DIGESTIVE SYSTEM

DIGESTIVE SYSTEM
Mr.MOHANRAJ SUBRAMANIAN

INTRODUCTION
The food we eat contains a variety of nutrients, which are used
for building new body tissues and repairing damaged tissues.
Food is also vital to life because it is our only source of chemical
energy.
However, most of the food we eat consists of molecules that are
too large to be used by body cells.
Foods must be broken down into molecules that are small
enough to enter body cells, a process known as digestion
(dis- apart; gerere = to carry).

The medical specialty that deals with the structure, function,
diagnosis, and treatment of diseases of the stomach and
intestines is called gastroenterology. (gastro- stomach; -
entero- intestines; -logy study of).
The medical specialty that deals with the diagnosis and
treatment of disorders of the rectum and anus is called
proctology (prok-TOL-oˉ-jeˉ; proct- rectum).

OVERVIEW OF DIGESTIVE SYSTEM
1. Two groups of organs compose the digestive system
 Gastrointestinal (GI) tract and Accessory digestive organs.
2. The gastrointestinal (GI) tract, or alimentary canal
(alimentary nourishment), is a continuous tube that extends
from the mouth to the anus through the thoracic and
abdominopelvic cavities.
3. Organs of the gastrointestinal tract include the mouth, most of
the pharynx, esophagus, stomach, small intestine, and large
intestine.
4. The length of the GI tract is about 5–7 meters (16.5–23 ft) in a
living person.

5. The muscles along the wall of the GI tract organs are in a state
of tonus (sustained contraction).
6. The accessory digestive organs include the teeth, tongue,
salivary glands, liver, gallbladder, and pancreas.
7. Teeth aid in the physical breakdown of food, and the tongue
assists in chewing and swallowing.
8. The other accessory digestive organs, however, never come into
direct contact with food.
9. They produce or store secretions that flow into the GI tract
through ducts; the secretions aid in the chemical breakdown of
food.

Overall, the digestive system performs six basic processes
1. Ingestion
2. Secretion
3. Mixing and propulsion
4. Digestion
5. Absorption
6. Defecation

Ingestion
1. This process involves taking foods and liquids into the mouth
(eating).
Secretion
1. Each day, cells within the walls of the GI tract and accessory
digestive organs secrete a total of about 7 liters of water, acid,
buffers, and enzymes into the lumen (interior space) of the
tract.

Mixing and propulsion
1. Alternating contractions and relaxations of smooth muscle in the
walls of the GI tract mix food and secretions and move them
toward the anus.
2. This capability of the GI tract to mix and move material along its
length is called motility

Digestion
1. Mechanical and chemical processes break down ingested food
into small molecules.
2. In mechanical digestion the teeth cut and grind food before it is
swallowed, and then smooth muscles of the stomach and small
intestine churn the food to further assist the process.
3. As a result, food molecules become dissolved and thoroughly
mixed with digestive enzymes.
4. In chemical digestion the large carbohydrate, lipid, protein, and
nucleic acid molecules in food are split into smaller molecules
by hydrolysis.

5. Digestive enzymes produced by the salivary glands, tongue,
stomach, pancreas, and small intestine catalyze these catabolic
reactions.
6. A few substances in food can be absorbed without chemical
digestion.
7. These include vitamins, ions, cholesterol, and water.
Absorption
1. The entrance of ingested and secreted fluids, ions, and the
products of digestion into the epithelial cells lining the lumen of
the GI tract is called absorption.
2. The absorbed substances pass into blood or lymph and circulate
to cells throughout the body.

Defecation
Wastes, indigestible substances, bacteria, cells sloughed from
the lining of the GI tract, and digested materials that were not
absorbed in their journey through the digestive tract leave the body
through the anus in a process called defecation. The eliminated
material is termed feces or stool.

Layers of the GI Tract
1. The wall of the GI tract from the lower esophagus to the anal
canal has the same basic, four-layered arrangement of tissues.
2. The four layers of the tract, from deep to superficial, are the
mucosa, submucosa, muscularis, and serosa/adventitia.

Mucosa
1. The mucosa, or inner lining of the GI tract, is a mucous
membrane.
2. It is composed of
 A layer of epithelium in direct contact with the contents of
the GI tract,
 A layer of connective tissue called the lamina propria, and
 A thin layer of smooth muscle (muscularis mucosa).

1. Every 5 to 7 days they slough off and are replaced by new cells.
2. Located among the epithelial cells are exocrine cells that secrete
mucus and fluid into the lumen of the tract, and several types of
endocrine cells, collectively called enteroendocrine cells (en-
ter-oˉ-EN-doˉ-krin), which secrete hormones.
Lamina Propria
1. The routes by which nutrients absorbed into the GI tract reach
the other tissues of the body.
2. The lamina propria also contains the majority of the cells of the
mucosa associated lymphatic tissue (MALT).
Epithelium

3. These prominent lymphatic nodules contain immune system
cells that protect against disease.
4. MALT is present all along the GI tract, especially in the tonsils,
small intestine, appendix, and large intestine.
Muscularis Mucosae
1. which increase the surface area for digestion and absorption.
SUB MUCOSA
1. The submucosa consists of areolar connective tissue that
binds the mucosa to the muscularis
2. It contains many blood and lymphatic vessels that receive
absorbed food molecules.

3. Submucosa is an extensive network of neurons known as the
submucosal plexus
4. The submucosa may also contain glands and lymphatic tissue.
Muscularis
1. The muscularis of the mouth, pharynx, and superior and
middle parts of the esophagus contains skeletal muscle that
produces voluntary swallowing.
2. Skeletal muscle also forms the external anal sphincter, which
permits voluntary control of defecation.
3. The rest of the tract, the muscularis consists of smooth muscle.
4. An inner sheet of circular fibers and an outer sheet of
longitudinal fibers.

1. The GI tract that are suspended in the abdominal cavity have a
superficial layer called the serosa.
Serosa

The gastrointestinal tract is regulated by an intrinsic set of nerves
known as the enteric nervous system and by an extrinsic set of
nerves that are part of the autonomic nervous system.
Enteric Nervous System
1. The enteric nervous system (ENS), the “brain of the gut,”
2. It consists of about 100 million neurons that extend from the
esophagus to the anus.
3. The neurons of the ENS are arranged into two plexuses: the
myenteric plexus and
submucosal plexus
Neural Innervation of the GI Tract

4. The myenteric plexus (myo- muscle), or plexus, is located
between the longitudinal and circular smooth muscle layers of
the muscularis.
5. The submucosal plexus, or plexus, is found within the
submucosa.
6. The plexuses of the ENS consist of motor neurons, interneuron,
and sensory neurons
7. The motor neurons of the myenteric plexus supply the
longitudinal and circular smooth muscle layers of the muscularis,
this plexus mostly controls GI tract motility (movement),

8. The motor neurons of the submucosal plexus supply the
secretory cells of the mucosal epithelium, controlling the
secretions of the organs of the GI tract.
9. The interneurons of the ENS interconnect the neurons of the
myenteric and submucosal plexuses
10. The sensory neurons of the ENS supply the mucosal epithelium
and contain receptors that detect stimuli in the lumen of the
GI tract.
11. The wall of the GI tract contains two major types of sensory
receptors.

(1) Chemoreceptors, which respond to certain chemicals in the
food present in the lumen, and
(2) Mechanoreceptors, such as stretch receptors, that are activated
when food distends (stretches) the wall of a GI organ.
Autonomic nervous system
Extrinsic set of nerves
Parasympathetic stimulation, increases secretion and activity by
stimulating ENS
Sympathetic stimulation, decreases secretions and activity by
inhibiting ENS.

 Emotions such as anger, fear, and anxiety may slow digestion
because they stimulate the sympathetic nerves that supply the GI
tract.
Gastrointestinal Reflex Pathways
 Many neurons of the ENS are components of GI
(gastrointestinal) reflex pathways that regulate GI secretion and
motility in response to stimuli present in the lumen of the GI
tract.

Largest serous membrane of the body
 Divided into
Parietal peritoneum – lines wall of cavity
Visceral peritoneum – covers some organs
Also called serosa
Space between is peritoneal cavity
5 major peritoneal folds
Greater omentum, falciform ligament, lesser
omentum, mesentery, and mesocolon
Weave between viscera binding organs together
PERITONEUM

Mouth
1. Oral or buccal cavity
2. Formed by cheeks, hard and sot palates, and tongue
3. Oral cavity proper is a space that extends from gums and teeth to
fauces (opening between oral cavity and oro pharynx)

Salivary Glands
1. A salivary gland is a gland that releases a secretion called
saliva into the oral cavity.
2. saliva is secreted to keep the mucous membranes of the mouth
and pharynx moist and to cleanse the mouth and teeth.
3. When food enters the mouth, however, secretion of saliva
increases, and it lubricates, dissolves, and begins the chemical
breakdown of the food.
4. The mucous membrane of the mouth and tongue contains many
small salivary glands that open directly, or indirectly via short
ducts, to the oral cavity.

5. These glands include labial, buccal and palatal glands in the lips,
cheeks, and palate, respectively.
6. There are three pairs of major salivary glands:
the Parotid,
Submandibular, and
Sublingual glands
7. The parotid glands (par- near; to- ear) are located inferior and
anterior to the ears, between the skin and the masseter muscle.
8. Each secretes saliva into the oral cavity via a parotid duct, open
into the vestibule opposite the second maxillary (upper) molar
tooth.

1. The submandibular glands are found in the floor of the
mouth; they are medial and partly inferior to the body of the
mandible.
2. Their ducts, the submandibular ducts, run under the
mucosa on either side of the midline of the floor of the mouth
and enter the oral cavity proper lateral to the lingual frenulum.
3. The sublingual glands are beneath the tongue and superior to
the submandibular glands. Their ducts, the lesser sublingual
ducts, open into the floor of the mouth in the oral cavity
proper.

Composition and Functions of Saliva
1. Chemically, saliva is 99.5% water and 0.5% solutes. Among
the solutes are ions, including sodium, potassium, chloride,
bicarbonate and phosphate.
2. Also present are some dissolved gases and various organic
substances, including urea and uric acid, mucus,
immunoglobulin A, the bacteriolytic enzyme lysozyme, and
salivary amylase, a digestive enzyme that acts on starch.
3. The water in saliva provides a medium for dissolving foods so
that they can be tasted by gustatory receptors.

4. Chloride ions in the saliva activate salivary amylase, an
enzyme that starts the breakdown of starch in the mouth into
maltose, maltotriose, and -dextrin.
5. Bicarbonate and phosphate ions buffer acidic foods that enter
the mouth.
6. saliva is only slightly acidic (pH 6.35–6.85)
7. Salivary glands (like the sweat glands of the skin) help remove
waste molecules from the body.
8. Immunoglobulin A (IgA) prevents attachment of microbes and
the enzyme lysozyme kills bacteria.

Salivation
1. The secretion of saliva, called salivation, is controlled by the
autonomic nervous system.
2. Saliva secreted daily vary considerably but average1000–1500
mL.
3. Parasympathetic stimulation promotes continuous secretion of a
moderate amount of saliva,
4. which keeps the mucous membranes moist and lubricates the
movements of the tongue and lips during speech.
.

5. Most components of saliva are reabsorbed, which prevents fluid
loss.
6. Sympathetic stimulation dominates during stress, resulting in
dryness of the mouth.
7. Chemicals in the food stimulate receptors in taste buds on the
tongue, and impulses are conveyed from the taste buds to two
salivary nuclei in the brain stem (superior and inferior
salivatory nuclei).
8. Returning parasympathetic impulses in fibers of the facial (VII)
and glossopharyngeal (IX) nerves stimulate the secretion of
saliva.

Tongue
1. Accessory digestive organ
2. Skeletal muscle covered by mucous membrane
3. Maneuvers food for chewing, shapes mass, forces food back
for swallowing
4. Lingual glands secrete salivary lipase.

Digestion in the mouth
Mechanical digestion in the mouth
1. Chewing or mastication
2. Food manipulated by tongue, ground by teeth, and mixed with
saliva
3. Forms bolus
Chemical digestion in the mouth
 Salivary amylase secreted by salivary glands acts on starches
1. Only monosaccharides can be absorbed
2. Continues to act until inactivated by stomach acid

 Lingual lipase secreted by lingual glands of tongue acts on
triglycerides
1. Becomes activated in acidic environment of stomach.
Esophagus
1. Secretes mucous, transports food – no enzymes produced, no
absorption
2. Mucosa – protection against wear and tear
3. Muscularis divided in thirds
Superior 1/3 skeletal muscle
Middle 1/3 skeletal and smooth muscle
Inferior 1/3 smooth muscle

4. 2 sphincters – upper esophageal sphincter (UES) regulates
movement into esophagus, lower esophageal sphincter (LES)
regulates movement into stomach
Deglutition
1. Act of swallowing
2. Facilitated by secretions of saliva and mucus
3. Involves mouth, pharynx, and esophagus
3 stages
1. Voluntary – bolus passed to oropharynx
2. Pharyngeal – involuntary passage through pharynx into
esophagus
3. Esophageal – involuntary passage through esophagus to
stomach .

 Peristalsis pushes bolus forward

STOMACH
1. Serves as mixing chamber and holding reservoir
2. 4 main regions
Cardia, fundus, body, pylorus
3. Same 4 layers
 Mucosa – gastric glands open into gastric pits
1. 3 types of exocrine gland cells – mucous neck cells (mucus),
parietal cells (intrinsic factor and HCl), and chief cells
(pepsinogen and gastric lipase)
2. G cell – endocrine cell – secretes gastrin

Mechanical and Chemical Digestion
1. Mechanical digestion
Mixing waves – gentle, rippling peristaltic movements –
creates chyme
2. Chemical digestion
1. Digestion by salivary amylase continues until inactivated by
acidic gastric juice
2. Acidic gastric juice activates lingual lipase
1. Digest triglycerides into fatty acids and diglycerides
3. Parietal cells secrete H+ and Cl-- separately but net effect is HCl
1. Kills many microbes, denatures proteins

1. Several minutes after food enters the stomach, gentle, rippling,
peristaltic movements called mixing waves pass over the
stomach every 15 to 25 seconds.
2. These waves macerate food, mix it with secretions of the gastric
glands, and reduce it to a soupy liquid called chyme (juice).
3. As digestion proceeds in the stomach, more vigorous mixing
waves begin at the body of the stomach and intensify as they
reach the pylorus.
4. As food reaches the pylorus, each mixing wave periodically
forces about 3 mL of chyme into the duodenum through the
pyloric sphincter, a phenomenon known as gastric emptying.

5. Foods may remain in the fundus for about an hour without
becoming mixed with gastric juice.
6. During this time, digestion by salivary amylase continues.
7. Soon, however, the churning action mixes chyme with acidic
gastric juice, inactivating salivary amylase and activating
lingual lipase, which starts to digest triglycerides into fatty
acids and diglycerides.

1. Parietal cells secrete hydrogen ions (H+ ) and chloride ions (Cl--)
separately into the stomach lumen, the net effect is secretion of
hydrochloric acid (HCl).
2. Proton pumps powered by H+ K+ ATPases actively transport H+
into the lumen while bringing potassium ions (K+) into the cell.
3. At the same time, Cl-- and K+ diffuse out into the lumen through Cl--
and K channels in the apical membrane.
4. The enzyme carbonic anhydrase, which is especially plentiful in
parietal cells, catalyzes the formation of carbonic acid (H2CO3)
from water H2O) and carbon dioxide (CO2).

5. As carbonic acid dissociates, it provides a ready source of H+ for
the proton pumps but also generates bicarbonate ions (HCO3).
6. As HCO3 builds up in the cytosol, it exits the parietal cell in
exchange for Cl via Cl/ HCO3 antiporters in the basolateral
membrane.
7. HCO3 diffuses into nearby blood capillaries.
8. This “alkaline tide” of bicarbonate ions entering the bloodstream
after a meal may be large enough to elevate blood pH slightly and
make urine more alkaline

1. HCl secretion by parietal cells can be stimulated by several
sources:
Acetylcholine (ACh) released by parasympathetic neurons,
Gastrin secreted by G cells, and
Histamine, which is a paracrine substance released by mast
cells in the nearby lamina propria.
2. Acetylcholine and gastrin stimulate parietal cells to secrete more
HCl in the presence of histamine.
3. Histamine acts synergistically, enhancing the effects of
acetylcholine and gastrin.

SMALL INTESTINE
1. Most digestion and absorption of nutrients occur in a long tube
called the small intestine.
2. Its length alone provides a large surface area for digestion and
absorption, and that area is further increased by circular folds,
villi, and microvilli.
3. The small intestine begins at the pyloric sphincter of the stomach
4. It averages 2.5 cm (1 in.) in diameter; its length is about 3 m (10
ft) in a living person.

1. The small intestine is divided into three regions
Duodenum - the shortest region,
Jejunum - means “empty,”
Ileum - (IL-e¯-um twisted)
Duodenum
1. It starts at the pyloric sphincter of the stomach and is in the form
of a C-shaped tube that extends about 25 cm.
Jejunum
1. The jejunum (je-JOO-num) is about 1 m (3 ft) long and
extends to the ileum.
Ileum
1. The final and longest region of the small intestine,
2. The ileum measures about 2 m (6 ft) and joins the large intestine
at a smooth muscle sphincter called the ileocecal sphincter.

Histology of the Small Intestine
1. Same 4 layers: mucosa, submucosa, muscularis, and serosa
2. The epithelial layer of the small intestinal mucosa consists of
simple columnar epithelium that contains many types of cells.
3. Absorptive cells, goblet cells, intestinal glands or crypts of
Lieberkühn .
4. Absorptive cells of the epithelium digest and absorb nutrients in
small intestinal chyme.
5. Goblet cells, which secrete mucus.
6. Mucosa contains many deep crevices lined with glandular
epithelium. Cells lining the crevices form the intestinal glands,
or crypts of Lieberkühn and secrete intestinal juice.

7. Besides absorptive cells and goblet cells, the intestinal glands
also contain paneth cells and enteroendocrine cells.
8. Paneth cells secrete lysozyme, a bactericidal enzyme, and are
capable of phagocytosis, regulating the microbial population in
the small intestine.
9. Three types of entero endocrine cells are found in the intestinal
glands. --- S cells, CCK cells or cholecystokinin and K cells.
10. S cells which secrete the hormones secretin.
11. The submucosa of the duodenum contains duodenal glands, also
called Brunner’s glands -- secrete an alkaline mucus that helps
neutralize gastric acid in the chyme.

villi ( tufts of hair)
1. villi (tufts of hair), which are fingerlike projections of the
mucosa that are 0.5–1 mm long.
2. The large number of villi (20–40 per square millimeter) vastly
increases the surface area of the epithelium available for
absorption and digestion.
3. Contains arteriole, venule, blood capillary, and lacteal
4. Nutrients absorbed by the epithelial cells covering the villus pass
through the wall of a capillary or a lacteal to enter blood or lymph.

Micro villi
1. Projects of apical membrane of absorptive cells
2. Brush border with brush border enzymes
Role of Intestinal Juice and Brush-Border Enzymes
1. Each day 1–2 liters (1–2 qt) of intestinal juice,
2. A clear yellow fluid,
3. Intestinal juice contains water and mucus and is slightly alkaline
(pH 7.6).
4. Pancreatic and intestinal juices provide a liquid medium that
aids the absorption of substances from chyme in the small
intestine.

5. The absorptive cells of the small intestine synthesize several
digestive enzymes, called brush-border enzymes
6. Among the brush-border enzymes are four carbohydrate
digesting enzymes called –
1. Dextrinase, maltase, sucrase, and lactase;
2. Protein-digesting enzymes called peptidases (aminopeptidase
and dipeptidase); and
3. Two types of nucleotide-digesting enzymes, nucleosidases and
phosphatases.

Mechanical Digestion
1. Governed by myenteric plexus
2. Segmentations
Localized, mixing contractions
Mix chyme and bring it in contact with mucosa for absorption
3. Migrating motility complexes (MMC)
Type of peristalsis
Begins in lower portion of stomach and pushes food forward

Chemical Digestion
1. In the mouth, salivary amylase converts starch (a polysaccharide)
to maltose (a disaccharide), maltotriose (a trisaccharide), and -
dextrins (short-chain, branched fragments of starch with 5–10
glucose units).
2. In the stomach, pepsin converts proteins to peptides (small
fragments of proteins),
3. lingual and gastric lipases convert some triglycerides into fatty
acids, diglycerides, and monoglycerides.
4. Chyme entering the small intestine contains partially digested
carbohydrates, proteins, and lipids.

5. The completion of the digestion of carbohydrates, proteins, and
lipids is a collective effort of pancreatic juice, bile, and
intestinal juice in the small intestine.

Digestion of Carbohydrates
1. Salivary amylase may continue in the stomach for a while, the
acidic pH of the stomach destroys salivary amylase and ends its
activity.
2. Only a few starches are broken down by the time chyme leaves
the stomach.
3. Starches not already broken down into maltose, maltotriose, and -
dextrins are cleaved by pancreatic amylase.
4. After amylase (either salivary or pancreatic) has split starch into
smaller fragments, a brush-border enzyme called -dextrinase acts
on the resulting -dextrins, clipping off one glucose unit at a time.

5. Three brush-border enzymes digest the disaccharides into
monosaccharides.
6. Sucrase breaks sucrose into a molecule of glucose and a
molecule of fructose;
7. lactase digests lactose into a molecule of glucose and a molecule
of galactose;
8. Maltase splits maltose and maltotriose into two or three
molecules of glucose.
9. Digestion of carbohydrates ends with the production of
monosaccharides, which the digestive system is able to absorb.

Digestion of Proteins
1. Protein digestion starts in the stomach, where proteins are
fragmented into peptides by the action of pepsin.
2. Enzymes in pancreatic juice—trypsin, chymotrypsin,
carboxypeptidase, and elastase—continue to break down
proteins into peptides.
3. Protein digestion is completed by two peptidases in the brush
border: aminopeptidase and dipeptidase.
4. Aminopeptidase cleaves off the amino acid at the amino end of
a peptide. Dipeptidase splits dipeptides (two amino acids
joined by a peptide bond) into single amino acids.

Digestion of Lipids
1. Enzymes that split triglycerides and phospholipids are called
lipases.
2. Three types of lipases that can participate in lipid digestion:
lingual lipase, gastric lipase, and pancreatic lipase.
3. Some lipid digestion occurs in the stomach through the action of
lingual and gastric lipases.
4. Most occurs in the small intestine through the action of
pancreatic lipase.
5. Triglycerides are broken down by pancreatic lipase into fatty
acids and monoglycerides.

Emulsification
1. A process in which the large lipid globule is broken down into
several small lipid globules
Amphipathic
1. Each bile salt has a hydrophobic (nonpolar) region and a
hydrophilic (polar) region.
2. The amphipathic nature of bile salts allows them to emulsify a
large lipid globule.
Digestion of Nucleic Acids
1. Pancreatic juice contains two nucleases: ribonuclease, which
digests RNA, and deoxy ribonuclease, which digests DNA.

2. Brush-border enzymes called nucleosidases and phosphatases
into pentoses, phosphates, and nitrogenous bases. These products
are absorbed via active transport.
Absorption in the Small Intestine
1. Passage of these digested nutrients from the gastrointestinal tract
into the blood or lymph is called absorption.
2. Absorption of materials occurs via diffusion, facilitated diffusion,
osmosis, and active transport.
3. About 90% of all absorption of nutrients occurs in the small
intestine.
4. 10% occurs in the stomach and large intestine.

LARGE INTESTINE
1. The large intestine is the terminal portion of the GI tract.
2. The overall functions of the large intestine are the completion of
absorption, the production of certain vitamins, the formation of
feces, and the expulsion of feces from the body.
Anatomy of the Large Intestine
1. The large intestine, which is about 1.5 m (5 ft) long and 6.5 cm
(2.5 in.) in diameter, extends from the ileum to the anus.
2. Structurally, the four major regions of the large intestine are the
cecum, colon, rectum, and anal canal.

Same 4 layers
Mucosa – mostly absorptive and goblet cells
1. No circular folds or villi
2. Does have microvilli
Muscularis
1. Longitudinal muscle modified to form teniae coli
2. Forms haustra – pouches
The opening of the anal canal to the exterior, called the anus, is
guarded by an internal anal sphincter of smooth muscle
(involuntary) and an external anal sphincter of skeletal muscle
(voluntary).

Mechanical Digestion
1. The passage of chyme from the ileum into the cecum is regulated
by the action of the ileocecal sphincter.
2. After a meal, a gastroileal reflex (gas-tro¯-IL-e¯-al) intensifies
peristalsis in the ileum and forces any chyme into the cecum.
3. One movement characteristic of the large intestine is haustral
churning.
4. Peristalsis also occurs, although at a slower rate (3–12
contractions per minute)
5. A final type of movement is mass peristalsis, a strong peristaltic
wave that begins at about the middle of the transverse colon and
quickly drives the contents of the colon into the rectum.

Chemical Digestion
1. The final stage of digestion occurs in the colon through the
activity of bacteria that inhabit the lumen. (fungi, protozoa,
and archaea).
2. Mucus is secreted by the glands of the large intestine, but no
enzymes are secreted.
3. Chyme is prepared for elimination by the action of bacteria,
4. Ferment any remaining carbohydrates and release hydrogen,
carbon dioxide, and methane gases.
5. These gases contribute to flatus (gas) in the colon, termed
flatulence.(Excessive)

6. Bacteria also convert any remaining proteins to amino acids and
break down the amino acids into simpler substances: indole,
skatole, hydrogen sulfide, and fatty acids.
7. Some of the indole and skatole is eliminated in the feces and
contributes to their odor; the rest is absorbed and transported to
the liver,
8. then these compounds are converted to less toxic compounds and
excreted in the urine.
9. Bacteria also decompose bilirubin to simpler pigments, including
stercobilin, which gives feces their brown color.
10. These bacteria also produce large amounts of vitamins,
especially vitamin K and biotin .

Absorption and Feces Formation in the Large Intestine
1. Chyme has remained in the large intestine 3–10 hours, it has
become solid or semisolid because of water absorption and is now
called feces.
2. Chemically, feces consist of water, inorganic salts, sloughed-off
epithelial cells from the mucosa of the gastrointestinal tract,
bacteria, products of bacterial decomposition, unabsorbed
digested materials, and indigestible parts of food.
3. 90% of all water absorption occurs in the small intestine, the large
intestine absorbs enough to make it an important organ in
maintaining the body’s water balance.

4. The large intestine also absorbs ions, including sodium and
chloride, and some vitamins
Defecation Reflex
1. The resulting distension of the rectal wall stimulates stretch
receptors, which initiates a defecation reflex that empties the
rectum.
2. The normal range of bowel activity varies from two or three
bowel movements per day to three or four bowel movements per
week

LIVER
1. The liver is the heaviest gland of the body, weighing about 1.4 kg
in an average adult.
2. The liver is inferior to the diaphragm and occupies most of the
right hypochondriac and part of the epigastric regions of the
abdominopelvic cavity.
Anatomy of the Liver
1. The liver is almost completely covered by visceral peritoneum
and is completely covered by a dense irregular connective tissue
Layer.
2. The liver is divided into two principal lobes—a large right lobe
and a smaller left lobe—by the falciform ligament, a fold of the
mesentery.

1. The falciform ligament extends from the undersurface of
the diaphragm between the two principal lobes of the liver
to the superior surface of the liver,
2. Helping to suspend the liver in the abdominal cavity.
3. In the free border of the falciform ligament is the ligamentum
teres (round ligament), a remnant of the umbilical vein of the
fetus.
4. This fibrous cord extends from the liver to the umbilicus.
5. The right and left coronary ligaments are narrow extensions
of the parietal peritoneum that suspend the liver from the diaphragm.

Histology of the Liver
Histologically, the liver is composed of several components
1. Hepatocytes (hepat- liver; -cytes cells)
2. Bile canaliculi (small canals).
3. Hepatic sinusoids
Hepatocytes
1. Hepatocytes are the major functional cells of the liver and perform
a wide array of metabolic, secretory, and endocrine functions.
2. Hepatocytes form complex three-dimensional arrangements called
hepatic laminae
3. Grooves in the cell membranes between neighboring hepatocytes
provide spaces for canaliculi into which the hepatocytes secrete
bile.

Bile canaliculi (small canals)
1. Small ducts between hepatocytes that collect bile produced by the
hepatocytes.
2. Bile canaliculi, bile passes into bile ductules and then bile ducts.
3. The bile ducts merge and eventually form the larger right and left
hepatic ducts, which unite and exit the liver as the common
hepatic duct.
4. The common hepatic duct joins the cystic duct (cystic bladder)
from the gallbladder to form the common bile duct.
5. Bile enters the small intestine to participate in digestion

Hepatic sinusoids
1. Hepatic sinusoids are highly permeable blood capillaries
between rows of hepatocytes that receive oxygenated blood
from branches of the hepatic artery.
2. Nutrient-rich deoxygenated blood from branches of the hepatic
portal vein.
3. Hepatic portal vein brings venous blood from the
gastrointestinal organs and spleen into the liver.
4. Hepatic sinusoids converge and deliver blood into a central
vein.

5. Central veins the blood flows into the hepatic veins, which drain
into the inferior vena cava.
6. Hepatic sinusoids are fixed phagocytes called stellate
reticuloendothelial (Kupffer) cells.
7. which destroy worn-out white and red blood cells, bacteria, and
other foreign matter in the venous blood draining from the
gastrointestinal tract.

1. The principal bile pigment is Bilirubin.
2. Each day, hepatocytes secrete 800–1000 mL of bile,
3. A yellow, brownish, or olive-green liquid.
4. It has a pH of 7.6–8.6
5. It consists mostly of water, bile salts, cholesterol, a phospholipid
called lecithin, bile pigments, and several ions.
6. The phagocytosis of aged red blood cells liberates iron, globin,
and bilirubin (derived from heme).
7. The iron and globin are recycled; the bilirubin is secreted into the
bile and is eventually broken down in the intestine.

.
8. One of its breakdown products—stercobilin —gives feces their
normal brown color.
9. Bile is partially an excretory product and partially a digestive
Secretion.
10. Bile salts, which are sodium salts and potassium salts of bile
acids (mostly chenodeoxycholic acid and cholic acid), play a
role in emulsification.
11. Bile salts also aid in the absorption of lipids following their
digestion.

Functions
1. Carbohydrate metabolism
2. Lipid metabolism
3. Protein metabolism
4. Processing of drugs and hormones
5. Excretion of bilirubin
6. Synthesis of bile salts
7. Storage (storage site for certain vitamins (A, B12, D, E, and K)
and minerals (iron and copper).
8. Phagocytosis.
9. Activation of vitamin D. The skin, liver, and kidneys participate
in synthesizing the active form of vitamin D.

1. The gallbladder (gall- bile) is a pear-shaped sac that is located in
a depression of the posterior surface of the liver.
2. It is 7–10 cm (3–4 in.) long and typically hangs from the
anterior inferior margin of the liver.
3. The parts of the gallbladder include the broad fundus, the body
the central portion; and the neck, the tapered portion.
4. Its main purpose is to collect and concentrate a digestive liquid
(bile) produced by the liver.
5. Bile is released from the gallbladder after eating, aiding
digestion.

6. Bile travels through narrow tubular channels (bile ducts) into
the small intestine.
7. Removal of the gallbladder is not associated with any
impairment of digestion in most people.

1. Gastro esophageal Reflux Disease (GERD)
2. Gallstones
3. Celiac Disease
4. Crohn's Disease
5. Ulcerative Colitis
6. Irritable Bowel Syndrome
7. Hemorrhoids
8. Diverticulitis
DISORDERS OF DIGESTIVE SYSTEM

Disorders of the G.I. tract may be localized to a particular structure
or organ or may be generalized throughout.
General Symptoms of G.I. Disease
1. Anorexia — loss of appetite
2. Nausea
3. Vomiting
4. Diarrhea, constipation
5. Bleeding — obvious or “occult” (i.e., detected by laboratory test)

Abnormalities of the esophagus
Dysphagia
1. Difficulty swallowing that may be caused by obstruction of the
esophagus or impaired motility of the esophageal walls.
2. Obstruction may be caused by tumors, congenital narrowing or
diverticula.
Achalasia
1. A condition caused by failure of the lower esophageal sphincter
(cardiac sphincter) to relax and allow food to enter the stomach.
2. It may be related to defects in neural input to the esophagus.

3. Achalasia is a chronic condition that causes distention of the
lower esophagus that may lead to chronic inflammation and
eventual ulceration of the esophagus.
Gastroesophageal reflux disease
1. Gastroesophageal reflux is a condition caused by the backflow of
stomach contents into the esophagus.
2. It results from weakness or incompetence of the lower
esophageal sphincter that normally blocks reflux of stomach
contents into the esophagus.
3. High acid content (low pH), reflux of stomach contents will
cause irritation and inflammation of the esophagus ( esophagitis)
that can lead to ulceration of the esophagus.

Sign & Symptoms
1. A burning sensation in your chest (heartburn), usually after
eating, which might be worse at night
2. Chest pain
3. Difficulty swallowing
4. Regurgitation of food or sour liquid
5. Sensation of a lump in your throat

Disorders of the stomach
Gastritis
1. Gastritis refers to inflammation of the gastric mucosa (stomach
lining).
2. It may present as an acute or chronic disorder.
Acute gastritis
1. Transient irritation and inflammation of the stomach lining
2. Caused by factors such as alcohol consumption, aspirin use and
stress.
Chronic gastritis
1. Chronic irritation and inflammation of the stomach lining.
2. Caused by bacterial infection, alcohol abuse or long-term aspirin
and nonsteroidal anti-inflammatory drug (NSAID) use.

Peptic ulcers
Peptic ulcer refers to erosion of the mucosa lining any portion
of the G.I. tract.
The causes of peptic ulcer disease include the following:
1. Infection with the bacteria
2. Helicobacter pylori occurs in 80 to 95% of patients with peptic
ulcer disease
3. Stress — Emotional, trauma, surgical.
4. Injury or death of mucus-producing cells.
5. Excess acid production in the stomach. The hormone Gastrin
stimulates the production of acid in the stomach;
6. Chronic use of aspirins and NSAIDs.

SYMPTOMS
Upper abdominal pain is a common symptom.
Gastrointestinal: belching, heartburn, indigestion, nausea, passing
excessive amounts of gas, or vomiting.
Whole body: fatigue, feeling full sooner than normal, or loss of
appetite

Zollinger–Ellison syndrome
1. Tumors of the gastrin-secreting endocrine cells of the pancreas
or, less frequently, the duodenal wall.
2. Leads to excessive acid production by the G.I. tract
3. Development of serious and aggressive peptic ulcers
4. Formation of ulcers in atypical locations
5. Complications can include perforation, hemorrhage an
obstruction

Disorders of the intestines
Irritable bowel syndrome
1. May be one of the most common G.I. disorders.
2. Symptoms of G.I. pain, gas, bloating and altered bowel
function (diarrhea or constipation).
3. symptoms are localized to the lower intestine and colon.
4. “Hyperreactivity” and excessive motility of the bowels
may be contributing factors

Inflammatory bowel disease
1. The term inflammatory bowel disease includes the conditions Crohn’s
disease and ulcerative colitis.
2. Both of these diseases are characterized by chronic inflammation of various
regions of the G.I. tract.
Crohn’s disease Ulcerative colitis
Age of onset 10–40 years 10–30 years
Location Large intestine Large or small intestine
Inflammation “Skip” lesions Uniform and continuous
Layers involved Mainly submucosal Mainly mucosal
Bloody stool Rare Common
Diarrhea Common Common
Malabsorption Rare Common
Cancer risk Increased Unchanged?
Abdominal pain Mild to severe Mild to severe

Crohn’s disease
1. The exact etiology of Crohn’s disease in unknown
2. possible role of pro-inflammatory cytokines in the pathogenesis
of this disorder.
3. The disease may affect any region of the G.I. tract but is most
commonly seen in the distal ileum and colon.
4. The inflammation of Crohn’s disease is particularly evident in the
submucosal layer of the intestine
5. The pattern of inflammation seen is a granulomatous
inflammation with distinct “cobblestone” appearance to the
mucosa.

SYMPTOMS
1. Diarrhea
2. Intestinal pain similar to indigestion
3. Fever
4. Weight loss from intestinal malabsorption
5. Nausea, anorexia, vomiting

Ulcerative colitis
1. Inflammatory disease of the rectum and colon
2. Primarily affects the submucosa layer of the intestines.
3. Exact etiology of ulcerative colitis is unknown, genetic and
immunological factors are likely contributors to the disease.
Symptoms
1. Chronic, bloody diarrhea
2. Fever, pain
3. Weight loss
4. Possible anemia from blood loss

Disorders of the gall bladder
Gallstone formation (cholelithiasis)
1. The gallstones that form in the gall bladder are hardened
precipitates of bile that contain predominantly cholesterol.
2. Size of gallstones can range from the size of a grain of sand to
several inches in diameter.
3. Factors such as aging, excess cholesterol, obesity, sudden
dietary
4. Changes or abnormal fat metabolism may contribute to
gallstone formation.

Symptoms
1. Acute and severe abdominal pain.
2. Nausea, vomiting, fever, chills.
3. Jaundice from obstruction of bile outflow.
Cholecystitis
1. Cholecystitis is an acute or chronic inflammation of the gall
bladder.
2. It is most commonly caused by the presence of gallstones in the
gall bladder, but may also result from infection or reduced blood
flow to the gall bladder

Disorders of the pancreas
1. Pancreatitis is inflammation of the pancreas that may be acute or
chronic.
Acute
1. Inflammation due to abnormal release of pancreatic enzymes or
reflux of duodenal contents into pancreatic tissues
2. Certain drugs such as thiazide diuretics, NSAIDS or sulfonamide
antibiotics.
Chronic
1. Alcohol, pancreatic tumors or cysts
Symptoms
 Epigastric pain, Fever, nausea, vomiting, Reduced bowel activity
Anorexia, Intestinal malabsorption, poor digestion of nutrients.

DIGESTIVE SYSTEM

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