Download Digestive system

Physiology of Digestive system
physiology for 2nd year students
By Assist. Prof.Dr. Majida Alqayim
Departement of physiology and pharmacology
College of Veterinary Medicine
University of Baghdad
Digestion
The digestive tract, alimentary
canal or gut is a hollow tube
stretching from the mouth to the
anus. It is the organ system concerned
with the treatment of foods.
At the mouth the large food molecules
are taken into the gut - this is
called ingestion. They must then be
broken down into smaller ones by
digestive enzymes - digestion, before
they can be taken from the gut into
the blood stream - absorption. The
cells of the body can then use these
small molecules - assimilation. The
indigestible waste products are
eliminated from the body by the act
of egestion
Four fundamental processes that take place are:
Transporting of food : Contractions of smooth muscle in the
wall of the tube that crush, mix and propel its contents
Digestion :
 mechanical breaking down of food into smaller parts “motility”
 chemical breaking down of food into smaller components that
can be absorbed “
Secretion: Delivery of enzymes, mucus, ions and the like into the
lumen, and hormones into blood.
Absorption: Transport of water, ions and nutrients from the
lumen, across the epithelium and into blood.
According to the type of feeding Some animals :
Herbivores eat plants.
Carnivores eat the herbivores.
Omnivores many animals feed on both animal
and vegetable material
Digestive system
The food that enters the mouth passes
to the oesophagus, then to
the stomach, small
intestine, cecum, large
intestine, rectum and finally
undigested material exits at the anus.
The liver and pancreas produce
secretions that aid digestion and
the gall bladder stores bile.
Herbivores have an appendix which
they use for the digestion of
cellulose. Carnivores have an
appendix but is not of any function
anymore due to the fact that their
diet is not based on cellulose
anymore.
• The mouth and phyranx:The tongue is a muscular organ moves food
around the mouth and rolls it into a ball for
swallowing.
Taste buds are located on the tongue and in dogs
and cats it is covered with spiny projections used
for grooming and lapping. The cow’s tongue is
prehensile and wraps around grass to graze it.
Teeth seize, tear and grind food
Motility within the Mouth- mastication(chewing):
is the mechanical digestion , cheeks, tongue, and
teeth involved in both voluntary and involuntary
grinding, ripping, and tearing of foodstuff tongue
compacts ground food into a "bolus
.
Secretion in the
mouth
The Salivary glands consist of
the parotid, submandibular, and sublingual glands as
well as numerous smaller buccal glands secreting.
•
There are two types of salivary glands:
• serous glands: These glands produce a secretion rich
in water, electrolytes, and enzymes. A great example
of a serous oral gland is the parotid gland.
• Mixed glands: These glands have both serous
cells and mucous cells, and include sublingual and
submandibular glands. Their secretion is mucinous
and high in viscosity
• Regulation of Salivary Secretion
• Salivation is controlled via the Sympathetic and para
sympathetic autonomic nervous system
•
from the salivary nuclei in the brain stem. largely
dependent upon cholinergic signalling from
• Salivary fluid secretion is increased by
parasympathetic nerves whilst the protein content of
saliva in the major (parotid, submandibular and
sublingual) salivary glands, is increased by sympathetic
nerves and the release of noradrenaline.
Factors that induce salivation include:
• Taste stimuli, especially sour taste
• Higher centers especially appetite anticipation, smells
and visual clues
•
signals from the stomach and upper GI tract,
particularly irritating stimuli.
•
•
.
The main salival component and their
functions
Functions of Saliva Components
Mucins
Lubricate food
Protect teeth against acid
Help protect against bacteria, viruses, fungi
Digestive Enzymes
a-Amylase – digests starches
Lipase – digests fats anti-bacterial and oral hygiene role,
Protease – digests proteins
Lysozyme, Peroxidases,
Lactoferrin, Histatins, Cystatins,
Anti-bacterial agents
thiocyanate
Secretory Immunoglobulin A,
Histatins, Cystatins
Anti-fungal, anti-viral agents
Bicarbonate ions, Phosphate
ions, Proteins
Buffer. Help protect teeth and soft tissues against acidic conditions
Calcium ions, Phosphate ions,
Proline-rich proteins
Help maintain mineral content of tooth enamel
Haptocorrin (also known as Rfactor):
Helps with the absorption of Vitamin B12
The phyranx
•
•
•
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Deglutition ( swallowing)
- Step 1: Oral phase- (Voluntary )control by brain cortex . A mass of
chewed, moistened food, a bolus, is moved to the back of the moth by the
tongue. In the pharynx, the bolus triggers an involuntary swallowing reflex
that prevents food from entering the lungs, and directs the bolus into the
esophagus.
Step 2: pharyngeal phase- brainstem reflex .Muscles in the esophagus
propel the bolus by waves of involuntary muscular contractions
(peristalsis) of smooth muscle lining the esophagus.
Step 3: Esophageal phase- brainstem reflex .The bolus passes through the
gastroesophogeal sphincter Lower Esophageal Sphincter(LES), into the
stomach by a primary peristaltic waves . Asecondary peristaltic waves
clears residual materials . LES innervated with parasympathetic nerves, in
both excitatory mediated by Acetylcholin, and inhibitory mediated by VIP
and No. Heartburn results from irritation of the esophagus by gastric
juices that leak through this sphincter.
Esophagus
secretion is mucous
Peristalsis motility
Microanatomy of the Digestive Tube
Tunic serosa •
Tunica muscularis •
Tunica submucosa •
Tunica mucosa
Gross and Microscopic Anatomy of
the simple Stomach
The stomach is an expanded section of the digestive tube between
the esophagus and small intestine.
regions of the stomach: Cardia
Fundus
Body
Antrum
Pylorus.
as food is liquefied in the stomach it passes through the pyloric canal
into the small intestine.
Two sphincters: 1- Lower esophagal sphincter
2- Pyloric sphincter
The wall of the stomach is
Tunic serosa
Tunica muscularis: three layers :longitudinal circular, andhas an
extra, oblique layer of smooth muscle inside the circular layer,
which aids in performance of complex grinding
Tunica submucosa
Tunica mucosa
In the empty state, the •
stomach is contracted and
its mucosa and submucosa
are thrown up into distinct
folds called rugae; when
distended with food, the
rugae are "ironed out" and
flat .
Gastric cells
Four major types of secretory epithelial cells cover the surface of
the stomach and extend down into gastric pits and glands:
Mucous cells: secrete an alkaline mucus that protects the
epithelium against shear stress and acid
Parietal cells: secrete hydrochloric acid
Chief cells: secrete pepsin, a proteolytic enzyme
G cells: secrete the hormone gastrin
There are differences in the distribution of these cell types among
regions of the stomach - for example, parietal cells are abundant
in the glands of the body, but virtually absent in pyloric glands.
The micrograph to the right shows a gastric pit invaginating into
the mucosa (fundic region of a raccoon stomach). Notice that all
the surface cells and the cells in the neck of the pit are foamy in
appearance - these are the mucous cells. The other cell types are
farther down in the pit and, in this image, difficult to distinguish.
Gastric secretion
Pepsin It is produced by the chief cells in its inactive form pepsinogen. Pepsinogen is then activated by the
stomach acid into its active form, pepsin. Pepsin breaks down the protein in the food into smaller particles,
such as peptide fragments and amino acids.
Hydrochloric acid (HCl): is produced by the parietal cells.
•
HCl functions : 1-denature the proteins ingested, to destroy any bacteria or virus that remains in the food. 2•
activate pepsinogen into pepsin.
Mechanism for HCl production:
•
The H+ is derived from H2O and CO2 reaction to form carbonic acid regulated by the enzyme carbonic
•
anhydrase.in the cell,
The Cl_ from the blood enter the cell in exchange with bicarbonate
•
•
Stimulation of Gastric Acid secretion.
• 1- Histamin release by enterochromaffin-like cells (ECL),
• 2- Gastrin, secreted by the G cells in the antrum of the
stomach
• 3- Acetylcholine release from the vagus nerve
•
• Intrinsic factor (IF): Intrinsic factor is produced by the parietal cells
of the stomach.. Intrinsic factor (IF) produced by the parietal cells
then binds Vitamin B12, creating a Vit. B12-IF complex. This
complex is then absorbed at the terminal portion of the ileum.
Mucin: secreting mucin and bicarbonate via its mucous cells, to •
protect lininig of the stomach
Gastrin: This is an important hormone produced by the "G cells" of •
the stomach.. Gastrin is an endocrine hormone and therefore
enters the bloodstream and eventually returns to the stomach
where it stimulates parietal cells to produce hydrochloric acid (HCl)
and Intrinsic factor (IF).
Gastric Lipase: Gastric lipase is an acidic lipase secreted by •
the gastric chief cells in the fundic mucosa in the stomach.
Function of the stomach
Mechanical digestion
Secreation of acid(HCJ) aid in activation of
pepsinogen
Digestion of proteins by active pepsine
Absorbtion of water and electrloytes
Synthesis of gastric intrinsic factor (GIF), is
a glycoprotein produced by the parietal cells of
thestomach. It is necessary for the absorption
of vitamin B12 (cobalamin

The Gastric Mucosal Barrier
The mucosa of the stomach is exposed to the highly corrosive
acidity of gastric juice. Gastric enzymes that can digest protein
can also digest the stomach itself. The stomach is protected from
self-digestion by the mucosal barrier.
This barrier has several components:
First, the stomach wall is covered by a thick coating of
bicarbonate-rich mucus. This mucus forms a physical barrier, and
its bicarbonate ions neutralize acid.
Second, the epithelial cells of the stomach's mucosa meet at
tight junctions, which block gastric juice from penetrating the
underlying tissue layers.
Third, stem cells quickly replace damaged epithelial mucosal
cells, when the epithelial cells are shed, and replaced every 3 to
6 days.
Regulation of
Gastric activity
(Secretion and
Motility
The nervous system and endocrine system collaborate to increase gastric secretion and motility when
food is eaten and to suppress them as the stomach empties. Gastric activity associated with eating
is divided into three phases:
• The cephalic phase (reflex phase) takes place before food enters the stomach. The smell, taste,
sight, or thought of food triggers this phase
• The gastric phase local neural and hormonal mechanisms triggered by the entry of food into the
stomach;- Distenation of stomach by the meal
Protein content of the meal ----- Increase Gastrin secretion ----- 1-Increase HCl secretion .
2- Increas the stomach smooth muscle activity (Gastric motility).
• The intestinal phase this phase is triggered by :
intestine distenation
PH< 2
Lipids
• 1- Neural mechanisms by reducing the vagal activity
• 2- Hormonal mechanisms by secretion of hormones : Secretin , Cholecystokinine, and Gastric
inhibitory peptide these hormones released by duodenum decrease the gastric activity
Gross and Microscopic
Anatomy of the Small Intestine
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•
•
•
The small intestine is the longest section of the digestive
tube and consists of three segments forming a passage
from the pylorus to the large intestine:
Duodenum: a short section that receives secretions
from the pancreas and liver via the pancreatic and
common bile ducts.
Jejunum: considered to be roughly 40% of the small gut
in man, but closer to 90% in animals.
Ileum empties into the large intestine; considered to be
about 60% of the intestine in man, but veterinary
anatomists usually refer to it as being only the short
terminal section of the small intestine. In most animals,
the length of the small intestine is roughly 3.5 times
body length - your small intestine, or that of a large dog,
is about 6 meters in length. Although precise boundaries
between these three segments of bowel are not
observed grossly but there are histologic differences
among duodenum, jejunum and ileum.
Intestinal wall
Mucosa
Throughout the small intestine,
mucosa is folded up into fingerlike
projections called villi, specialized for
maximizes the surface area available
for nutrient absorption. Between the
villi are infoldings known as crypts.
The villus epithelial cells are also
notable for the extensive microvilli
that characterize their apical
membranes. These microvilli are
endowed with a dense glycocalyx (the
brush border) that probably protects
the cells to some extent from the
effects of digestive enzymes. Some
digestive enzymes are also actually
part of the brush border, being
membrane-bound proteins.
Types of intestinal cells
Enterocytes, the epithelial cells which mature
into absorptive epithelial cells that cover the villi.
These are the cells that take up and deliver into
blood virtually all nutrients from the diet.
However, two other major cell types populate the
small intestinal epithelium:
Enteroendocrine cells which, as part of the enteric
endocrine system sense the lumenal environment
and secrete hormones such as cholecystokinin and
gastrin into blood.
Goblet cells, which secrete a lubricating
mucus into the intestinal lumen.
The panath cellsThey are identified
microscopically by their location just below the
intestinal stem cells in the intestinal
glands (crypts of Lieberkühn) and the
large eosinophilic refractile granules that occupy
most of their cytoplasm. These granules consist
of several anti-microbial compounds and other
compounds that are known to be important in
immunity and host-defense. When exposed
to bacteria or bacterial antigens, Paneth cells
secrete some of these compounds into
the lumen of the intestinal gland, thereby
contributing to maintenance
of the gastrointestinal barrier.
Intestinal Stem Cells – found at the intestinal
crypt base; differentiate to give rise to all other
intestinal epithelial cell types
• Intestinal epithelial renewing
• The normal intestinal epithelium is
replaced every three days or so.
extracellular matrix.
• Stem cells that give rise to both
crypt and villus epithelial cells
reside toward the base of the
crypts and are responsible for
completely renewing the
epithelium every few days or so.
• Daughter cells undergo several
rounds of cell division in the crypts
then migrate out onto the villi,
where they are eventually shed
and lost in the stool.
Secretions of intestine
Intestinal juice
Intestinal juice refers to the clear to pale yellow watery
secretions from the glands lining the small intestine walls.
Which containing :
1- fluid and electrolyttes ,and ions to neutrelize the acidity of
the HCl from the stomach
2- Digestive enzyme: digestion of neutrients
3- Mucous: Protection of intestinal epithelial cells
4- Hormones:regulation th eintestinal secretions and motility.
Sources of intestinal juice:
Intestinal glands, The Brunner's glands , crypts of lieberkühn,. –
Pancreatic juice
Bile secretion
Secretory Glands of intestine
.
Located over the entire surface of the small intestine are small pits called
crypts of lieberkühn,.These crypts lie between the intestinal villi. In the
duodeum these glands called Brunne’s gland . The function of the mucus
secreted by brunner’s glands is :
1- to protect the duodenal wall from digestion by the highly acid gastric juice
emptying from the stomach.
2- In addition, the mucus contains a large excess of bicarbonate ions
neutralizing the hydrochloric acid entering the duodenum from the
stomach.
Secretion of mucus
Secretion of mucus is by exocytosis of secretory granules. Interestingly,
goblet cells have two pathways for secretion:
Constitutive or basal secretion: low level, unregulated and essentially
continuous secretion
Stimulated secretion: regulated exocytosis of granules in response to
extracellular stimuli.
Stimulations for mucus secretion:
(1) vagal stimulation, which causes increased brunner’s glands secretion
concurrently with increase in stomach secretion
(2) gastrointestinal hormones, especially secretin
(3) tactile or irritating stimuli on the duodenal mucosa
Digestive Enzymes in the Small
Intestinal Secretion.
Enzyme
Substrate
Catalytic Function or Products
Enteropeptidase
Trypsinogen
Trypsin
Aminopeptidases
Polypeptides
Cleave amino terminal amino acid from peptide
Carboxypeptidases Polypeptides
Cleave carboxyl terminal amino acid from peptide
Endopeptidases
Polypeptides
Cleave between residues in midportion of peptide
Dipeptidases
Dipeptides
Two amino acids
Maltase
Maltose, maltotriose, dextrins
Glucose
Lactase
Lactose
Galactose and glucose
Sucrasea
Sucrose; also maltotriose and Fructose and glucose
maltose
-Dextrinasea
-Dextrins, maltose
maltotriose
Glucose
Trehalase
Trehalose
Glucose
Nuclease and
related enzymes
Nucleic acids
Pentoses and purine and pyrimidine bases
Various peptidases
Di-, tri-, and tetrapeptides
Amino acids
GIT as endocrine gland
Action of the major digestive hormones
Secretin - is in the duodenum. This hormone responds •
to the acidity of the chyme.
Secretin stimulate: •
the secretion of sodium bicarbonate in the pancreas •
the bile secretion in the liver. •
Cholecystokinin (CCK) - is in the duodenum and •
stimulates the release of digestive enzymes in the
pancreas and stimulates the emptying of bile in the
gall bladder. This hormone is secreted in response to
fat in chyme.
Gastric inhibitory peptide (GIP) - is in the duodenum •
and decreases the stomach churning in turn slowing
the emptying in the stomach. Another function is to
induce insulin secretion.
Motilin - is in the duodenum and increases the •
migrating myoelectric complex component of
gastrointestinal motility and stimulates the
production of pepsin
Intestinal function
Chemical digestion by digestive enzymes:
 Pancreatic secretion
 Intestinal secretion
 Biliary secretion
Absorption
Mixing the chime with digestive enzymes via
contraction of the muscle walls, is the force that
propels matter through the small intestine. It is a slow
process, allowing the food matter to mix with the
digestive juices.
Immunology