1. (A Project of National Medical Centre Karachi)
THE DIGESTIVE SYSTEM
BY:
SHAHZAD BASHIR
LECTURER, NMC ION.
Acknowledgment:
Gail Jenkins, Christopher Kemnitz, Gerard Tortora
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2. Objectives.
At the completion of this unit, learners will be able to:
Define the digestive system and list its functions
Identify the various organs of digestive system
Describe the anatomy & physiology of digestive organs
Discuss the role of accessory organs in digestion
Discuss digestion of food with in
◦ Mouth
◦ Stomach
◦ Small intestines
◦ Large intestines
Discuss the absorption of nutrients in the digestive system
Discuss the process of defecation.
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3. Chapter Overview
1 Gastrointestinal (GI) Tract
2 Accessory Organs of the Head
3 Swallowing
4 Stomach
5 Accessory Organs of the Abdomen
6 Small Intestine
7 Large Intestine
8 Phases of Digestion
9 Food Molecules
10 Metabolism
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4. Essential Terms
Digestion
Process of mechanically or chemically breaking
down food
Absorption
Passage of small molecules into blood and
lymph
Digestive system
Organs which carry out process of digestion
and absorption
Metabolism
All the chemical reactions of the body
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5. Introduction.
Digestive System.
The digestive system contributes to
homeostasis by breaking down food into
forms that can be absorbed and used by body
cells.
1. Composed of GI tract and accessory organs.
2. Breaks down ingested food for use by the body.
3. Digestion occurs by mechanical and chemical
mechanisms.
4. Excretes waste products or feces through process
of defecation.
5. The length of the GI tract is about 5–7 meters (16.5–
23 ft) in a living person.
6. It is longer in a cadaver (about 7–9meters or 23–29.5
ft) because loss of muscle tone.
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6. Concept.1
Gastrointestinal (GI) Tract
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7. GI Tract / Alimentary Canal
Continuous tube from mouth to anus
Mouth
Pharynx
Esophagus
Stomach
Small intestine
Large intestine
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8. Accessory Digestive Organs
Provide mechanical and chemical
mechanisms to aid digestion
Teeth
Tongue
Salivary glands
Liver
Gallbladder
Pancreas
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10. Functions of Digestive System
1. Ingestion
2. Secretion
3. Mixing and propulsion
• Motility
4. Digestion
• Mechanical and chemical
5. Absorption
6. Defecation
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11. Layers of GI Tract
Same in all areas of GI tract
From deep to superficial:
Mucosa
Sub mucosa
Muscularis
Serosa
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13. Layers of GI Tract
Mucosa
◦ Epithelium
Type varies
◦ Lamina propria – areolar connective
tissue
MALT – mucus-associated lymphatic tissue
◦ Muscularis mucosae – smooth muscle
Sub mucosa
◦ Areolar connective tissue
◦ Blood and lymphatic vessels
◦ Neurons – sub mucosal plexus
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14. Layers of GI Tract
Muscularis
◦ Skeletal and smooth muscle
◦ Neurons – myenteric plexus
Serosa
◦ Areolar and simple squamous epithelium
◦ Visceral peritoneum
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15. Peritoneum
Parietal peritoneum
Visceral peritoneum
Peritoneal cavity
Retroperitoneal
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20. Folds of Peritoneum
Greater omentum
◦ Adipose tissue
Falciform ligament
◦ Liver to anterior abdominal wall
Lesser omentum
Mesentery
◦ Small intestine to posterior abdominal
wall
Mesocolon
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21. Neural Innervation of GI Tract
Regulated by autonomic nervous system
Enteric division
◦ Myenteric plexus / plexus of Auerbach
Major function is to regulate the movement of GIT
◦ Submucosal plexus / plexus of Meissner
Major function is the regulation of secretory function of
GIT.
Able to function independently from rest of nervous
system
Linked to CNS by extrinsic sympathetic and
parasympathetic nerves
Sympathetic nerves decrease GI secretions & motility
Parasympathetic nerves increase GI secretion and
motility
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22. Concept.2
Accessory Organs of the
Head
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23. ACCESSORY ORGANS ALIMENTARY CANAL
Salivary glands
Secrete saliva, which contains Mouth
enzymes that initiate breakdown Mechanical breakdown of food; begins
of carbohydrates chemical digestion of carbohydrates
Pharynx
Connects mouth with esophagus
Esophagus
Peristalsis pushes food to stomach
Stomach
Liver Secretes acid and enzymes; mixes food
Produces bile, which with secretions to begin enzymatic
emulsifies fat digestion of proteins
Gallbladder
Stores bile and introduces it Small intestine
into small intestine Mixes food with bile and pancreatic juice;
final enzymatic breakdown of food
Pancreas molecules; main site of nutrient absorption
Produces and secretes pancreatic
juice, containing digestive enzymes
and bicarbonate ions, into small Large intestine
intestine Absorbs water and electrolytes to form
feces
Rectum
Regulates elimination of feces
4/6/2013 Anus
Shahzad Bashir
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24. Mouth Parts of Digestive System
Mouth formed by several parts:
Cheeks
Lips / labia
Labial frenulum
Orbicularis
Vestibule
Oral cavity proper
Fauces
Hard and soft palate
Uvula
Palatoglossal and palatopharyngeal arch
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26. Tongue
Skeletal muscle and mucous membrane
Helps form floor of oral cavity
Extrinsic muscles: Hyoglossus, genioglossus,&
Styloglossus muscles.
Intrinsic muscles: Longutudinalis superior,
Inferior, Transversus linguae, Verticalis linguae.
Lingual frenulum
Papillae
◦ Fungiform
◦ Filiform
◦ Circumvallate
◦ Foliate
Lingual glands
◦ Lingual lipase
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27. Tongue
• The tongue is a Epiglottis
thick, muscular Lingual tonsils
organ that Root
occupies the floor Palatine tonsil
of the mouth and
nearly fills the Body
oral cavity when Papillae
the mouth is
closed
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28. Salivary Glands
Release saliva to oral cavity
3 pairs of salivary glands
Parotid:
◦ Largest salivary gland, weight about 20- 30
gram in adult. Open secretion through
Stensen’s duct(35 to 40mm long).
Submandibular:
◦ Weight about 8-10 gram, Secretion through
Wharton’s duct (40mm long)
Sublingual:
◦ Smallest gland, weight about 2-3g. Saliva open
into 5 -15 small ducts called ducts of Ravinus.
Largest duct is “Bartholin’s duct”.
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29. Composition of Saliva
Volume: 1000-1500ml is secreted/day.
About 1ml/minute.
pH of saliva is slightly acidic 6.35 to 6.85.
99.5 % water
0.5% other solutes
◦ Ions
◦ Mucus
◦ Immunoglobulin A
◦ Enzymes
Salivation controlled by autonomic
nervous system
Stimulated by various mechanisms
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31. Teeth
External regions
1. Crown
2. Root
3. Neck
Internal components
1. Enamel
2. Dentin: Majority of teeth. Calcified connective tissue that
gives tooth is basic shape and rigidity.
Cementum
3. Pulp cavity
PulpRoot canals
Apical foramen
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33. Teeth
Dentitions
Deciduous teeth – first set
Permanent teeth – secondary
Carry out mechanical digestion by mastication
Creates bolus
Salivary amylase
Breakdown starch
Lingual lipase
Breakdown triglycerides
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34. Incisors
Canine (cuspid)
Premolars
(bicuspids)
Molars
Molars
Premolars
(bicuspids)
Canine (cuspid)
(a) Incisors
Second
premolars
Central Lateral Canines First
incisors incisors premolars
(b)
4/6/2013 ShahzadNick Koudis/Getty Images
b: © Bashir
34

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37. Concept.3
Swallowing
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38. Pharynx
Composed of skeletal muscle
Lined by mucous membrane
Nasopharynx
Oropharynx
Laryngopharynx
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39. Esophagus
Collapsible muscular tube through esophageal
hiatus of diaphragm(25cm long)
Mucosa
◦ Submucosa contains areolar connective tissue
Muscularis
◦ Skeletal muscle
◦ Upper and lower esophageal sphincter
Adventitia
◦ Attaches esophagus to nearby structures
Secrets mucus and transports food
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41. Deglutition
Definition:
The act or process of swallowing.
Stages of swallowing
Voluntary
◦ Mouth to oropharynx
Pharyngeal
◦ Deglutition center in medulla oblongata and pons
◦ Closing of epiglottis
◦ Involuntary
Esophageal
◦ Involuntary
◦ Peristaltic contractions
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45. Concept.4
Stomach
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46. Stomach
The stomach is a J-shaped hollow muscular organ
situated on the left side in the abdominal cavity,&
in Epigastric, Umbilical and left hypochondriac
region of abdomen, about 25-30 centimeters long.
Serves as mixing chamber and storage area for
ingested food
When empty, its volume is 50ml & normally it can
expand to accommodate 1 to 1.5 liters of solids &
liquids.
However it is also capable of expanding still
further upto 4 liters.
Rugae allow for increased volume.
The food is stored for a long period, i-e. for 3 to 4
hours & emptied into the intestine slowly.
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47. Stomach.
Total secretion of stomach per day is 2000-3000ml.
4 main regions
1. Cardia
2. Fundus
3. Body
4. Pylorus
◦ Pyloric antrum and canal
◦ Pyloric sphincter
◦ Lesser and greater curvatures
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49. Stomach Histology
1. Mucosa
◦ Surface mucous cells
◦ Lamina propria
◦ Muscularis mucosae
◦ Gastric glands and pits
◦ Parietal cells
◦ Chief cells
◦ G cells
2. Submucosa – areolar connective tissue
3. Muscularis
◦ 3 layers of smooth muscle
4. Serosa
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53. Mechanical and Chemical Digestion
Mixing waves caused by peristaltic
movement
Chyme released in process of gastric
emptying
Proton pumps bring H+ into the lumen
Carbonic anhydrase forms carbonic acid to
provide H+ and bicarbonate ions (HCO3-)
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55. Mechanical and Chemical Digestion
Chemical digestion stimulated by nervous
system
Parasympathetic neurons release acetylcholine
◦ Works with gastrin
◦ HCl released in presence of histamine
Pepsin begins digestion of proteins
◦ Stomach protected by alkaline mucus secretion
Gastric lipase digests triglycerides
Few molecules absorbed by stomach
◦ Water, ions, short-chain fatty acids, alcohol
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58. Regulation of Gastric Secretions
Release into
1 Parasympathetic bloodstream
preganglionic
nerve fiber (in Stimulation
vagus nerve)
4 Gastrin stimulates
gastric glands to
release more gastric
juice
2 Parasympathetic
postganglionic
impulses stimulate
the release of Bloodstream
gastric juice from 3 Impulses
gastric glands stimulate
the release
of gastrin 4/6/2013 Shahzad Bashir 58

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60. Mixing and Emptying Actions
Stomach
Food
entering
Duodenum
Pyloric sphincter Chyme Pyloric sphincter
contracted relaxed
(a) (b) (c)
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61. 4 Nerve impulses
inhibit peristalsis
in stomach wall
From CNS
Vagus
nerve
To CNS
1 Duodenum
fills with chyme
3 Sensory nerve
impulses travel
to central
2 Sensory stretch nervous system
receptors are
stimulated 4/6/2013 Shahzad Bashir 61

62. Concept.5
Accessory Organs of
the Abdomen
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63. Pancreas
The pancreas (pan- all; -creas flesh), a
retroperitoneal gland that is about 12–15 cm
(5–6 in.) long and 2.5 cm (1 in.) thick, lies
posterior to the greater curvature of the
stomach. Produces secretions to aid digestion
Head
Body
Tail
Pancreatic duct /duct of Wirsung
◦ Hepatopancreatic ampulla
◦ Sphincter of the heatopancreatic ampulla
(sphincter of (Oddi)
Regulates passage of pancreatic juice and bile
Accessory duct (duct of Santorini)
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67. Histology of Pancreas
Glandular epithelial cells
◦ 99% exocrine clusters
◦ Secrete pancreatic juice
Fluid and enzymes
Pancreatic islets (islets of Langerhans)
◦ 1% endocrine cells
◦ Hormones
Glucagon(Secrete by Alpha cell)
Insulin(Secrete by Beta cell)
Somatostatin (Secrete by Delta cell)
◦ Pancreatic polypeptide (Secrete by F cell)
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68. Pancreatic Juice
1200-1500 mL/day
pH 7.1-8.2
Water
Salts
Sodium bicarbonate
Enzymes
◦ Pancreatic amylase
◦ Trypsin
Entereokinase
◦ Chymotrypsin
◦ Carboxypeptidase
◦ Elastase
◦ Pancreatic lipase
◦ Ribonuclease and deoxyribonuclease
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69. Regulation of Pancreatic Secretion
Acidic chyme
1 enters
duodenum
4 Pancreatic juice rich in
bicarbonate ions passes down
pancreatic ducts to the
duodenum
5 Bicarbonate ions
neutralize acidic
chyme
3
Secretin stimulates
pancreas to secrete
bicarbonate ions
2 Intestinal mucosa
releases secretin
into bloodstream
Bloodstream
Hormonal signals
released into bloodstream
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Stimulation of effector organ 69

70. Liver and Gallbladder
Liver is the heaviest gland of the body
located in abdominal cavity, right, left
hypochondriac and part of the epigastric
regions immediately beneath the
diaphragm.
Largest gland at 1.4 kg (~3 lb)
Gallbladder
Closely associated with liver
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71. Anatomy of Liver
Right and left lobe separated by
falciform ligament
Quadrate lobe
Caudate lobe
Round ligament (ligamentum teres)
◦ Remnant of umbilical vein
Coronary ligaments
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73. Liver Structure
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Inferior vena cava Gallbladder Quadrate lobe
Coronary ligament Right lobe
Left lobe
Cystic duct
Falciform
ligament Hepatic duct
Hepatic artery
Hepatic portal
Right lobe vein
Left lobe
Round
ligament Bile duct
Gallbladder
Inferior vena cava Caudate lobe
(a) (b)
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74. Histology of Liver
Lobule
◦ Hepatocytes radiating from central vein
◦ Sinusoids
Reticuloendothelial (Kupffer) cells
◦ Stationary phagocytes
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79. Bile duct Bile canaliculi Bile ductule Kupffer cell Hepatic cells
Branch Branch Hepatic Central vein
of hepatic of hepatic sinusoids (blood flow
portal vein artery out of liver)
Blood flow
into liver
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80. Bile Duct System
Bile secreted by hepatocytes
Bile canaliculi
Bile ducts
Right and left hepatic ducts
Common hepatic duct
Common bile duct
Gallbladder for temporary storage of
bile
Cystic duct
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82. Blood Supply of Liver
Hepatic artery provides oxygenated blood
Hepatic portal vein provides deoxygenated
blood
◦ Nutrients, drugs, toxins, microbes
Hepatic artery and vein carry blood to
sinusoids
◦ Substances exchanged by hepatocytes
◦ Blood drains to central vein and eventually hepatic
vein
Portal triad
◦ Hepatic portal vein
◦ Hepatic artery
◦ Bile duct
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84. Regulation of Bile Release
Cystic duct
Common
hepatic duct
Gallbladder 3 CCK stimulates muscular layer
of gallbladder wall to contract
Bile duct
1 Chyme with
4 Bile passes down the cystic duct and bile duct to duodenum
fat enters
duodenum
5 Hepatopancreatic sphincter relaxes and bile enters duodenum
Pancreatic
duct
Cells from the
2
intestinal mucosa
secrete the hormone
cholecystokinin (CCK)
into the bloodstream
Duodenum
Hormonal
signals released
into bloodstream
Bloodstream Stimulation of
effector organ
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85. Bile
800-1000 mL/day
pH 7.6 – 8.6
Water
Bile acids
Bile salts
◦ Emulsification
Cholesterol
Lecithin
Bile pigments
◦ Bilirubin
Stercobilin
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86. Liver Functions
Metabolism of:
◦ Carbohydrates
◦ Lipids
◦ Proteins
Process drugs and hormones
Excrete bilirubin
Synthesize bile salts
Storage
◦ Glycogen
◦ Vitamins
◦ Minerals
Phagocytosis
Activate Vitamin D
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87. Gallbladder
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88. Concept.6
Small Intestine
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89. Small Intestine
Most digestion and absorption of
nutrients occur in a long tube called the
small intestine.
2.5 cm (1 in.) in diameter.
3 m (10 ft) its length (living)
6.5 m (21 ft) without muscle tone (after
death).
Duodenum
Jejunum
Ileum
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90. Duodenum
The shortest region, is retroperitoneal.
It starts at the pyloric sphincter of the
stomach and extends about 25 cm (10
in.) until it merges with the jejunum.
Duodenum means “12”.
It is so named because it is about as
long as the width of 12 fingers.
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91. Jejunum.
The jejunum is about 1 m (3 ft) long
and extends to the ileum.
Jejunum means “empty,” which is
how it is found at death.
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92. Ileum
The final and longest region of the small
intestine, the ileum.
(Ileum = twisted)
Measures about 2 m (6 ft) and joins the
large intestine at a smooth muscle
sphincter called the ileocecal sphincter.
(Peyer’s patches) are also present in the
ileum.
Ileocecal sphincter
◦ Connection to large intestine
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95. Histology of Small Intestine
Mucosa
Cell types
◦ Absorptive
◦ Goblet
◦ Endocrine
◦ Paneth
Lysozyme
Intestinal glands (crypts of Lieberkühn)
◦ S cells
Hormone secretin
◦ CCK cells
Hormone – cholecystokinin (CCK)
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97. Microvilli
Absorptive cell
(absorbs nutrients)
Blood Capillary
Goblet cell
(secretes
Lacteal mucus)
Enteroendocrine cell
(secretes the
hormones secretin,
cholecystokinin, or
GIP)
Paneth cell
(secretes lysozyme
and is capable
of phagocytosis)
4/6/2013 Shahzad Bashir 97

98. Structure of the
Small Intestinal Wall
Lumen
Simple columnar
epithelium Villus
Lacteal
Intestinal
Villus gland
Blood capillary
network
Goblet cells
Intestinal gland
© The McGraw-Hill Companies, Inc./Al Telser, photographer
Arteriole
Venule
Lymph vessel
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99. (a)
Plicae
circulares
Villi
Submucosa
Circular muscle Muscular
Longitudinal muscle layer
Serosa
99
(b) 4/6/2013 Shahzad Bashir

100. Secretions of the Small Intestine
• In addition to mucous-secreting goblet cells, there are many
specialized mucous-secreting glands (Brunner’s glands) that
secrete a thick, alkaline mucus in response to certain stimuli
• Enzymes in the membranes of the microvilli include:
• Peptidase – breaks down peptides into amino acids
• Sucrase, maltase, lactase – break down disaccharides into
monosaccharides
• Lipase – breaks down fats into fatty acids and glycerol
• Enterokinase – converts trypsinogen to trypsin
• Somatostatin – hormone that inhibits acid secretion by
stomach
• Cholecystokinin – hormone that inhibits gastric glands,
stimulates pancreas to release enzymes in pancreatic juice,
and stimulates the gallbladder to release bile
• Secretin – stimulates the pancreas to release bicarbonate
ions in pancreatic juice
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101. Regulation of
Small Intestinal Secretions
• Regulation of small intestine secretion occurs
by:
• Mucus secretion is stimulated by the
presence of chyme in the small intestine
• Distension of the intestinal wall activates
nerve plexuses in the wall of the small
intestine
• Parasympathetic reflexes triggering the
release of intestinal enzymes
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103. Absorption of the Small Intestine
• Villi increase the surface area for absorption
• Small intestine absorption is so effective that very little
reaches the organ’s distal end, noting that:
• Monosaccharides and amino acids absorb:
• Through facilitated diffusion and active transport
• Absorbed into blood
• Large proteins are broken down and absorbed into
villi
• Fatty acids and glycerol absorb by:
• Several steps involved as noted
• Absorbed into lymph and blood
• Electrolytes and water absorb:
• Through diffusion, osmosis, and active transport
• Absorbed into blood
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105. O O O O
H H H H H H H H
Maltase
H2O
O
HO OH HO OH HO OH
Maltase
Maltose + Water Glucose + Glucose
Disaccharide Monosaccharides
R O H R O
H R O H O H
Dipeptidase
H N C C N C C OH H2O H N C C OH H N C C OH
H H R H
H
Dipeptide + Water Dipeptidase Amino acid + Amino acid
(from protein digestion)
O H
C17H35 C C
HO C H
H OH
C17H35 COO C H O
Lipase
C17H35 COO C H 3H2O C17H35 C C HO C H
H OH
C17H35 COO C
O
H
C17H35 C C HO C H
OH H
Fat + Water Lipase 4/6/2013 Shahzad Bashir +
Fatty acids 105
Glycerol

106. Fats collect in clusters
3
encased in protein to
form chylomicrons
Nucleus
Chylomicrons
4
leave epithelial cell
2 Fatty acids are and enter lacteal
used to synthesize
fats in endoplasmic Chylomicrons
reticulum
5 Lymph in lacteal
transports chylomicrons
away from intestine
1
Fatty acids
resulting from fat
digestion enter
epithelial cell
Fatty acids Endoplasmic
reticulum Lymph
Epithelial
cell Lacteal
Lumen of
intestine
To blood 106
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107. Large Intestine.
The large intestine is the terminal portion
of the GI tract.
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.
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108. Parts of the Large Intestine
Muscular layer
Mucous membrane
Serous layer
Transverse colon
Hepatic flexure Splenic
flexure
Ascending colon
Tenia coli
Epiploic
appendage Descending colon
Ileum
Ileocecal sphincter
Orifice of appendix
Haustra
Cecum
Appendix
Rectum Sigmoid colon
4/6/2013canal
Anal Shahzad Bashir 108

109. Anatomy of Large Intestine.
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.
Structurally, the four major regions of
the large intestine are the cecum,
colon, rectum, and anal canal.
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110. Anatomy of Large Intestine.
Ileocecal sphincter (valve), which allows
materials from the small intestine to pass
into the large intestine.
Hanging inferior to the ileocecal valve is
the cecum, a small pouch about 6 cm (2.4
in.) long.
Attached to the cecum is a twisted,
coiled tube, measuring about 8 cm (3 in.)
in length, called the appendix or
vermiform appendix (vermiform worm-
shaped; appendix = appendage).
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111. TRANSVERSE COLON
DESCENDING
ASCENDING COLON
COLON
Ileum
Ileocecal
sphincter
(valve)
CECUM
SIGMOID
COLON
VERMIFORM APPENDIX
RECTUM
ANUS
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112. Anatomy of Large Intestine.
The open end of the cecum merges
with a long tube called the colon
( food passage), which is divided into
ascending, transverse, descending,
and sigmoid portions.
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113. Functions of the Large Intestine
1. Haustral churning, peristalsis, and mass
peristalsis drive the contents of the colon
into the rectum.
2. Bacteria in the large intestine convert
proteins to amino acids, break down
amino acids, and produce some B
vitamins and vitamin K.
3. Absorbing some water, ions, and
vitamins.
4. Forming feces.
5. Defecating (emptying the rectum).
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114. Feces
• Feces is composed of materials not digested or
absorbed, and include:
• Water
• Electrolytes
• Mucus
• Bacteria
• Bile pigments altered by bacteria provide the color
• The pungent odor is produced by bacterial
compounds including:
• Phenol
• Hydrogen sulfide
• Indole
• Skatole
• Ammonia
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115. Lifespan Changes
• Changes to the digestive system are slow and
slight, and eventually include:
• Teeth may become sensitive
• Gums may recede
• Teeth may loosen, break or fall out
• Heartburn may become more frequent
• Constipation may become more frequent
• Nutrient absorption decreases
• Accessory organs age but typically not
necessarily in ways that effect health
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116. REFERENCES
Tortora, G. J. & Grabowski, S.R. (2000).
Principles of anatomy and physiology (12th
ed.). New York: John Wiley & sons Inc.
Eline, N.M, Essential of human anatomy &
physiology (9th ed.). New York: John Wiley
& sons Inc.
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117. End of Chapter
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