinal tract
 The upper GI tract consists of the mouth, pharynx,
esophagus, and stomach.
 The mouth contains the buccal mucosa, which
contains the openings of the salivary glands; the
tongue; and the teeth.
 Behind the mouth lies the pharynx, which leads to a
hollow muscular tube, the esophagus.
 Peristalsis takes place, which is the contraction of
muscles to propel the food down the esophagus
which extends through the chest and pierces the
diaphragm to reach the stomach.
 The stomach, in turn, leads to the small intestine.

15. Lower gastrointestinal tract
 The lower GI tract comprises the intestines and anus.
 Bowel or intestine
small intestine, which has three parts:
○ duodenum
○ jejunum
○ ileum
large intestine, which has three parts:
○ caecum (the vermiform appendix is attached to the cecum).
○ colon (ascending colon, transverse colon, descending colon and
sigmoid flexure)
○ rectum
 anus

16. Common anatomical features of GIT.
Histology
 The GI tract has a uniform general histology with
some differences which reflect the specialization
in functional anatomy.The GI tract can be divided
into 4 concentric layers:
 mucosa
 submucosa
 Muscularis externa (the external muscle layer)
 adventitia or serosa

17. Mucosa
 The mucosa is the innermost layer of the GI tract,
surrounding the lumen, or space within the tube.
This layer comes in direct contact with the food (or
bolus), and is responsible for absorption and
secretion, important processes in digestion.
 The mucosa can be divided into:
 epithelium
 lamina propria
 muscularis mucosae

18. Submucosa
 The submucosa consists of a dense irregular layer of connective
tissue with large blood vessels, lymphatics and nerves branching into
the mucosa and muscularis.

19. Muscularis externa
 The muscularis externa consists of a
circular inner muscular layer and a
longitudinal outer muscular layer. The
circular muscle layer prevents the food
from going backwards and the longitudinal
layer shortens the tract. The coordinated
contractions of these layers is called
peristalsis.
Adventitia/Serosa
 The adventitia consists of several layers
of epithelia

20. The pH of the organs of GI tract

21. Lengths of various regions of the human GI
tract :
Region Mean (cm)
Over all length (nose to
anus)
Duodenum
Jejunum & Ileum
Colon
451
22
255
100

22. Stomach
 The stomach functions both as a reservoir and as a
digestive organ. It empties its contents in small
portions (suitable for continued digestion) into the
small intestine.
 Anatomically, the stomach is divided into
 a cardiac part,
 fundus,
 body or corpus, and
 a pyloric part (pyloric antrum and pyloric canal)
 Histologically, most of the layers of the wall of the
stomach appear similar in its different parts. Regional
differences are mainly restricted to the appearance of
the gastric mucosa

23. The Mucosa (epithelium, lamina propria, muscularis
mucosae)
 The mucosa is thrown into longitudinal folds (gastric
folds or rugae), which disappear when the stomach is
fully distended. On the mucosal surface we see small,
funnel-shaped depressions (gastric pits). Almost the
entire mucosa is occupied by simple, tubular gastric
glands which open into the bottom of the gastric pits.

24. Pyloric glands
 Pyloric glands are more coiled than principal glands,
and they may be branched.
 The lamina propria is formed by a very cell-rich loose
connective tissue (fibroblasts, lymphocytes, plasma
cells, macrophages, eosinophilic leukocytes and mast
cells). The muscularis mucosae of the stomach
contains both circular and longitudinal layers of
muscle cells. Its organization is somewhat variable
depending on the location in the stomach.
 Large blood vessels, lymph vessels and nerves are
located in the submucosa which consists of loose
connective tissue.
 Note that the muscularis externa consists of three
layers of muscles: an inner oblique layer, a middle
circular layer and an outer longitudinal layer.

26. Small Intestine
 The small intestine is divided into three
parts
1 duodenum (25-30 cm),
2 jejunum (about first two-fifths of the rest),
3 ileum.

27. Structure of small intestine
Peritoneum:
a double layer peritoneum called the mesentery
attach the jejunum and ileum to posterior
abdominal wall
The Mucosa
 The mucosa of the small intestine has various
structural features which considerably increase
the luminal surface area and consequently
support the main function of the small intestine -
the absorption of the degraded components of
the food.

29. .
 The entire intestinal mucosa forms intestinal villi , which
is tiny figure like projection of mucosal layer in to
intestinal lumen, about 0.5 to 1 mm long.
 The wall consist of columnar epithelial cells, or
enterocytes, with tiny microvilli on their free borders.
Blood supply : The superior artery supplies the whole
of the intestine and venous drainage is by the superior
mesenteric vein which joins other veins to form the
portal vein.

30. The Submucosa
 The submucosa contains glands only in
the duodenum. Submucosal glands of the
duodenum are also called Brunner's
glands.

31. Large intestine

32. Large Intestine
 The large intestine constitutes the terminal part of the
digestive system. It is divided into three main sections:
caecum, colon, and rectum with the anal canal. The
primary function of the large intestine is the reabsorption
of water and inorganic salts.
 The surface of the mucosa is relatively smooth as there
are no plicae circulares or intestinal villi. Crypts of
Lieberkühn are present and usually longer and straighter
than those of the small intestine. .
 There is only little lamina propria squeezed between the
glands. The muscularis mucosae again forms two layers.
 Considerable amounts of fat may be found in the
submucosa.
 The appearance of the muscularis externa is different
from that of the small intestine.

34. References
 1. Fundamentals of Biopharmaceutics &
Pharmacokinetics by V.Venkateswarlu
 2. Biopharmaceutics & Pharmacokinetics by
D.M. Brahmankar
 3.Google images
 4. Google. COM
 5.Ross and Wilson.