Gastrointestinal Physiology and Biliary System Explained

GASTROINTESTINAL PHYSIOLOGY
AND BILLIARY SYSTEM
OGBE S.E.
DEPARTMENT OF PHYSIOLOGY
BINGHAM UNIVERSITY
(MLS/OPTOMETRY/PH LECTURE NOTES)

INTRODUCTION
• The gastrointestinal (digestive) system is the organ
system that processes food, extracts nutrients from it,
and eliminates the residue.
• It does this in four stages:
 Ingestion, the selective intake of food;
 Digestion, the mechanical and chemical breakdown of
food into a form usable by the body;
 Absorption, the uptake of nutrient molecules into the
epithelial cells of the digestive tract and then into the
blood or lymph
 Defecation, the elimination of undigested residue.

INTRODUCTION
• Unlike plants, which can form organic molecules using
inorganic compounds, humans and other animals must
obtain their basic organic molecules from food
• Some of the ingested food molecules are needed for:
I. Their energy (caloric) value—obtained by the reactions
of cell respiration and used in the production of ATP—
II. The remaining food molecule is used to make
additional tissue.
• Most of the organic molecules that are ingested are
similar to the molecules that form human tissues.
• These are generally large molecules ( polymers), which
are composed of subunits ( monomers).
• Within the GIT, the digestion of these large molecules
into their monomers occurs by means of hydrolysis
reactions.

INTRO..
• The monomers thus formed are transported across the wall
of the small intestine into the blood and lymph in the
process of absorption.
• Digestion and absorption are the primary functions of the
digestive system.
• Enzymes that digest food are also capable of digesting a
person’s own tissues this is because they have similar
composition
• This however does not occur, because a variety of
protective devices inactivate digestive enzymes in the body
and keep them away from the cytoplasm of the cells.
• The fully active digestive enzymes are normally confined to
the lumen (cavity) of the gastrointestinal tract.
• The lumen of the gastrointestinal tract is open at both ends
(mouth and anus), and is thus continuous with the
environment.

FUNCTION OF THE GIT
1. Motility. This refers to the movement of food through
the processes of
a. Ingestion: Taking food into the mouth.
b. Mastication: Chewing the food and mixing it with
saliva.
c. Deglutition: Swallowing food.
d. Peristalsis: Rhythmic, wavelike contractions that
move food through the gastrointestinal tract.
2. Secretion. This includes both exocrine and endocrine
secretions.
a. Exocrine secretions: Water, hydrochloric acid,
bicarbonate, and many digestive enzymes
b. Endocrine secretions: The GIT secrete a number of
hormones that help to regulate the digestive system.

3. Digestion. This refers to the breakdown of
food molecules into their smaller subunits, which
can be absorbed.
4. Absorption. This refers to the passage of
digested end products into the blood or lymph.
5. Storage and elimination. This refers to the
temporary storage and subsequent elimination of
indigestible food molecules.

PHYSIOLOGIC ANATOMY OF THE GIT
• The digestive system can be divided into the tubular
gastrointestinal (GI) tract, or alimentary canal, and
accessory digestive organs.
• The GI tract is approximately 9 m (30 ft) long and extends
from the mouth to the anus.
• It traverses the thoracic cavity and enters the abdominal
cavity at the level of the diaphragm.
• The anus is located at the inferior portion of the pelvic
cavity.
• The organs of the GI tract include the oral cavity, pharynx,
esophagus, stomach, small intestine, and large intestine
• The accessory digestive organs include the teeth, tongue,
salivary glands, liver, gallbladder, and pancreas

Layers of the Gastrointestinal Tract
• The GI tract from the esophagus to the anal canal is
composed of four layers, or tunics.
• Each tunic contains a dominant tissue type that
performs specific functions in the digestive process.
• The four tunics of the GI tract, from the inside out, are
• the mucosa,
• submucosa,
• muscularis,
• serosa

Mucosa
• The mucosa, which lines the lumen of the GI tract, is the absorptive
and major secretory layer.
• It consists of a simple columnar epithelium supported by the
lamina propria,
• The LP is a thin layer of areolar connective tissue containing
numerous lymph nodules, which are important in protecting against
disease
• External to the lamina propria is a thin layer of smooth muscle
called the muscularis mucosae.
• This is the muscle layer responsible for the numerous small folds in
certain portions of the GI tract.
• These folds greatly increase the absorptive surface area.
• Specialized goblet cells in the mucosa secrete mucus throughout
most of the GI tract.

Submucosa
• This a highly vascular layer of connective tissue
that serves the mucosa.
• Absorbed molecules that pass through the
columnar epithelial cells of the mucosa enter into
blood and lymphatic vessels of the submucosa.
• The submucosa also contains glands and nerve
plexuses:
• The submucosal plexus (Meissner’s plexus)
provides an autonomic nerve supply to the
muscularis mucosae.

muscularis
• The muscularis (muscularis externa) is responsible for segmental
contractions and peristaltic movement through the GI tract.
• The muscularis has an inner circular and an outer longitudinal
layer of smooth muscle.
• Contractions of these layers move the food through the tract
and physically pulverize and mix the food with digestive
enzymes.
• The myenteric plexus (Auerbach’s plexus), located between the
two muscle layers, provides the major nerve supply to the GI
tract.
• It includes both the sympathetic and parasympathetic fibers and
ganglia of the autonomic nervous system
Serosa
• The outer serosa completes the wall of the GI tract.
• It is a binding and protective layer consisting of areolar connective
tissue covered with a layer of simple squamous epithelium.

GIT MOTILITY
• Contraction of the smooth muscle within the walls of
the digestive organs accomplishes movement of
material through most of the digestive tract.
• The exceptions are at the ends of the tract:
 the mouth through the early part of the esophagus at
the beginning
 the external anal sphincter at the end—where motility
involves skeletal muscle rather than smooth muscle
activity.
• Accordingly, the acts of chewing, swallowing, and
defecation have voluntary components, because
skeletal muscle is under voluntary control.

GIT MOTILITY
• By contrast, motility accomplished by smooth
muscle throughout the rest of the tract is
controlled by complex involuntary mechanisms.
• The processes involved in motility in the GIT are:
i. Ingestion
ii. Mastication/chewing
iii. Deglutition/swallowing
iv. Peristalsis

MOTILITY OF THE MOUTH:
MASTICATION
• Entry to the digestive tract is through the mouth or oral
cavity.
• The opening is formed by the muscular lips, which help
procure, guide, and contain the food in the mouth.
• The first step in the digestive process is mastication, or
chewing,
• The motility of the mouth that involves the slicing,
tearing, grinding, and mixing of ingested food by the
teeth.
• The teeth are firmly embedded in and protrude from the
jawbones.
• The exposed part of a tooth is covered by enamel, the
hardest structure of the body.
• Enamel forms before the tooth’s eruption by special cells
that are lost as the tooth erupts.

MASTICATION
• The main muscles of chewing or mastication are the masseter and
temporalis, which powerfully bring the lower jaw up against the upper
jaw,
• The pterygoids, which open the jaws, keep them aligned, and moves
them sideways, and backwards, and forwards for grinding.
• The trigeminal (Vth cranial) nerve controls the muscles of mastication
• Teeth are specialized for different tasks as follows:
i. Incisors have flat, sharp edges for cutting tough foods, such as meat
and hard fruits.
ii. Canines have pointed, sharp ends for gripping food, particularly
meat, and tearing away pieces.
iii. Premolars and molars have flattened, complex surfaces that capture
tiny bits of food, such as grains, and allow them to be crushed
between the surfaces of two opposed teeth.
As people get older, the grinding surfaces of the molars are gradually worn
down.

MASTICATION
• The functions of chewing are:
i. To grind and break food into smaller pieces to
facilitate swallowing and to increase the food
surface area on which salivary enzymes will act,
ii. To mix food with saliva,
iii. To stimulate the taste buds.
• The third function not only gives rise to the
pleasurable subjective sensation of taste,
• It reﬂexly increases salivary, gastric, pancreatic, and
bile secretion to prepare for the arrival of food.

Motility of Pharynx and Esophagus:
Deglutition/Swallowing
• The motility associated with the pharynx and esophagus
is swallowing.
• Most of us think of swallowing as the limited act of
moving food out of the mouth into the esophagus.
• However, swallowing actually is the entire process of
moving food from the mouth through the esophagus
into the stomach.
• Swallowing is initiated when a bolus, or ball of chewed
or liquid food, is voluntarily forced by the tongue to the
rear of the mouth and into the pharynx.
• The pressure of the bolus stimulates pharyngeal
pressure receptors, which send aﬀerent impulses to the
swallowing center located in the medulla of the brain
stem.

Deglutition/Swallowing
• The swallowing center then reﬂexly activates in the
appropriate sequence the muscles of the pharynx
and esophagus by way of the:
a. trigeminal, (cranial nerves V)
b. facial, (cranial nerves VII)
c. glossopharyngeal, (cranial nerves IX
d. hypoglossal nerves (cranial nerves XII).
• Swallowing is the most complex reﬂex in the body.
• Multiple highly coordinated responses are triggered
in a specific all-or-none pattern over a period of time
to accomplish the act of swallowing.
• Swallowing is initiated voluntarily, but once begun it
cannot be stopped

The gastroesophageal sphincter
• Except during swallowing, the gastroesophageal
sphincter stays contracted to keep a barrier between the
stomach and the esophagus
• This reduces the chance of reﬂux of acidic gastric
contents into the esophagus.
• If gastric contents do ﬂow backward despite the
sphincter, the acidity of these contents irritates the
esophagus, causing the esophageal discomfort known as
heartburn.
• As the peristaltic wave sweeps down the esophagus, the
gastroesophageal sphincter relaxes so that the bolus can
pass into the stomach.
• After the bolus has entered the stomach, the swallow is
complete and this lower esophageal sphincter again
contracts.

Gastric Motility
• Gastric motility is a complex process that involves multiple
regulatory inputs.
• Ingested food is pulverized and mixed with gastric
secretions to produce a thick liquid mixture known as
chyme.
• The stomach contents must be converted to chyme before
they can be emptied into the duodenum.
• Gastric motility is divided into four phases:
I. Filling phase
II. Storage phase
III. Mixing phase
IV. Emptying phase

The two regions of the stomach: body and
antrum.

I. Gastric filling
• The volume of an empty stomach is about 50ml, however, it can
expand to as much as 1000ml (1 liter) during meal
• The stomach can accommodate such a 20-fold change in volume
with little change in tension in its walls and little rise in
intragastric pressure through the following mechanism:
i. The interior of the stomach is thrown into deep folds.
ii. During a meal, the folds get smaller and nearly ﬂatten out as
the stomach relaxes slightly with each mouthful,
iii. This reﬂex relaxation of the stomach as it is receiving food is
called receptive relaxation; it allows the stomach to
accommodate the meal with little rise in pressure.
iv. If more than a liter of food is consumed, however, the stomach
becomes over-distended and the person experiences
discomfort.
v. Receptive relaxation is triggered by the act of eating and is
mediated by the vagus nerve.

II. Gastric Storage
• A group of pacemaker cells located in the upper fundus region of
the stomach generate slow-wave potentials
• This potential, sweeps down the length of the stomach toward
the pyloric sphincter at a rate of three per minute.
• Because the muscle layers are thin in the fundus and body, the
peristaltic contractions in this region are weak.
• When the waves reach the antrum, they become stronger and
more vigorous, because the muscle there is thicker
• Feeble mixing movements occur in the body and fundus of the
stomach
• Food delivered to the stomach from the esophagus is stored in
the relatively quiet body without being mixed.
• The fundic area usually does not store food but contains only a
pocket of gas.
• Food is gradually fed from the body into the antrum, where
mixing does take place.

III. Gastric Mixing
• The strong antral peristaltic contractions mix the food
with gastric secretions to produce chyme.
• Each antral peristaltic wave propels chyme forward
toward the pyloric sphincter
• Tonic contraction of the pyloric sphincter normally keeps
it almost, but not completely, closed.
• The opening is large enough for water and other ﬂuids to
pass through with ease but too small for the thicker
chyme to pass through except when a strong antral
peristaltic contraction pushes it through.
• Even then, of the 30 ml of chyme that the antrum can
hold, usually only a few milliliters of antral contents are
pushed into the duodenum with each peristaltic wave.

III. Gastric Mixing
• Before more chyme can be squeezed out, the peristaltic
wave reaches the pyloric sphincter and causes it to contract
more forcefully,
• This seals oﬀ the exit and blocks further passage into the
duodenum.
• The bulk of the antral chyme that was being propelled
forward but failed to be pushed into the duodenum is
abruptly halted at the closed sphincter and is tumbled back
into the antrum,
• It propels forward and tumbles back again as the new
peristaltic wave advances.
• This tossing back and forth thoroughly mixes the chyme in
the antrum.

IV Gastric emptying
• The antral peristaltic contractions are the driving force for gastric
emptying
• The amount of chyme that escapes into the duodenum with each
peristaltic wave before the pyloric sphincter tightly closes depends
largely on the strength of antral peristalsis.
• The intensity of antral peristalsis can vary markedly under the
inﬂuence of various signals from the duodenum.
• The duodenum must be ready to receive the chyme and can delay
gastric emptying by reducing the strength of antral peristalsis until the
duodenum is ready to accommodate more chyme.
• The four most important duodenal factors that inﬂuence gastric
emptying are fat, acid, hypertonicity, and distension.
• The presence of one or more of these stimuli in the duodenum
activates appropriate duodenal receptors,
• This thus triggers either a neural or a hormonal response that puts
brakes on antral peristaltic activity, thereby slowing the rate of gastric
emptying:

Gastric emptying and mixing as a result of antral
peristaltic contractions.

Control of gastric motility
• Acid: Because the stomach secretes HCl, highly
acidic chyme is emptied into the duodenum
• it is neutralized by sodium bicarbonate (NaHCO3)
secreted into the duodenal lumen primarily from the
pancreas.
• unneutralized acid in the duodenum inhibits further
emptying of acidic gastric contents until complete
neutralization can be accomplished
• Hypertonicity: As molecules of protein and starch
are digested in the duodenal lumen, large numbers
of amino acid and glucose molecules are released.
• If absorption of these amino acid and glucose
molecules does not keep pace with the rate at which
protein and carbohydrate digestion proceeds,

• These large numbers of molecules remain in the chyme and
increase the osmolarity of the duodenal contents.
• Thus, the amount of food entering the duodenum for
further digestion into a multitude of additional osmotically
active particles is reduced until absorption processes have
had an opportunity to catch up
Distension: Too much chyme in the duodenum inhibits the
emptying of even more gastric contents,
• This gives the distended duodenum time to cope with the
excess volume of chyme it already contains before it gets
any more.

Motility in the small intestine
• The small intestine is the site where most digestion and
absorption take place.
• Once the luminal contents pass beyond the small intestine, and
no further digestion and absorption of ingested nutrients can
occur
• The large intestine though absorbs small amounts of salt and
water
• The small intestine lies coiled within the abdominal cavity,
extending between the stomach and the large intestine.
• It is arbitrarily divided into three segments—the duodenum, the
jejunum, and the ileum.
• Small-intestine motility includes segmentation and the migrating
motility complex.

Gross anatomy of the small inetstine

I. Segmentation contractions
• The small intestine’s primary method of motility during
digestion of a meal, both mixes and slowly propels the
chyme.
• Segmentation consists of oscillating, ring-like
contractions of the circular smooth muscle along the
small intestine’s length;
• Between the contracted segments are relaxed areas
containing a small bolus of chyme.
• The contractile rings occur every few centimeters,
dividing the small intestine into segments like a chain of
sausages.
• These contractile rings do not sweep along the length of
the intestine as peristaltic waves do.

Segmentation contractions
• Rather, after a brief period, the contracted segments relax, and
ring-like contractions appear in the previously relaxed areas
• The new contraction forces the chyme in a previously relaxed
segment to move in both directions into the now relaxed adjacent
segments.
• A newly relaxed segment therefore receives chyme from both the
contracting segment immediately ahead of it and the one
immediately behind it.
• Shortly thereafter, the areas of contraction and relaxation alternate
again.
• In this way, the chyme is chopped, churned, and thoroughly mixed.
• The chyme slowly progresses forward because the frequency of
segmentation declines along the length of the small intestine

Segmentation.
• Segmentation consists of ring-like
contractions along the length of
the small intestine.
• Within a matter of seconds, the
contracted segments relax and
the previously relaxed areas
contract.
• These oscillating contractions
thoroughly mix the chyme within
the small-intestine lumen.

Segmentation contractions
• The pacemaker cells in the duodenum spontaneously depolarize
faster than those farther down the tract
• Segmentation contractions occurring in the duodenum at a rate
of 12 per minute, compared to only 9 per minute in the terminal
ileum.
• Because segmentation occurs with greater frequency in the
upper part of the small intestine than in the lower part, more
chyme, on average, is pushed forward than is pushed backward.
• As a result, chyme is moved slowly from the upper to the lower
part of the small intestine, being shuffled back and forth to
accomplish thorough mixing and absorption in the process.
• This slow propulsive mechanism is advantageous because it
allows ample time for the digestive and absorptive processes to
take place.
• The contents usually take 3 to 5 hours to move through the small
intestine.

Migrating motility complex
• When most of the meal has been absorbed, segmentation
contractions cease and are replaced between meals by the
migrating motility complex, or “intestinal housekeeper.”
• This between-meal motility consists of weak, repetitive
peristaltic waves that move a short distance down the
intestine before dying out.
• The waves start at the stomach and migrate down the
intestine; that is, each new peristaltic wave is initiated at a
site a little farther down the small intestine.
• These short peristaltic waves take about 100 to 150
minutes to gradually migrate from the stomach to the end
of the small intestine,

Migrating motility complex
• with each contraction sweeping any remnants of the
preceding meal plus mucosal debris and bacteria
forward toward the colon
• After the end of the small intestine is reached, the cycle
begins again and continues to repeat itself until the next
meal.
• The migrating motility complex is regulated between
meals by the hormone motilin, which is secreted during
the unfed state by endocrine cells of the small-intestine
mucosa.
• When the next meal arrives, segmental activity is
triggered again, and the migrating motility complex
ceases.
• Motilin release is inhibited by feeding

Ileocecal juncture
• At the juncture between the small and the large
intestines, the last part of the ileum empties into the
cecum
• Two factors makes this juncture act as a barrier
between the small intestine and large intestine:
i. The anatomic arrangement is such that valve-like
folds of tissue protrude from the ileum into the
lumen of the cecum.
• When the ileal contents are pushed forward, this
ileocecal valve is easily pushed open,
• The folds of tissue are forcibly closed when the cecal
contents attempt to move backward.

Ileocecal juncture
ii. The smooth muscle within the last several
centimeters of the ileal wall is thickened, forming a
sphincter that is under neural and hormonal control.
• Most of the time, this ileocecal sphincter remains at
least mildly constricted.
• Relaxation of the sphincter is enhanced by release of
gastrin at the onset of a meal, when increased gastric
activity is taking place.
• This relaxation allows the undigested fibers and
unabsorbed solutes from the preceding meal to be
moved forward as the new meal enters the tract

Control of the ileocecal valve and sphincter.
• Pressure on the cecal side pushes
the valve closed and contracts the
sphincter,
• This prevents bacteria-laden
colonic contents from
contaminating the nutrient-rich
small intestine.
• The valve–sphincter opens and
allows ileal contents to enter the
large intestine
• This is due to:
 pressure on the ileal side of the
valve
 the hormone gastrin secreted as a
new meal enters the stomach.

Large intestinal motility
• The large intestine consists of the colon, cecum, appendix, and
rectum
• The cecum forms a blind-ended pouch below the junction of the
small and large intestines at the ileocecal valve.
• The small, fingerlike projection at the bottom of the cecum is the
appendix, a lymphoid tissue that houses lymphocytes.
• The colon, which makes up most of the large intestine, is not
coiled like the small intestine but consists of three relatively
straight parts—
i. the ascending colon,
ii. the transverse colon,
iii. the descending colon.
• The end part of the descending colon becomes S shaped,
forming the sigmoid colon and then straightens out to form the
rectum

Anatomy of the large intestine

• Most of the time, movements of the large intestine are
slow and non-propulsive, as is appropriate for its
absorptive and storage functions.
• The colon’s main motility is haustral contractions
initiated by the autonomous rhythmicity (BER) of colonic
smooth muscle cells.
• These contractions, which throw the large intestine into
pouches or sacs called haustra, are similar to small-
intestine segmentations but occur less frequently.
• Thirty minutes may elapse between haustral
contractions, whereas segmentation contractions in the
small intestine occur at rates of between 9 and 12 per
minute.
•

• The location of the haustral sacs gradually
changes as a relaxed segment that has formed a
sac slowly contracts while a previously contracted
area simultaneously relaxes to form a new sac.
• These movements are non-propulsive; they
slowly shuffle the contents in a back-and-forth
mixing movement that exposes the colonic
contents to the absorptive mucosa.
• Haustral contractions are largely controlled by
locally mediated reﬂexes involving the intrinsic
plexuses.

SALIVA
• This a digestive secretion of the oral cavity.
• It has a pH of 6.8 to 7.0.
• About 800 and 1500 milliliters of saliva is secreted daily
• COMPOSITION OF SALIVA
• It is a hypotonic solution of 97.0% to 99.5% water and the
following solutes:
 salivary amylase, an enzyme that begins starch digestion in the
mouth
 lingual lipase, an enzyme that is activated by stomach acid and
digests fat after the food is swallowed;
 mucus, which binds and lubricates the food mass and aids in
swallowing
 lysozyme, an enzyme that kills bacteria
 immunoglobulin A (IgA), an antibody that inhibits bacterial
growth
 electrolytes, including sodium, potassium, chloride, phosphate,
and bicarbonate salts.

SALIVARY GLAND
• There are two kinds of salivary glands, intrinsic and
extrinsic.
• The intrinsic salivary glands are an indefinite number of
small glands dispersed amid the other oral tissues.
• They include lingual glands in the tongue, labial glands on
the inside of the lips, and buccal glands on the inside of the
cheeks.
• They secrete saliva at a fairly constant rate irrespective
food presence, but in relatively small amounts.
• This saliva contains lingual lipase and lysozyme and serves
to moisten the mouth and inhibit bacterial growth.

SALIVARY GLAND CONTD……
• The extrinsic salivary glands are three pairs of larger, more discrete organs
located outside of the oral mucosa.
• They communicate with the oral cavity by way of ducts, they include:
i. Parotid Gland:
• This is the largest salivary gland
• It is located just beneath the skin anterior to the earlobe.
• Its duct passes superficially over the masseter, pierces the buccinator, and
opens into the mouth opposite the second upper molar tooth
• The facial nerve courses through this gland
• It secretes most of the serous secretion that contains ptyalin (an α-
amylase)

SALIVARY GLAND CONTD……
ii. Submandibular (submaxillary) gland
• It is located halfway along the body of the mandible, medial to its
margin, just deep to the mylohyoid muscle.
• Its duct empties into the mouth at a papilla on the side of the
lingual frenulum, near the lower central incisors.
• It secretes both serous and mucus (which contains mucin)
secretions
iii. Sublingual gland
• It is located in the floor of the mouth.
• It has multiple ducts that empty into the mouth posterior to the
papilla of the submandibular duct
• It secretes both serous and mucus (which contains mucin)
secretions

Diagram showing the extrinsic salivary
glands
NB: the mandible has been removed to expose the sublingual
gland

FUNCTIONS OF SALIVA
a. Saliva moistens the mouth
b. Digests a little starch and fat
c. Cleanses the teeth
d. Inhibits bacterial growth
e. Dissolves molecules so they can stimulate the
taste buds
f. Moistens food and binds particles together to
aid in swallowing.

Nervous Regulation of Salivary
Secretion
• The presence of food stimulates tactile, pressure, and taste
receptors in the mouth, transmit signals to a group of salivatory
nuclei in the medulla oblongata and pons.
• These nuclei also receive input from higher brain centers, so
even the odor, sight, or thought of food stimulates salivation.
• The salivatory nuclei send signals to the glands by way of
autonomic fibers in the facial and glossopharyngeal nerves.
• In response to such stimuli as the aroma, taste, and texture of
food, the parasympathetic nervous system stimulates the glands
to produce abundant, thin saliva rich in enzymes.
• The appetite area of the brain, which partially regulates these
effects, is located in proximity to the parasympathetic centers of
the anterior hypothalamus.
• This appetite area acts in response to signals from the taste and
smell areas of the cerebral cortex or amygdala

Nervous Regulation of Salivary
Secretion contd…
• Irritation of the stomach and esophagus by spicy foods, stomach
acid, or toxins also stimulates salivation
• This perhaps serves to dilute and rinse away the irritants.
• Sympathetic stimulation, by contrast, briefly enhances salivation
but its primary effect is to produce less abundant, thicker saliva
with more mucus.
• This is why the mouth may feel sticky or dry under conditions of
stress.
• Sympathetic stimulation constricts the blood vessels of the
salivary glands.
• Since saliva begins as a filtrate from the blood capillaries, this
vasoconstriction reduces saliva output.
• Dehydration similarly reduces salivation by reducing capillary
filtration.

DISORDERS OF SALIVATION
• Anticholinergic drugs are the most common cause of decreased
saliva
production and dry mouth, also known as xerostomia.
• Less common causes include autoimmune damage to salivary
glands in Sjogren’s syndrome and sarcoidosis.
• Xerostomia is a serious condition, because chewing and swallowing
rely on adequate saliva, as does maintaining teeth in good
condition.
• Occasionally stones can form in the salivary glands, causing
obstruction, pain and swelling in the proximal part of the gland.
• The mumps virus, for unknown reasons, preferentially attacks the
salivary glands, pancreas, ovaries and testicles,
• Inflammation of the parotid gland causes the typical swollen cheeks
appearance of mumps.

The stomach
• The stomach is a J-shaped organ relatively vertical in tall people, and more
horizontal in short people
• The lesser curvature of the stomach extends the short distance from
esophagus to duodenum along the medial to superior aspect
• The greater curvature extends the longer distance from esophagus to
duodenum on the lateral to inferior aspect.
• The stomach is divided into four regions:
• (1) The cardiac region (cardia) is a small area immediately inside the
cardiac orifice.
• (2) The fundic region (fundus) is the domeshaped portion superior to the
esophageal attachment.
(3) The body (corpus) makes up the greatest part of the stomach inferior
to the cardiac orifice.
• (4) The pyloric region is a slightly narrower pouch at the inferior end; it is
subdivided into a funnel-like antrum12 and a narrower pyloric canal

The Stomach Wall
• The stomach wall has tissue layers similar to those of the
esophagus, with some variations.
• The mucosa is covered with a simple columnar glandular epithelium
mucin, which swells with water and becomes mucus after it is
secreted.
• The mucosa and submucosa are flat and smooth when the stomach
is full.
• As it empties, these layers form conspicuous longitudinal wrinkles
called gastric rugae.
• The lamina propria is almost entirely occupied by tubular glands,
• The muscularis externa has three layers, rather than two—an outer
longitudinal, middle circular, and inner oblique layer

The Stomach Wall
• The gastric mucosa is pocked with depressions called gastric pits
lined with the same columnar epithelium as the surface
• Cells near the bottom of the gastric pits divide and produce new
epithelial cells that continually migrate upward and replace old
epithelial cells that are sloughed off into the chyme.
• Two or three tubular glands open into the bottom of each gastric
pit and span the rest of the lamina propria.
• In the cardiac and pyloric regions they are called cardiac glands and
pyloric glands, respectively.
• In the rest of the stomach, they are called gastric glands.
• These three glands differ in cellular composition:
 the cardiac and pyloric glands secrete mainly mucus;
 the gastric glands secretes acid and enzyme.
• Hormones are secreted throughout the stomach

The Stomach Wall
• Collectively, they have the following cell types:
 Mucous cells, which secrete mucus, predominate in the cardiac and pyloric glands.
• In gastric glands, they are called mucous neck cells and are concentrated in the narrow neck of the
gland, where it opens into the gastric pit.
 Regenerative (stem) cells, found in the base of the pit and neck of the gland, divide rapidly and
produce a continual supply of new cells.
• Newly generated cells migrate upward to the gastric surface as well as downward into the glands to
replace the dead cell.
 Parietal cells, found mostly in the upper half of the gland, secrete hydrochloric acid and intrinsic
factor.
They are found mostly in the gastric glands, but a few occur in the pyloric glands.
 Chief cells, so-named because they are the most numerous, secrete chymosin and lipase in infancy
and pepsinogen throughout life.
• They dominate the lower half of the gastric glands but are absent from cardiac and pyloric glands.
 Enteroendocrine cells, concentrated especially in the lower end of a gland, secrete hormones and
paracrine

SECRETIONS OF THE STOMACH:
GASTRIC SECRETION
• The gastric glands produce 2 to 3L of gastric juice per day
• It is composed mainly of water, hydrochloric acid, and pepsin
• The mucosa of the stomach has two types of tubular glands
i. The oxyntic gland (gastric glands) which secretes hydrochloric acid,
mucus pepsinogen, and intrinsic factors
ii. The pyloric glands which secretes mucus and gastrin
OXYNTIC GLAND
This gland has three cell types; they include:
a. Mucous neck cells, which secrete mainly mucus
b. Peptic (or chief) cells, which secrete large quantities of pepsinogen
c. Parietal (or oxyntic) cells, which secrete hydrochloric acid and intrinsic
factor.

Hydrochloric Acid
• Gastric juice has a high concentration of hydrochloric
acid (HCl) and a pH as low as 0.8.
• This concentration can be very deleterious to the skin.
• Parietal cells contain carbonic anhydrase (CAH), which
catalyzes the first step in the following reaction:
CAH
CO2 + H2O → H2CO3 → HCO3- + H+
• Parietal cells pump the H+ from this reaction into the
lumen of a gastric gland by an active transport called H+ - K+
ATPase.
• This is an antiport that uses the energy of ATP to pump
H+ out and K+ into the cell.

Hydrochloric Acid contd………….
• HCl secretion does not affect the pH within the parietal
cell because H+ is pumped out as fast as it is generated.
• The bicarbonate ions (HCO3- ) are exchanged for chloride
ions (Cl-) from the blood plasma
• The Cl- is pumped into the lumen of the gastric gland to
join the H+.
• HCl thus accumulates in the stomach while bicarbonate
ions accumulate in the blood.
• Because of the bicarbonate, blood leaving the stomach
has a higher pH when digestion is occurring than when
the stomach is empty.
• This high-pH blood is called the alkaline tide.

Secretion of gastric acid by parietal
cells.

FUNCTIONS OF HCl
• Although HCl does not actually digest anything, it performs these
specific functions that aid digestion:
1. HCl activates the enzyme precursor pepsinogen to an active enzyme,
pepsin, and provides an acid medium that is optimal for pepsin activity.
2. It aids in the breakdown of connective tissue and muscle fibers,
reducing large food particles into smaller particles.
3. It denatures protein; that is, it uncoils proteins from their highly folded
final form, thus exposing more of the peptide bonds for enzymatic attack.
4. Along with salivary lysozyme, HCl kills most of the microorganisms
ingested with food, although some do escape and continue to grow and
multiply in the large intestine.

Pepsinogen
• The major digestive constituent of gastric secretion is pepsinogen,
• It is an inactive enzymatic molecule produced by the chief cells.
• Pepsinogen is stored in the chief cells’ cytoplasm within secretory
vesicles known as zymogen granules and released by exocytosis
• When pepsinogen is secreted into the gastric lumen, HCl cleaves oﬀ
a small fragment of the molecule, converting it to the active form of
the enzyme pepsin
• Once formed, pepsin acts on other pepsinogen molecules to
produce
more pepsin.
• A mechanism such as this, whereby an active form of an enzyme
activates other molecules of the same enzyme, is called an
autocatalytic (“self-activating”) process

Pepsinogen contd….
• Pepsin initiates protein digestion by splitting certain
amino acid linkages in proteins to yield peptide
fragments (small amino acid chains)
• It works most eﬀectively in the acid environment
provided by HCl.
• Because pepsin can digest protein, it must be stored
and secreted in an inactive form so it does not digest
the proteins of the cells in which it is formed.
• Pepsin is maintained in the inactive form of pepsinogen
until it reaches the gastric lumen, where it is activated
by HCl secreted into the lumen by a diﬀerent cell type.

Control of Gastric Secretion
• The rate of gastric secretion can be inﬂuenced by
• (1) factors arising before food ever reaches the stomach
• (2) factors resulting from the presence of food in the stomach
• (3) factors in the duodenum after food has left the stomach.
• CEPHALIC PHASE The cephalic phase of gastric secretion refers to the
increased secretion of HCl and pepsinogen.
• This occurs in feedforward fashion in response to stimuli acting in the
head even before food reaches the stomach
• The thought of food, taste, smell, chewing and deglutition increases
gastric secretion by vagal nerve activity in two ways:
I. Vagal stimulation of the intrinsic plexuses promotes increased
secretion of ACh, which in turn leads to increased secretion of HCl
and pepsinogen by the secretory cells.
II. Vagal stimulation of the G cells within the pyloric gland area (PGA)
causes the release of gastrin, which in turn further enhances
secretion of HCl and pepsinogen, with the eﬀect on HCl being
potentiated by gastrin promoting the release of histamine

GASTRIC PHASE
• The gastric phase of gastric secretion begins when food actually
reaches the stomach.
• Stimuli acting in the stomach—namely protein, especially peptide
fragments; distension; caﬀeine; and alcohol—increase gastric secretion
by overlapping eﬀerent pathways.
• The most potent stimulus for gastric secretion is protein, its presence
in the stomach, stimulates chemoreceptors that activate the intrinsic
nerve plexuses, which in turn stimulate the secretory cells.
• Protein brings about activation of the extrinsic vagal fibers to the
stomach.
• Vagal activity further enhances intrinsic nerve stimulation of the
secretory cells and triggers the release of gastrin.
• Protein also directly stimulates the release of gastrin. Gastrin, in turn,
is a powerful stimulus for further HCl and pepsinogen secretion and
also calls forth release of histamine, which further increases HCl
secretion.
• Through these synergistic and overlapping pathways, protein induces
the secretion of a highly acidic, pepsin-rich gastric juice, which
continues the digestion of the protein that first initiated the process

INTESTINAL PHASE
• The intestinal phase of gastric secretion
encompasses the factors originating in the
small intestine that influence gastric secretion.
• Whereas the other phases are excitatory, this
phase is inhibitory.
• The intestinal phase is important in helping
shut off the flow of gastric juices as chyme
begins to be emptied into the small intestine

Stimulation of gastric secretions

Inhibition of gastric secretions

GIT HORMONES AND THEIR ROLES
• The GI tract is both an endocrine gland and a target for the action of various
hormones.
• Indeed, the first hormones to be discovered were gastrointestinal hormones.
• In 1902 two English physiologists, Sir William Bayliss and Ernest Starling,
discovered that the duodenum produced a chemical regulator.
• They named this substance secretin, and proposed in 1905 that it was but one
of many yet undiscovered
chemical regulators produced by the body.
• Bayliss and Starling coined the term hormones for this new class of regulators.
• In that same year other investigators discovered that an extract from the
stomach antrum stimulated gastric secretion.
• The hormone gastrin was thus the second hormone to be discovered.
• The chemical structures of gastrin, secretin, and the duodenal hormone
cholecystokinin (CCK) were determined in
the 1960s.
• More recently, a fourth hormone produced by the small intestine, gastric
inhibitory peptide (GIP), has been added to the list of proven GI tract
hormones.
• The effects of these and other gastrointestinal hormones are summarized the
next slide

Effects of Gastrointestinal
Hormones

PANCREATIC SECRETIONS
• The pancreas is an elongated gland that lies behind and below
the stomach, above the first loop of the duodenum
• This mixed gland contains both exocrine and endocrine tissue.
• The predominant exocrine part consists of grapelike clusters of
secretory cells that form sacs known as acini
• These acini, connect to ducts that eventually empty into the
duodenum.
• The smaller endocrine part consists of isolated islands of
endocrine tissue, the islets of Langerhans, which are dispersed
throughout the pancreas.
• The most important hormones secreted by the islet cells are
insulin and glucagon
• Both exocrine and endocrine secretions are involved with the
metabolism of nutrient molecules.
• They have diﬀerent functions under the control of diﬀerent
regulatory mechanisms.

Figure showing the exocrine and endocrine portions
of the pancreas

The exocrine pancreas
• secretes a pancreatic juice consisting of two components:
I. Pancreatic enzymes actively secreted by the acinar cells that form
the acini
II. An aqueous alkaline solution actively secreted by the duct cells that
line the pancreatic ducts.
• The aqueous (watery) alkaline component is rich in sodium
bicarbonate (NaHCO3).
• Pancreatic enzymes are stored within zymogen granules after being
produced, then are released by exocytosis as needed.
• These pancreatic enzymes are important because they can almost
completely digest food in the absence of all other digestive secretions.
• The acinar cells secrete three diﬀerent types of pancreatic enzymes
capable of digesting all three categories of foodstuﬀs:
I. proteolytic enzymes for protein digestion
II. pancreatic amylase for carbohydrate digestion,
III. pancreatic lipase for fat digestion.

I. Pancreatic Proteolytic Enzymes
• The three major pancreatic proteolytic enzymes are trypsinogen,
chymotrypsinogen, and procarboxypeptidase, each of which is secreted
in an inactive form.
• When trypsinogen is secreted into the duodenal lumen, it is activated
to its active enzyme form, trypsin, by enterokinase (also known as
enteropeptidase)
• Trypsin then autocatalytically activates more trypsinogen.
• Like pepsinogen, trypsinogen must remain inactive within the pancreas
to prevent this proteolytic enzyme from digesting the proteins of the
cells in which it is formed.
• Trypsinogen remains inactive, therefore, until it reaches the duodenal
lumen, where enterokinase triggers the activation process, which then
proceeds autocatalytically.
• As further protection, the pancreas also produces a chemical known as
trypsin inhibitor, which blocks trypsin’s actions if spontaneous
activation of trypsinogen inadvertently occurs within the pancreas.

Pancreatic Proteolytic Enzymes
Contd..
• Chymotrypsinogen and procarboxypeptidase, are
converted by trypsin to their active forms, chymotrypsin
and carboxypeptidase, within the duodenal lumen.
• Thus, once enterokinase has activated some of the trypsin,
trypsin then carries out the rest of the activation process.
• Each of these proteolytic enzymes attacks diﬀerent peptide
linkages.
• The end products that result from this action are a mixture
of small peptide chains and amino acids.
• Mucus secreted by the intestinal cells protects against
digestion of the
small-intestine wall by the activated proteolytic enzymes.

PANCREATIC AMYLASE
• Like salivary amylase, pancreatic amylase contributes to carbohydrate
digestion by converting polysaccharides into the disaccharide maltose.
• Amylase is secreted in the pancreatic juice in an active form because
active amylase does not endanger the secretory cells.
• These cells do not contain any polysaccharides
• PANCREATIC LIPASE
• Pancreatic lipase is extremely important because it is the only enzyme
secreted throughout the entire digestive system that can digest fat.
• In humans, insignificant amounts of lipase are secreted in the saliva
and gastric juice— lingual lipase and gastric lipase.)
• Pancreatic lipase hydrolyzes dietary triglycerides into monoglycerides
and free fatty acids, which are the absorbable units of fat.
• Like amylase, lipase is secreted in its active form because there is no
risk of pancreatic self-digestion by lipase.
• Triglycerides are not a structural component of pancreatic cells.

Enzymes Contained in Pancreatic
Juice

Phases of Pancreatic Secretion
• Pancreatic secretions occurs in three phases: the cephalic phase,
the gastric phase, and the intestinal phase.
• Cephalic phase:
• Nervous signals from the brain causes the vagal nerve endings of
the pancreas to release acetylcholine
• This causes moderate amounts of enzymes to be secreted into
the pancreatic acini
• This accounts for about 20% of the total secretion of pancreatic
enzymes after meal
• Despite the amount of pancreatic enzymes secreted, only small
quantity get to flow through the pancreatic duct into the small
intestine.
• This is because small amounts of water and electrolytes are
secreted along with the enzymes

Gastric phase
• There is continuation of the nervous stimulation of enzyme
secretion in this phase
• This accounts for about 5 – 10% of pancreatic enzymes secreted
after a meal
• But, again, only small amounts secreted after a meal reach the
duodenum because of continued lack of significant fluid
secretion
• Intestinal Phase.
• After chyme leaves the stomach and enters the small intestine
where pancreatic secretion becomes copious
• The presence of chyme in the duodenum, results in the release
of enterogastrone (secretin and cholecystokinin)
• They play central role in controlling pancreatic secretion

ROLE OF SECRETIN IN PANCREATIC SECRETION
• Some factors such as fat, acid, hypertonicity, and distension
stimulates the release of enterogastrones
• The primary stimulus for secretin release is the presence of
acid in the duodenum
• Secretin, in turn, is carried by the blood to the pancreas,
where it stimulates the duct cells to markedly increase their
secretion of a NaHCO3-rich aqueous ﬂuid into the duodenum
• This mechanism provides a control system for maintaining
neutrality of the chyme in the intestine
• The amount of secretin released is proportional to the
amount of acid that enters the duodenum, so the amount of
NaHCO3 secreted parallels the duodenal acidity

ROLE OF CCK IN PANCREATIC SECRETION
• Cholecystokinin is important in regulating pancreatic digestive
enzyme secretion.
• The main stimulus for release of CCK from the duodenal mucosa
is the presence of fat and, to a lesser extent, protein products.
• The circulatory system transports CCK to the pancreas where it
stimulates the pancreatic acinar cells to increase digestive
enzyme secretion.
• Among these enzymes are lipase and the proteolytic enzymes,
which appropriately further digest the fat and protein that
initiated the response and also help digest carbohydrate.
• In contrast to fat and protein, carbohydrate does not have any
direct inﬂuence on pancreatic digestive enzyme secretion
• All three types of pancreatic digestive enzymes are packaged
together in the zymogen granules, so all the pancreatic enzymes
are released together on exocytosis of the granules.

Figure showing the hormonal control of pancreatic exocrine
secretion.

ROLE OF CCK IN PANCREATIC SECRETION
• Therefore, even though the total amount of enzymes released varies depending on
the type of meal consumed (the most being secreted in response to fat), the
proportion of enzymes released does not vary on a meal-to-meal basis.
• That is, a high protein meal does not cause the release of a greater proportion of
proteolytic enzymes.
• Evidence suggests, however, that long term adjustments in the proportion of the
types of enzymes produced may occur as an adaptive response to a prolonged
change in diet.
• For example, with a long-term switch to a high protein diet, a greater proportion of
proteolytic enzymes are produced.
• Cholecystokinin may play a role in pancreatic digestive enzyme adaptation to
changes in diet.
• Just as gastrin is trophic to the stomach and small intestine, CCK and secretin exert
trophic eﬀects on the exocrine pancreas to maintain its integrity

Regulation of pancreatic Secretion
• Pancreatic juice are secreted in response to parasympathetic (vagal)
stimulation and inhibited by sympathetic stimulation.
• It is stimulated by the hormones cholecystokinin (CCK), gastrin, and
secretin.
• The duodenum secretes CCK in response to acid and fat arriving from
the stomach.
• CCK triggers three responses:
I. Secretion of pancreatic enzymes
II. Relaxation of the hepato-pancreatic sphincter, which allows bile and
pancreatic juice to be released into the duodenum.
• Gastrin from the stomach and duodenum stimulates gallbladder
contraction and pancreatic enzyme secretion, but only half as strongly as
CCK does.
• Acidic chyme also stimulates the duodenum to secrete secretin
• Secretin stimulates the hepatic bile ducts and pancreatic ducts to
secrete bicarbonate, so the bile and pancreatic juice both help to
neutralize stomach acid in the duodenum.

BILLIARY SECRETIONS
• The liver is a reddish brown gland located
immediately inferior to the diaphragm
• It fills most of the right hypochondriac and
epigastric regions.
• It is the body’s largest gland, weighing about 1.4
kg (3 pounds).
• It has a tremendous variety of functions, but only
one of them, the secretion of bile, contributes to
digestion.

THE BILE
• The gallbladder is a pear-shaped sac on the underside
of the liver that serves to store and concentrate bile.
• Bile is a yellow-green fluid containing minerals,
cholesterol, neutral fats, phospholipids, bile pigments,
and bile acids.
• The principal pigment is bilirubin, derived from the
decomposition of hemoglobin.
• Bacteria of the large intestine metabolize bilirubin to
urobilinogen, which is responsible for the brown color
of feces.
• In the absence of bile secretion, the feces are grayish
white and marked with streaks of undigested fat
(acholic feces).

THE BILE
• Bile acids (bile salts) are steroids synthesized from
cholesterol.
• Bile acids and lecithin, a phospholipid, aid in fat
digestion and absorption.
• All other components of the bile are wastes destined
for excretion in the feces.
• When these waste products become excessively
concentrated, they may form gallstones
• Bile gets into the gallbladder by first filling the bile
duct, then overflowing into the gallbladder.
• Between meals, the gallbladder absorbs water and
electrolytes from the bile and concentrates it by a
factor of 5 to 20 times.

THE BILE
• The liver secretes about 500 to 1,000mL of bile per day.
• About 80% of the bile acids are reabsorbed in the
ileum, returned to the liver, where the hepatocytes
absorb and resecrete them.
• This route of secretion, reabsorption, and resecretion is
called the enterohepatic circulation
• This reuses the bile acids two or more times during the
digestion of an average meal.
• The 20% of the bile that is not reabsorbed is excreted
in the feces.
• This is the body’s only way of eliminating excess
cholesterol.
• The liver synthesizes new bile acids from cholesterol to
replace the quantity lost in the feces.

DIGESTION AND ABSORPTION
IN THE GIT

DIGESTION AND ABSORPTION OF CARBOHYDRATES
• Carbohydrates are ingested as starches and sugars,
which are longer or shorter polymers of
monosaccharides.
• Plant starch is a complex, branched polysaccharide of
glucose linked by α1–4 and α1–6 glycosidic linkages
• Cane sugar (sucrose) is a disaccharide composed of
glucose and fructose.
• Lactose, the major sugar in milk, is composed of glucose
and galactose.
• Humans cannot digest β1–4 glycosidic linkages in
cellulose, the major polysaccharide in plant cell walls,
which is also known as dietary fibre or roughage.

Digestion and absorption of carbohydrates:
• during the process of mastication, there is a mixture of
food with saliva which contains ptyalin (α- amylase) enzyme
• The starch is being hydrolyzed by this enzyme into
disaccharide (maltose) and other small glucose polymers
• Because the process of mastication is short-lived, only a
few (about 5%) of ingested starch gets hydrolyzed
• The digestion of starch continues fundus and body of the
stomach for up to an hour before it gets mixed with gastric
secretions
• the salivary amylase becomes deactivated in the presence
of acid from the gastric secretions of the stomach
• This accounts for about 30 – 40% of starch been hydrolyzed
to form maltose

Digestion and absorption of carbohydrates:
• The pancreas secretes large quantity of α- amylase enzyme
similar to that secreted by the saliva
• As the stomach empties the chyme into the duodenum, the
chyme mixes with pancreatic juice thus digesting the
carbohydrate
• This accounts for almost all the carbohydrates being hydrolyzed
to maltose and other small glucose polymers
• Dietary carbohydrates are presented to the small intestine for
absorption mainly in the forms of the disaccharides maltose (the
product of polysaccharide digestion), sucrose, and lactose.
• The enterocytes lining the villi of the small intestine contain four
enzymes (lactase, sucrase, maltase, and α-dextrinase)

Digestion and absorption of carbohydrates
contd…
• These enzymes further reduce their corresponding disaccharides
into the absorbable monosaccharide units of glucose (mostly),
galactose, and fructose.
• Glucose and galactose are both absorbed by secondary active
transport, in by symport carriers, such as the sodium and glucose
cotransporter (SGLT)
• The SGLT on the luminal membrane transport both the
monosaccharide and Na+ from the lumen into the interior of the
intestinal cell.
• Glucose (or galactose), having been concentrated in the cell by
these symporters, leaves the cell down its concentration gradient
by facilitated diﬀusion to enter the capillary network within the
villus.
• Fructose is absorbed into the blood solely by facilitated diﬀusion.

DIGESTION AND ABSORPTION OF
PROTEINS
• Long chains of amino acids bound together by peptide linkages make up
dietary proteins
• The stomach secretes the enzyme, pepsin which is usually active at PH of
2.0 – 3.0
• Pepsin unlike other digestive enzyme has the ability to digest collagen
which is an important constituent in the intracellular connective tissue of
meat
• The action of pepsin on collagen fibers of meat, makes the other meat
proteins to be digested easily by the digestive enzymes of the GIT
• The hydrolysis of the peptide linkages between amino acids, leads to the
breakdown of proteins
• The action of pepsin is to initiate the process of protein digestion by
converting the proteins to proteoses, peptones, and a few polypeptides
• Pepsin accounts for about 10 – 20% of total protein digestion

Protein digestion contd…
• The upper small intestine, duodenum and the jejunum are the site of most
protein digestion
• This is due to the influence of proteolytic enzymes from pancreatic
secretion
• The proteolytic enzymes: trypsin, chymotrypsin, carboxypolypeptidase,
and proelastase breaks down the partially digested protein molecules of
the chyme that leaves the stomach
• Trypsin and chymotrypsin split protein molecules into small polypeptides;
• Carboxypolypeptidase cleaves individual amino acids from the carboxyl
ends of the polypeptides
• Proelastase is converted to elastase which the digests elastin fibers that
partially hold meats together
• The percentage of proteins that are digested to their constituent amino
acids by the pancreatic juices are few
• Most remain as dipeptides and tripeptides

Protein digestion contd…
• The intestinal lumen of the small intestine contains enterocytes that
line the villi
• The enterocytes that lines the intestinal lumen of the small intestine
(duodenum and jenunum) completes the process of protein digestion
• These cells have a brush border that consists of hundreds of microvilli
projecting from the surface of each cell.
• Multiple peptidase (aminopolypeptidase and dipeptidases) are located
at the membrane of the microvilli, they protrude through these
mebranes and makes contact with the intestinal fluid
• These peptidase splits polypeptides to dipeptides and tripeptides and
few amino acids which are taken into the enterocytes through the
microvillar membrane
• Within the cytosol of the enterocytes, the tripeptide and the
dipeptides are further broken down to single amino acid by yet
another enzyme known as the peptidase enzyme
• These amino acids passes through the other side of the enterocytes
and thus into the blood
• This accounts for about 99% of protein digestion

DIGESTION AND ABSORPTION OF FATS
• Neutral fat also known as triglycerides are the most
abundant fats of our diet which are more animal origin
and a few plant origin
• Triglycerides are composed of three fatty acid side
chain, nucleus and glycerol
• Phospholipids, cholesterol and cholesterol esters are
found in small quantities in our diet
• Phospholipids and cholesterol esters contains fatty acid
and thus considered as fats
• Cholesterol is a sterol compound however, it exhibits
the physical and chemical properties of fat

Digestion and absorption of fats
• The lingual lipase that is secreted by the lingual gland,
digests triglcerides in the stomach
• This digestion accounts for about 10%bof the total fat
ingested
• The fat globules are broken down to small sizes to enable
water soluble enzymes act on the globule surface
• This process is known as emulsification of fat this is the
first step in fat digestion
• This process starts in the stomach where the fat mixes with
stomach digestion products
• The duodenum is the site for most fat digestion in the
presence of bile

• The bile contains bile salts and phospholipid lecithin which
does the fat emulsification
• The bile salts and lecithin has polar part which makes them
soluble in water while most of the portions of these
molecules are fat soluble
• The fat-soluble portions of these liver secretions dissolve in
the surface layer of the fat globules, with the polar portions
projecting.
• The polar projections reduces the surface tension of the fat
because they are soluble in the watery fluid surrounding
thus making the fat soluble
• The lipase enzymes are water-soluble compounds and can
attack the fat globules only on their surfaces

• Consequently, the pancreatic lipase secreted by the pancreas
digests all triglycerides within minutes of their release to free fatty
acids and 2 monoglycerides
• Another enzyme is the enteric lipase which is secreted by the small
intestine it functions as well in fat digestion but usually the
pancreatic lipase would have digested all the triglycerides in the
intestine
• The free fatty acids and monoglycerides dissolves in the central lipid
portions of bile micelles and a carried together with the chyme
• They are carried to the surface of the microvilli of the small
intestine where they penetrate into the recesses among the
microvilli
• At this point, the fatty acids and monoglycerides diffuse out of the
micelles into the interior of the epithelial cell memberane

• The bile micelles still remain in the cyhme where they absorb most
of the monoglycerides and fatty acids
• This function of the bile micelles is describes as “ferrying” function.
About 97% of fat is absorbed in the presence of bile micelles while
in its absence about 40 – 50% are absorbed
• The fatty acids and monoglycerides after entering the epithelial cell,
are taken up by the smooth endoplasmic reticulum to form
triglycerides
• These triglycerides are released in the form of chylomicrons
through the base of epithelial cells
• They flow upwards through the thoracic lymph duct amd empty
into the circulating blood

Gastrointestinal Physiology and Biliary System Explained

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