Download Digestive system

Digestive System
Dr.Spandana Charles
Digestive System


The system by which ingested food is acted upon by
physical and chemical means to provide the body with
absorbable nutrients and to excrete waste products.
This system includes

The alimentary canal extending from the mouth to the
anus,

and the accessory digestive organs like the teeth,
tongue,salivary glands, gallbladder, liver and pancreas.
Figure 23.1 Alimentary canal and related accessory digestive organs.
Mouth (oral cavity)
Tongue*
Parotid gland
Sublingual gland
Submandibular gland
Salivary
glands*
Pharynx
Esophagus
Stomach
Pancreas*
(Spleen)
Liver*
Gallbladder*
Transverse colon
Small
intestine
Duodenum
Jejunum
Ileum
Descending colon
Ascending colon
Cecum
Sigmoid colon
Anus
Rectum
Appendix
© 2013 Pearson Education, Inc.Anal canal
Large
intestine
3
Digestive Processes

Six essential activities
1.
2.
3.
4.
5.
6.
Ingestion
Mechanical break down
Propulsion
Digestion
Absorption
Defecation
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Figure 23.2 Gastrointestinal tract activities.
Ingestion
Food
Mechanical
breakdown
Pharynx
• Chewing (mouth)
• Churning (stomach)
• Segmentation
(small intestine)
Digestion
Esophagus
Propulsion
• Swallowing
(oropharynx)
• Peristalsis
(esophagus,
stomach,
small intestine,
large intestine)
Stomach
Absorption
Lymph
vessel
Small
intestine
Large
intestine
Blood
vessel
Mainly H2O
Feces
Defecation
Anus
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Peritoneum and Peritoneal Cavity

Peritoneum - serous membrane of abdominal
cavity



Visceral peritoneum on external surface of most
digestive organs
Parietal peritoneum lines body wall
Peritoneal cavity


Between two peritoneums
Fluid lubricates mobile organs
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Figure 23.5b The peritoneum and the peritoneal cavity.
Mesentery
resorbed
and lost
Alimentary
canal organ
Alimentary canal organ in
a retroperitoneal position
Some organs lose their mesentery and move,
becoming retroperitoneal,
during
development.
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Inc.
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Blood Supply: Splanchnic Circulation

Branches of aorta serving digestive organs



Hepatic, splenic, and left gastric arteries
Inferior and superior mesenteric arteries
Hepatic portal circulation


Drains nutrient-rich blood from digestive organs
Delivers it to the liver for processing
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8
Histology of the Alimentary Canal
Four basic layers (tunics)
Mucosa-



Secretes mucus, digestive enzymes, and hormones

Absorbs end products of digestion
Submucosa


Contains blood and lymphatic vessels, lymphoid follicles, and
submucosal nerve plexus
Muscularis externa

Responsible for peristalsis and segmentation movements.

Contains inner circular and outer longitudinal muscle layers.

Myentric nerve plexus exists between the two muscle layers.
Serosa

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Pearson Education, Inc.
Serosa is the visceral
peritoneum.
9
Enteric Nervous System



Intrinsic nerve supply of alimentary canal – enteric neurons.
Major nerve supply to GI tract wall; control motility
 Submucosal nerve plexus
 Regulates glands and smooth muscle in the mucosa
 Myenteric nerve plexus
 Controls GI tract motility
Long ANS fibers synapse with enteric plexuses
 Sympathetic impulses inhibit digestive activities

Parasympathetic impulses stimulate digestive activities
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10
Mouth





Oral (buccal) cavity
Bounded by lips, cheeks, palate, and tongue
The lips and cheek contain orbicularis oris and buccinator
muscles.
Hard palate – formed by palatine bones and palatine
processes of maxillae. Slightly corrugated to help create
friction against tongue.
Soft palate - formed mostly of skeletal muscle. Closes off
nasopharynx during swallowing. Uvula projects downward
from its free edge
Figure 23.7b Anatomy of the oral cavity (mouth).
Upper lip
Gingivae
(gums)
Superior
labial
frenulum
Palatine
raphe
Palatoglossal
arch
Hard
palate
Soft
palate
Uvula
Palatopharyngeal
arch
Posterior wall
of oropharynx
Palatine
tonsil
Tongue
Lingual frenulum
Sublingual
fold with
openings of
sublingual
ducts
Opening of
Submandibular
duct
Gingivae (gums)
Oral vestibule
Inferior labial
frenulum
Lower lip
Anterior view
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Tongue


Skeletal muscle
Functions include





Repositioning and mixing food during chewing
Formation of bolus
Initiation of swallowing, speech, and taste
Lingual frenulum: attachment to floor of
mouth
Surface bears filiform, foliate, fungiform and
circumvallate papillae(also known as taste
buds, help in identifying taste)
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Teeth



Tear and grind food for digestion
Primary and permanent dentitions formed by
age 21
20 deciduous teeth erupt (6–24 months of
age)


Roots resorbed, teeth fall out (6–12 years of age)
as permanent teeth develop
32 permanent teeth

All but third molars in by end of adolescence

Third molars at 17–25, or may not erupt
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Classes of Teeth

Incisors


Canines


Fanglike teeth that tear or pierce
Premolars (bicuspids)


Chisel shaped for cutting
Broad crowns, rounded cusps – grind/crush
Molars

Broad crowns, rounded cusps – best grinders
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Figure 23.11 Longitudinal section of a canine tooth within its bony socket (alveolus).
Enamel
Dentin
Crown
Dentinal
tubules
Pulp cavity
(contains
blood vessels
and nerves)
Neck
Gingival
sulcus
Gingiva
(gum)
Cement
Root canal
Root
Periodontal
ligament
Apical
foramen
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Bone
Salivary Glands




Produce most saliva; lie outside oral cavity
Parotid- Anterior to ear; external to masseter muscle.
Duct opens next to second upper molar. Mumps is
inflammation of parotid.
Submandibular- Medial to body of mandible. Duct
opens at base of tongue.
Sublingual- Present under the tongue. Opens into
the floor of the mouth.
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Saliva
Function of saliva:
 Cleanses mouth

Dissolves food chemicals for taste

Moistens food; compacts into bolus

Begins breakdown of starch with enzymes
Composition of Saliva:
 97–99.5% water, slightly acidic
+
+
–
2–, HCO
 Electrolytes—Na , K , Cl , PO
4
3–

Salivary amylase -Breaks down starch to
oligosaccharides
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
Mucin,IgA,Lysosyme, Defensins
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Control of Salivation


1500 ml/day
Extrinsic salivary glands activated by
parasympathetic nervous system when



Ingested food stimulates chemoreceptors and
mechanoreceptors in mouth 
Salivatory nuclei in brain stem send impulses along
parasympathetic fibers in cranial nerves VII and IX
Strong sympathetic stimulation inhibits salivation and
results in dry mouth (xerostomia)
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Mastication/Chewing




Cheeks and closed lips hold food between
teeth
Tongue mixes food with saliva; compacts food
into bolus
Teeth cut and grind
Mastication is partly voluntary and partly
reflexive.

pressure receptors in cheeks, gums and tongue
initiate a stretch reflex which is responsible for
reflexive chewing.
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Pharynx

Food passes from mouth  oropharynx 
laryngopharynx




Allows passage of food, fluids, and air
Epiglottis closes the laryngeal inlet during
swallowing
Soft palate prevents ingested food from entering
the nasal cavity.
Skeletal muscle layers: inner longitudinal, outer
pharyngeal constrictors
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Esophagus






Flat muscular tube from laryngopharynx to
stomach.
Upper esophageal sphincter separates the
pharynx from the esophagus.
Pierces diaphragm at esophageal hiatus
Joins stomach at cardial orifice
Lower esophageal sphincter(LES) surrounds
the junction of esophagus with the stomach.
Heart burn is when the stomach acid
regurgitates into esophagus
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Deglutition


Involves tongue, soft palate, pharynx,
esophagus
Buccal phase


Voluntary contraction of tongue
Pharyngeal-esophageal phase


Involuntary – primarily vagus nerve
Control center in the medulla and lower pons
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Stomach: Gross Anatomy

In upper left quadrant; temporary storage; chemical
digestion of bolus to chyme

Cardial part (cardia) -Surrounds cardial orifice

Fundus-Dome-shaped region beneath diaphragm

Body-Midportion

Pyloric part - continuous with duodenum through
pyloric valve (sphincter controlling stomach
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emptying).
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Stomach: Gross Anatomy

In upper left quadrant; temporary storage; chemical
digestion of bolus to chyme

Cardial part (cardia) -Surrounds cardial orifice

Fundus-Dome-shaped region beneath diaphragm

Body-Midportion

Pyloric part - continuous with duodenum through
pyloric valve (sphincter controlling stomach
© 2013 Pearson Education, Inc.
emptying).
25
Figure 23.14a Anatomy of the stomach.
Cardia
Fundus
Esophagus
Muscularis
externa
• Longitudinal layer
• Circular layer
• Oblique layer
Serosa
Body
Lumen
Lesser
curvature
Rugae of
mucosa
Greater
curvature
Duodenum
Pyloric sphincter
(valve) at pylorus
Pyloric
canal
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Pyloric
antrum
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Gastric Glands

Cell types
 Mucous neck cells- Secrete thin, acidic mucus
 Parietal cells- Secrete Hcl and intrinsic factor.
 Chief cells -Secrete Pepsinogen and Lipases



Hcl in stomach converts pepsinogen to pepsin
Pepsin breaks down proteins to proteases and
peptones
Enteroendocrine cells – Secrete chemical
messengers/hormones like Histamine, serotonin
somatostatin and gastrin.
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Figure 23.15c Microscopic anatomy of the stomach.
Pepsinogen
Pepsin
HCI
Mitochondria
Parietal cell
Chief cell
Enteroendocrine
cell
Location of the HCl-producing parietal cells
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Education,
Inc.in a gastric
and pepsin-secreting
chief
cells
gland
28
Mucosal Barrier




Harsh digestive conditions in stomach
Has mucosal barrier to protect
 Thick layer of bicarbonate-rich mucus
 Tight junctions between epithelial cells
 Prevent juice seeping underneath tissue
Gastritis-Inflammation caused by anything that breaches
mucosal barrier
Peptic or gastric ulcers- Erosions of stomach wall
 Most caused by Helicobacter pylori bacteria
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Digestive Processes in the Stomach



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
Physical digestion
Denaturation of proteins by HCl
Enzymatic digestion of proteins by pepsin (and
milk protein by rennin in infants)
Lingual lipase digests some triglycerides
before digested as any other protein
Delivers chyme to small intestine
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Regulation of Gastric Secretion


About 3litres of gastric juice is produced per day.
Three phases of gastric secretion
 Cephalic (reflex) phase – conditioned reflex triggered by
aroma, taste, sight, thought
 Gastric phase – lasts 3–4 hours; ⅔ gastric juice released
 Stimulated by distension, peptides, low acidity, gastrin
(major stimulus)
 Enteroendocrine G cells stimulated by peptides,rising
ph release gastrin
 Gastrin facilitates release of enzyme and HCl release
 Stimulation of vagus nerve also increases Hcl secretion
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Regulation of Gastric Secretion

Intestinal phase

Stimulatory component


Partially digested food enters small intestine  brief
intestinal gastrin release
Inhibitory effects (enterogastric reflex) 


Chyme with H+, fats, peptides, irritating substances inhibition-tightening of pyloric sphincter - no more food
entry to small intestine.
If small intestine is forced to accept more chyme it
causes dumping syndrome-(nausea and vomiting).
Hormones such as secretin, cholecystokinin and VIP
secreted by small intestine also inhibit gastric juice
secretion.
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Response of the Stomach to Filling

Stretches to accommodate incoming food


Pressure constant until 1.5 L food ingested
Gastric accommodation


Plasticity (stress-relaxation response) of smooth muscle
Peristaltic waves move toward pylorus at rate
of 3 per minute

Peristaltic waves are initiated by Enteric pace
maker cells also called interstitial cells of Cajal.

Peristaltic waves most vigorous near pylorus.
Chyme is either delivered in ~3 ml spurts to
duodenum, or is forced backward into stomach
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Homeostatic Imbalance

Vomiting (emesis) caused by




Extreme stretching
Intestinal irritants, e.g., bacterial toxins, excessive
alcohol, spicy food, certain drugs
Chemicals/sensory impulses  emetic center
of medulla
Excessive vomiting  dehydration, electrolyte
and acid-base imbalances (alkalosis)
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Small Intestine: Gross Anatomy



Major organ of digestion and absorption
2-4 m long; from pyloric sphincter to ileocecal
valve
Subdivisions

Duodenum -curves around head of pancreas




Shortest part( about 25cm)
Bile duct from liver and pancreatic duct from pancreas
drain into it.
Jejunum – about 2.5 meters long
Ileum – about 3.6 meters long
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Structural Modifications

Increase surface area of proximal part for nutrient
absorption

Circular folds (plicae circulares)- force chyme to
slowly spiral through the lumen.

Villi- Extensions (~1 mm high) of mucosa with
capillary bed and lacteal for absorption

Microvilli- contain enzymes for carbohydrate and
protein digestion
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Figure 23.22a Structural modifications of the small intestine that increase its surface area for digestion and absorption.
Vein carrying
blood to
hepatic portal
vessel
Muscle
layers
Lumen
Circular
folds
Villi
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37
Intestinal Juice




1-2 L secreted daily in response to distension
or irritation of mucosa
Slightly alkaline; isotonic with blood plasma
Largely water; enzyme-poor (enzymes of small
intestine only in brush border); contains mucus
Facilitates transport and absorption of nutrients
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Liver
Largest gland in body
 Four lobes—right, left, caudate, and quadrate lobe
Liver lobules
Hexagonal structural and functional units
Composed of plates of hepatocytes (liver cells)
Central vein in longitudinal axis
Portal triad at each corner of lobule.

Branch of hepatic artery supplies oxygen
 Branch of hepatic portal vein brings nutrientrich blood
 Bile duct receives bile from bile canaliculi

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Figure 23.25c Microscopic anatomy of the liver.
Interlobular veins
(to hepatic vein)
Central vein
Sinusoids
Bile canaliculi
Plates of
hepatocytes
Bile duct (receives
bile from bile
canaliculi)
Fenestrated
lining (endothelial
cells) of sinusoids
Stellate macrophages
in sinusoid walls
Bile duct
Portal venule
Portal arteriole
Portal triad
Portal vein
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


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
Liver produces bile which helps in digestion.
Bile leaves the liver through the common hepatic duct.
Bile is stored in the gallbladder till the need arises.
The cystic duct drains bile from the gallbladder.
Composition of bile:yellow-green, alkaline solution
containing
 Bile salts - cholesterol derivatives that function in fat
emulsification and absorption
 Bilirubin - pigment formed from heme.Bacteria break
down bilirubin in intestine to form stercobilin - brown
color of feces
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Figure 23.21 The duodenum of the small intestine, and related organs.
Right and left
hepatic ducts
of liver
Cystic duct
Common hepatic duct
Bile duct and sphincter
Accessory pancreatic duct
Mucosa
with folds
Tail of pancreas
Pancreas
Gallbladder
Jejunum
Major duodenal
papilla
Hepatopancreatic
ampulla and sphincter
Main pancreatic duct and sphincter
Duodenum
Head of pancreas
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Inc.
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Pancreas

Location




Mostly retroperitoneal, deep to greater curvature of
stomach
 Head encircled by duodenum; tail abuts spleen
Endocrine function-Pancreatic islets secrete insulin
and glucagon
Exocrine function- Acini (clusters of secretory cells)
secrete pancreatic juice
To duodenum via main pancreatic duct
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Pancreatic Juice




1200 – 1500 ml/day
Watery alkaline solution (pH 8) neutralizes
chyme
Electrolytes (primarily HCO3–)
Enzymes


Amylase, lipases, nucleases secreted in active
form but require ions or bile for optimal activity
Proteases secreted in inactive form, activated in
the duodenum.

Trypsinogen

Chymotrypsinogen
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Digestion in the Small Intestine

Chyme from stomach contains




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Partially digested carbohydrates and proteins
Undigested fats
Entry of chyme into the stomach causes
duodenum to release CCK and secretin.
CCK causes gall bladder to contract and the
hepatopancreatic sphincter to relax , so that
the bile and the pancreatic juices enter the
duodenum.
Secretin stimulates the pancreas to increase
production of pancreatic juice.
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Digestion in small intestine


Starch digestion
 Pancreatic amylase (small intestine)  breaks
down carbohydrates that escaped salivary
amylase  oligosaccharides
 Brush border enzymes (dextrinase, glucoamylase,
lactase, maltase, sucrase)  oligosaccharides 
monosaccharides
Protein digestion
 Pancreatic proteases-Trypsin, chymotrypsin, and
carboxypeptidase

Brush border enzymes-Aminopeptidases,
carboxypeptidases, and dipeptidases
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Digestion of Lipids

Pre-treatment—emulsification by bile salts


Broken down into smaller particles
Enzymes—pancreatic lipases
 Fatty acids and monoglycerides
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Motility of the Small Intestine

Segmentation




Most common motion of small intestine
Initiated by intrinsic pacemaker cells
Mixes/moves contents toward ileocecal valve
Intensity altered by long & short reflexes;
hormones



Parasympathetic  ; sympathetic 
Wanes in late intestinal (fasting) phase
Migrating motor complex/Hunger pangs
Initiated by the hormone motilin

Meal remanants, bacteria and debris are moved to
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the large intestine.

To be continued....
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