This document summarizes gastrointestinal physiology, including the structure and layers of the GI tract wall, motility, digestion, absorption, and hormonal and neural control. It describes how ingested food passes through the mouth, esophagus, stomach, and intestines. The four layers of the GI wall are outlined. Motility throughout the GI tract is controlled by peristalsis, segmentation, and sphincters. Hormones and the enteric nervous system regulate digestion and emptying between sections.

1. GASTROINTESTINAL PHYSIOLOGY
 Ingestion of food is controlled by hunger and appetite.
In health the body controls the rate at which ingested
food moves through the GI system, so that digestion
and absorption are optimised. The first residue from a
meal will pass through the GI system in 7-10 hours, but
some may take up to 5 or 6 days to pass through.

2.  Ingested food pass through the mouth, esophagus,
stomach, small intestine, (duodenum, jejunum, and
ileum), and the large intestine (colon) before exiting the
body.
 THE GI WALL HAS FOUR LAYERS.
 The basic structure of the GIT wall is similar in the
stomach and intestine though with some variations
between different sections of the GIT.
 An inner layer—mucosa
 a middle layer--- submucosa
 An outer layer---muscle
 Cover-----------serosa

3. I. THE MUCOSA
 The inner lining of the GIT created from;
 A. A single layer of epithelial cells
 B. Lamina propria, subepithelial connective tissue that
holds the epithelium in place
 C. Muscularis mucosa, a thin layer of smooth muscle .
 Several modifications have evolved to increase the amount
of surface area that is contained within the lumen.
 1st entire wall is crumpled into folds –the rugae in the
stomach and plicae in the intestine. Intestinal mucosa also
projects into the lumen in small fingerlike extensions-villi

4.  Additional surface area is created by tubular
invaginations of the surface that extend down into the
suppository connective tissue. These invaginations are
gastric glands in the stomach and crypts in the
intestine. Some of the deepest invaginations form
secretory submucosal glands that open into the
lumen through ducts
 The surface area of each individual cell is increased by
microvilli along apical membrane

5. II. THE SUBMUCOSA
 Middle layer composed of connective tissue with larger
blood and lymph vessels. It also contains, submucosal
plexus, (meissner’s) one of the 2 major nerve networks
of the enteric nervous system. The enteric NS is a
unique division of the nervous system that helps
coordinate digestive functions

6. iii. MUSCULATURE
 The outer wall of the intestinal tract consists mostly of
2 layers of smooth muscle, an inner circular layer,
and an outer longitudinal layer. Contraction of the
circular layer decreases the diameter of the lumen ,
and contraction of the longitudinal layer shortens the
tube.
 Myenteric plexus (Auebach’s) which is the second
nerve network of the Enteric NS lies between the two
muscle layers
 Iv. THE SEROSA
This is the outer covering of the GIT

7. Principles of GI Function
 Neural Control of GI Function
 The gut is controlled by its own nervous system the
Enteric nervous system,
 Autonomic nervous system (parasympathetic and
sympathetic)
 Endocrinal Control of GI Function
 Several hormones.

8. ENTERIC NERVOUS SYSTEM
 Contains all the neural elements required for complex
integrative function and behaves like a “little brain “ in the
generation and modulation of phasic patterns of neuronal
activity. It programs and regulates all GI functions. The 2
peripheral plexuses of the ENS are
 the submucosal (Meissner’s) plexus located within the
submucosa , is more involved with local conditions and
controls local secretion, absorption and local movements
 Myenteric (Auerbach’s) plexus located between the circular
and outer longitudinal muscle layer. MOTILITY throughout
the whole gut. Stimulation of the plexus increases the tone
and velocity and intensity of the contractions. Inhibition
helps relax the sphincters

9.  The mucosa and epithelium have sensory nerve
endings that feed signals to the both layers of the
enteric plexus as well as sending information back to
the sympathetic pre- vertebral ganglia, the spinal corn
and to the brain stem. Numerous transmitters seem to
be involved, the more important of which are
acetylcholine and norepinephrine. The former excites,
the latter inhibits it.

10. EXTRINSIC NERVES
 I. PARASYMPATHETIC FIBERS are supplied by the vagus
nerve and pelvic nerves which are of sacral origin.
Parasympathetic fibers are cholinergic and innervate both
plexuses of the enteric NS. Increased parasympathetic
activity increases smooth muscle activity. Motility and
secretion is increased, there is a reduction in constriction of
sphincters. An increase in parasympathetic activity
promotes digestive and absorptive processes.
 The proximal half of the nervous system is innervated from
the cranial parasympathetic nerve fibers via the vagal nerve.
The distal half is innervated via Sacral Parasympathetic
nerves, which gives supply to the sigmoid colon, rectum and
anus, and are important in controlling defecation

11.  SYMPATHETIC INNERVATION
 The fibers originate in the sympathetic ganglia of T-5 to L-2
and terminate on the enteric nervous plexus, but also a few
nerves terminate in the mucosa it self
 SYMPATHETIC FIBERS innervation of the GI is
noradrenergic postganglionic. Increased sympathetic
discharge inhibit acetylcholine secretion from cholinergic
neurons.
 Some sympathetic fibers innervate smooth muscle cells
directly and some innervate splanchnic blood vessels and
act to cause vasocostriction, leading to decreased motility
and secretions, increase in constriction of sphincters.

12. Afferent Sensory Innervation
 Numerous afferent sensory fibers innervate the gut.
Some have their cell bodies in the enteric plexus, and
some in the spinal cord. As well as sending information
concerning irritation and over distension, they can
also pick up the presence of chemical signals in the gut.
80% of the fibers in the vagus nerve are afferent, and
these send signals all the way to the medulla for
processing

13. Gastrointestinal Reflexes
 GI reflexes can be considered;
 1. Local
 2. Regional
 3. Systemic
 Local reflexes are processed entirely within the enteric system and
control secretion, local motility, and mixing contractions.
 Regional reflexes go to the sympathetic ganglia, and are important for
reflexes at a distant, such as the gastro- colic reflex causing
evacuation of the colon, and messages from the intestine to the stomach
to inhibit emptying, the entero- gastric reflex, or the colono- ilial
reflex that inhibits emptying of the ilial contents into the colon.
 Systemic reflexes are processed in the spinal cord or brainstem and
will control overall activity f the GI system, for example pain reflexes
that will inhibit the entire GI system.

14. Hormone Source Stimulus Stomach
Motility and
Secretion
Pancreas Gall bladder
1. Secretin S cells lining
the
duodenum
Acid entering
duodenum
Inhibits Stimulates
fluid secretion
(HCO3
-)
2. CCK Cells lining the
duodenum
Fat and amino
acids entering
duodenum
Inhibits
emptying
Stimulates
enzyme
secretion
1. Contraction
2. Relaxation
sphincter
(Oddi)
3. Gastrin G cells of
stomach
Antrum
Duodenum
Stomach
distension
Parasymp
Peptides
Stomach acid
inhibits
Stimulates
4. GIP Duodenum Fat, CH0,
amino acids
Inhibits
CCK = Cholecystokonin, GIP = Gastric inhibitory peptide (glucose insulintropic peptide)
Note: In a non-acid producing stomach (e.g, chronic gastritis), the reduced negative feedback increases circulating
gastrin.
All four hormones stimulate insulin release.

15. MOTILITY
 In the GIT serves 2 purposes
 I. Moving food from mouth to the anus
 II.Mechanically mixing food to maximize exposure to
digestive enzymes and the absorptive epithelium.

16.  Most of the intestinal tract is composed of single unit
muscle whose cells are electrically connected by gap
junctions
 Certain intestinal muscle cells act as pacemaker and exhibit
spontaneous depolarization which is similar to pacemaker
of the heart. These generate slow wave potentials at a rate of
8-11 per minute. When the slow wave reaches threshold, it
fires a battery of AP’s that spread through gap junctions to
adjacent muscle cells. The longitudinal layer of muscle
conducts these action potentials along the length of the GI
tract, creating a wave of contractions.

17.  Smooth muscle is unique in that it can contract even
without a significant change in membrane potential.
This occurs when a chemical ligand such as hormones
or drugs combine with membrane receiptors that either
open Ca++ channels in the muscle cell membrane, or
cause release of Ca++ from the SR. This type of
contraction is termed pharmacomechanical coupling.

18. SMOOTH MUSCLE
CHARACTERISTICS
 Smooth, also called involuntary or un-striated muscle
is usually found in the walls of the hollow organs, and
have many unique characteristics. Contraction of these
cells is due an influx of Ca++ ions. In the gut three
types of contraction are seen;
 Tonic sustained contractions such as occur in
sphincters
 Peristaltic contractions
 Segmental contractions
 By a variety of these contractions, food and chyme is
moved through the bowel.

19. TONIC, SUSTAINED CNTRACTION
 These type of contractions occur in rings or bands of
muscles- sphincters that separate different sections of
the digestive system.
There are several sphincters , upper and lower esophageal
sphincters which close off the 2 ends of esophagus
-the pyloric sphincter located between the stomach and
the small intestine.
-the sphincter of oddi- which controls the flow of bile and
pancreatic juices into the small intestine

20.  -ileocecal sphincter found between the small intestine
and the large intestines.
 -internal and external sphincters
 The upper esophageal and the external anal sphincter
are composed of skeletal muscle, and the remaining 5
are smooth muscles
 All sphincters are tonically contracted . When they
relax material is able to pass from one segment of the
GI to another

21. PERITALTIC CONTRACTIONS
 These are progressive waves of contractions that move
from one section of the intestinal tract to another.
-this movement is responsible for the rapid forward
propulsion of material through the tract.
The forward movement of material with peristaltic
contraction occurs at a speed of between 2 and
25 cm/sec

22.  In the peristaltic Reflex, peristaltic waves are triggered
in isolated segment of the intestine by distention of the
wall. This reflex is mediated strictly through the enteric
NS- peristalsis is also subject to external control by
hormones, paracrines and ANS

23. SEGMENTAL CONTRACTION
 These are mixing contraction that knead material back
and forth without propelling it forward at a very fast
rate . In this segment contraction alternative segments
of intestine contract and relax, propelling material
short distances in both directions.
 Segmental contractions – churn the intestines
contracts back and forth mixing them and keeping
them in contact with absorptive epithelium.

24. MECHANICS OF CONTRACTION
 Smooth or un-striated muscle cells contract by altering
their shape. They contain numerous actin-myosin
bundles. Some of the strands attach to the cell, they are
all anchored to the dense bodies in the cytoplasm of the
cell. On activation the actin strands slide over the
myosin causing shortening of the actin-myosin bundle.

25. MOTILITY THROUGHOUT THE GI
SYSTEM
 The passage of food through the gut, its conversion to
chyme, and finally feces is all under involuntary
control. Only the first part –ingestion and swallowing,
and the last part – defecation are under voluntary
control.

26. MASTICATION
 Chewing is extremely important part of the digestive
progress especially for fruits and vegetables as these
have indigestible cellulose coats which must be
physically broken down . Also digestive enzymes only
work on the surfaces of food particles, so the smaller
particle, the more efficient the digestive process

27. SWALLOWING
 Swallowing is coordinated by the swallowing or
deglutition center located in the lower pons. Impulses
are carried by the Trigeminal, Glossopharangeal, and
Vagus nerves.
 STAGES has 3 stages
 The tongue pushes a bolus of food against the soft
palate triggering the swallowing reflex.
 The soft palate is pulled upwards preventing reflux of
food into the nasal cavities.
 The vocal cords are strongly approximated

28. SWALLOWING
 The tongue pushes a bolus of food against the soft palate
triggering the swallowing reflex.
 The soft palate is pulled upwards preventing reflux of food
into the nasal cavities
 The vocal cords are strongly approximated
 The larynx is pulled upwards, closing the epiglottis,
preventing food entering the trachea. The esophageal
sphincter relaxes.
 The muscular wall of the pharynx contracts beginning
superiorly, pushing the food into the esophagus
 Peristaltic waves assisted by gravity push the food down the
esophagus.

29. ESOPHAGUS
 Food is carried down the esophagus by peristaltic
contractions. If these are insufficient to move all the
food, stronger secondary peristaltic waves develop.
These are initiated by both the myenteric plexus and
centrally
 The muscle at the lower end of the esophagus thickens
and is called the lower esophageal sphincter. This is
usually tonically contracted, but relaxes when the
peristaltic wave reaches it, allowing passage of food
into the stomach.

30. STOMACH
 Food entering the stomach passes into the fundus of
the stomach where it is stored. Weak peritaltic waves
known as mixing waves originate in the upper stomach
and pass down to the antrum. These waves become
stronger as they approach the antrum, and as they
push the food against a closed pylorus they also act as
mixing waves. Food in the antrum of the stomach is
also thoroughly mixed with segmental contractions.
The mixed fluid contents are called chyme, and
amounts of this are pushed through the pylorus into
the duodenum with the stronger peritaltic contractions.

31. Control of stomach emptying
 T he rate of emptying of the stomach is controlled by
various factors originating in the duodenum and stomach,
of which the duodenal factors are the most important.
 Gastric factors include increased volume of food in the
stomach and stretching of the stomach wall. The hormone
Gastrin also appears to promote stomach entering
 Duodenal factors serve mainly to inhibit entering , thereby
ensuring that the intestine is not overwhelmed by sudden
influx of acidic chyme. They include nervous reflexes and
hormones. The nerve reflexes are transmitted both by the
enteric nervous system and through extrinsic nerves via the
pre-vertebral sympathetic ganglia.

32. Control of Stomach Emptying cont..
 Factors that inhibit emptying include
 Distention of the duodenum
 The degree of acidity of the duodenal chyme
 The osmolarity of the chyme
 Irritation of the duodenum
The reflexes are particularly sensitive to acidity and irritation which
case rapid inhibition of the stomach entering
Hormones that inhibit emptying include cholecystokinin, secretin,
Gastric Inhibitory Peptide (GIP) Secretin is secreted in response
to acidity in the duodenum, Cholecytokinin and GIP response to
the presence of fats in the chyme
Al these factors ensure that the rate of stomach emptying is limited
to what the small intestine can process.

33. SMALL BOWEL
 In the small intestine mixing with segmental
contractions continues and the food is slowly passed
through the intestine , finally passing through the
ileocaecal sphincter to the large intestine, to a large
extent the separation of segmental contractions from
peritaltic contractions is artificial as both serve to
move chyme forward and both add to mixing. Chyme
moves down the small intestine at a rate about 1
cm/min, so will reach the ileocaecal junction in 3-5
hours. If often stays there till the next meal the
gastroileal reflex intensifies peristalsis in the distal
ileum forcing chyme through the ileocaecal valve.

34. Small Bowel cont.
 Intensity of peristalsis is controlled by both neuronal
reflexes and hormones. Neuronal factors include
distension of the intestine wall, but also distension of
the stomach will also cause increased small intestine
peristalsis. Both of these reflexes are mediated by the
mesenteric plexus.
 Hormonal factors increasing peristalsis include
gastrin, CCK, insulin, motulin and serotonin.
 Glucagon and secretin inhibit peristasis.

35. Ileocaecal valve
 T he prime function of the ileoceacal valve is to prevent
reflux of fecal contents into the small intestines. The
valve protrudes into the caecum, thus increased ceacal
pressure will cause occlusion. Furthermore, thee
muscle is thickened for a few centimeters from the
distal end of the ileum, and this acts as a functional
sphincter.
 Increased pressure or irritation in the distal ileum will
cause relaxation, increased caecal pressure or irritation
will cause contriction.

36. LARGE BOWEL and DEFECATION
 The principle function of the large intestine is to
remove water and electrolytes from the chyme, and to
store the resultant faeces until it can be eliminated.
 In the colon the longitudinal muscle coat is condensed
into three narrow bands called taenia coli.
Thus mixing movements of the circular muscle coat , so
called haustrations predominate . These will also slowly
move the contents towards the rectum.

37.  Much of the movement comes from haustrations, but is
the third type of contraction called mass movement
which sends substantial amounts of material forward.
These typically occur 2-3 times a day, usually after a
meal- the so called gastrocolic reflex, and last for
about 20 minutes. They are responsible for the final
formation of the faeces and the filling of the rectum.
 Filling of the rectum is a signal for the relaxation of the
Internal anal sphincter. However the External anal
sphincter is under voluntary control.

38.  Although the myenteric defecation signal only weakly
relaxes the Internal anal sphincter, the stronger signal
comes from parasympathetic reflexes synapsing in the
sacral cord .These can be inhibited centrally, and when
time to defecate is convenient, the inhibition is
released, and the external anal sphincter, under
voluntary control is relaxed.
 The sequence of defecation is often initiated
voluntarily; the epiglotis is closed a deep breath, and
contraction of the abdominal muscles increase intra-abdominal
pressure.

39. Coordination of motility
 All these actions are coordinated, and are under control
of hormones, and the autonomic nervous system as
well as the enteric nervous system, the result is that in
health food products and chyme are moved forward at
the optimal rate to allow for efficient digestion and
absorption.

40. GASTROINTESTINAL SECRETIONS
 Secretions in the GI tract
 About 9 L of fluid pass through the GI system each day,
and only about 2 L are ingested, the rest represent
secretions from the system itself. About 3.5, L is
secreted from the exocrine glands, the salivary glands,
the pancreas and the liver, and other half is secreted
from by the epithelial cells of the digestive tract it self.
Nearly all this fluid is absorbed, so the pellets of feces
only contain a significant amount of fluid in diarrhea.

41.  To put this in perspective a 70 kg man has 42 L of fluid,
so the secretions represent about a sixth of the body’s
volume. The circulation contains about 3.5 liters, so
these secretions represent twice the body’s circulating
volume! Failure of absorption of the intestinal
secretions can thus lead to rapid dehydration and
electrolyte imbalance.
 The secretions consist of digestive enzymes, mucous
and substantial amounts of fluid and ions.

42.  Daily Volume pH
 Saliva 1000 6.0- 7.0
 Stomach 1500 1.0-3.0
 Brunners Gland 200 8.0-9.0
 Pancreas 1000-1500 8.0- 8.3
 Bile 1000 7.8
 Small Intestine 1800 7.5-8.0
 Large Intestine 200 7.5-8.0
 Total 6700-7200

43.  Types of glands
 Several different types of gland and are found in the GIT
 Single cell mucous glands and goblet cells.
 Pit glands. Invaginations of the epithelia into the
submucosa. In the Small intestine these are called Crypts
of Lieberkuhn
 Deep tubular glands. These are found in the stomach – the
gastric glands, and the upper duodenum-Brunners
glands.
 Complex glands, the salivary glands, the pancreas, and the
liver. The salivary glands and the pancreas are compound
acinous glands.

44. Mechanisms of stimulation
 Stimulation occurs due to local effects; autonomic
stimulation; and hormones
 Local effects
 The mechanical presence of food causes stimulation
not only locally but also adjacent regions. This may
either be a direct effect, or via the enteric nervous
system.

45.  Autonomic Stimulation
 Stimulation of parasympathetic nerves serve to
increase secretion. Stimulation of sympathetic nerves
may increase some secretions, but usually diminish
blood flow, which will usually decrease overall secretion
 Hormones
 Several different hormones affect secretions

46.  Digestive Enzymes
 Digestive enzyme are secreted by glandular cells which
will store the enzyme in secretory vesicles until they
are released. These cells are characterised by a robust
rough endoplasmic reticulum and numerous
mitochondria. Passage of materials from the
ribosomes, through the endoplasmic reticulum and
Golgi body to the secretory vesicles takes about 20
minutes.

47.  Water and Eletrolyte secretions
 Glandular secretions must also secrete water
 and electrolytes to go along with the organic substances.
 In its resting state the membrane resting potential is about -
30 -40 mV.
 Neural stimulation causes an influx of -ve chloride ions
decreasing resting potential by 10-20 mV
 Sodium ions follow down the electrical gradient. Cell
contents become hyper osmotic
 Water follows. Intracellular pressure increases.
 Increased pressure opens ports on the apical side of cell
flushing water and electrolytes

48.  Mucus Secreting Cells
 Mucous is viscous secretion used for protection and
lubrication. It consists mainly of Glycoproteins. It is
made by mucous cells in the stomach and Goblet
cells in the small intestine. Up to 25% of the intestinal
epithelial cells are goblet cells. In the mouth about 70%
of the mucous is secreted by the minor salivary glands.

49.  Mucous has the following properties:
 Adherent properties, it sticks well to surfaces
 Enough body to prevent contact of most food particles
with tissue.
 Lubricates well- has a low resistance to slippage
 Strongly resistant to digestive enzymes
 Neutralizing properties..As well as a buffer like effect,
mucous can also contain large quantities of
bicarbonate.

50. Electrolytes and Fluids
 A large portion of the 7 liters is composed of water and ions.
The ionic composition varies from region to region.
 The acini of the salivary glands secrete a sodium and
chloride rich secretion, this is then turned to a potassium,
bicarbonate rich secretion as it travels down the lumen and
ducts of the glands.
 The oxyntic cells of the stomach secrete hydrochloric acid
 The mucous cells of the stomach secrete mucous rich in
bicarbonates
 The pancreatic ducts and ductules secrete a solution rich
in bicarbonate
 The Crypts of Liberkuhn of the intestine secrete a solution
almost indistinguishable from intestinal fluid

51. MOUTH
 The salivary glands consist of the Parotid,
submandibular, and sublingual as well as numerous
smaller buccal glands secreting both serous and
mucoid secretions. The parotid secretions are mainly
serous, the buccal glands mucus, and the sublingual
and submandibular are a mixture of the two. The acini
secrete proteins and a fluid similar in consistency to
interstitial fluid, and the ducts exchange the sodium for
potassium and bicarbonate for chloride leaving saliva
rich in potassium and bicarbonate.
 The saliva secrete between 800- 1500 mls a day.

52.  The sodium ions are actively reabsorbed, and the
potassium ions are actively secreted at the luminal side
of the cell with an excess of sodium reabsorption
causing a – 70m V gradient. This causes passive
reabsorption of chloride ions. Bicarbonate ions are both
passively exchanged, and actively secreted in exchange
for chloride.
 The saliva contains enzyme ptyalin, an amylase for
breaking down carbohydrates as well as lipase.

53.  Anti bacterial action of Saliva
 The mouth contains numerous bacteria, and an
important function of saliva is oral hygiene. The saliva
contains thiocyanate, a potent antibacterial. The lipase
in saliva will also breakdown bacteria cell walls and
facilitates the passage of thiocyanate into bacteria.
 The enzyme lipase is not very important for the
digestion of food, most of fat digestion occurs with the
pancreatic enzymes, but is important in its
antibacterial and oral hygiene role

54. Regulation of Salivary Secretion.
 Salivation is controlled via the parasympathetic system
from the salivary nuclei in the brain stem. Factors that
induce salivation include;
 Taste stimuli, especially sour taste
 Higher centers especially appetite anticipation, smells
and visual clues
 In response to signals from the stomach and upper GI
tract, particularly irritating stimuli.
 Salivation can also occur as a prelude to vomiting.

55. Esophagus
 Esophageal secretions are entirely mucous in
character, and assist passage of food as well as
protecting the lower end of the esophagus from gastric
reflux.

56. Stomach
 The adult stomach secretes about 1500 ml in a normal day
consisting of hydrochloric acid, bicarbonate rich mucous,
and the digestive hormone precursor pepsinogen.
 Pepsinogen is activated to pepsin by the acidity of the stomach. G
cells also secrete the hormone gastrin.
 The gastric pits of the stomach open on to branching glands;
pyloric glands in the antral part of the stomach; gastric or
oxyntic glands in the fundus and body of the stomach.
 The parietal or oxyntic cells secrete hydrochloric acid; the peptic
or chief cells secrete pepsinogen; the mucous cells secrete a
bicarbonate rich mucous; and the G cells secrete the hormone
Gastrin.

57. Hydrochloric Acid secretion
 The oxyntic or parietal cell contains a large number of
intracellular canaliculi. The pH of the secreted acid is
0.8, and has hydrogen ion concentration of about 3
million times that of arterial blood. To achieve this level
of concentration requires a lot of energy, about 1500
calories per liter of secretion

58.  Carbon dioxide and water enter the cell and combine to
form carbonic acid under the influence of enzyme
carbonic anhydrase.
 Bicarbonate is actively excreted at the basal side of the
cell and is exchanged for chlorine.
 Potassium is exchanged for hydrogen ions at the apical
side of the cell.
 Chlorine ions are also actively secreted.

59.  The chief cells also secrete intrinsic factor, a substance
essential for the absorption of vitamin 12B in the small
intestine. In chronic gastritis, this may not be secreted,
and the medical condition pernicious anemia will
develop.
 Mucous secretion rich in alkaline bicarbonate protects
the stomach from the acid of the gastric juice