1. Digestive system
• Note:
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meant for better understanding of text.
These in no way replace the text books .All
chapters in the text books must be
thoroughly studied.

2. Digestive system
25-2

3. Digestive System: Overview
• The alimentary canal or gastrointestinal
(GI) tract digests and absorbs food
• Alimentary canal – mouth, esophagus,
stomach, small intestine, and large
intestine
• Accessory digestive organs – teeth,
tongue, gallbladder, salivary glands, liver,
and pancreas

4. Digestive Process
• There are six essential activities:
–Ingestion
–propulsion
–mechanical digestion
–Chemical digestion
–Absorption
–defecation

5. Gastrointestinal Tract Activities
• Ingestion – taking food into the digestive tract
• Propulsion – swallowing and peristalsis
– Peristalsis – waves of contraction and relaxation
of muscles in the organ walls
• Mechanical digestion – chewing, mixing, and
churning food
• Chemical digestion – catabolic breakdown of food
• Absorption – movement of nutrients from the GI
tract to the blood or lymph
• Defecation – elimination of indigestible solid
wastes

6. Histology of the Alimentary Canal
• From esophagus to the anal canal the
walls of the GI tract have the same four
tunics
–From the lumen to outward they are
the:
– Mucosa
– Submucosa
– Muscularis externa
– Serosa

7. Histology of the Alimentary Canal

8. Regulation of Digestive Tract
• Motility and secretion of the digestive tract are controlled by
neural, hormonal, and paracrine mechanisms
• Neural control
– Short (myenteric) reflexes – stretch or chemical stimulation acts through
myenteric plexus
• Stimulates parastaltic contractions of swallowing
– Long (vagovagal) reflexes - parasympathetic stimulation of digestive
motility and secretion
• Hormones
– Chemical messengers secreted into bloodstream, and stimulate distant
parts of the digestive tract
• Paracrine secretions
– Chemical messengers that diffuse through the tissue fluids to stimulate
nearby target cells
25-8

9. Enteric Nervous System
• Enteric nervous system – a nervous network in the esophagus,
stomach, and intestines that regulated digestive tract motility,
secretion, and blood flow
– thought to have over 100 million neurons
– more than the spinal cord
– functions completely independently of the central nervous system
• CNS exerts a significant influence on its action
• composed of two networks of neurons
– Submucosal (Meissner) plexus – in submucosa
• controls glandular secretion of mucosa
• controls movements of muscularis mucosae
– Myenteric (Auerbach) plexus – parasympathetic ganglia and nerve fibers
between the two layers of the muscularis interna
• controls peristalsis and other contractions of muscularis externa
• enteric nervous system contains sensory neurons that monitor
25-9
tension in gut wall and conditions in lumen

10. Mouth
• Oral or buccal cavity:
–Is bounded by lips, cheeks, palate, and
tongue
–Has the following organs:
–Teeth
–Salivary glands
–tongue

11. Mouth or Oral Cavity
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Upper lip
Vestibule Superior
labial
frenulum
Palatine raphe
Palatoglossal Hard palate
arch and palatine
Palatopharyngeal rugae
arch
Palatine tonsil Soft palate
Tongue Uvula
Lingual frenulum
Salivary duct
Lower lip
Figure 25.4
orifices:
Sublingual
Submandibular
Inferior labial
frenulum
25-11

12. Tongue
• Occupies the floor of the mouth and fills the oral cavity when
mouth is closed
• Functions include:
– Gripping and repositioning food during chewing
– Mixing food with saliva and forming the bolus
– Initiation of swallowing, and speech
Lingual Frenulum – median fold that attaches the
body to the floor of the mouth
• Sulcus terminalis – groove that separates the
tongue into two areas:
– Anterior 2/3 residing in the oral cavity
– Posterior third residing in the oropharynx

13. Tongue
• Superior surface bears four types of papillae
– Filiform – give the tongue roughness and provide
friction . Do not contain taste buds
– Fungiform – scattered widely over the tongue and
give it a reddish hue. Contains lots of taste buds.
– Circumvallate – V-shaped row in back of tongue
– Foliate –pleatlike shaped
#Study:- Ankyloglossia

14. Tongue
Figure 23.8

15. Salivary Glands
• Three pairs of glands – parotid,
submandibular, and sublingual
– Parotid – lies anterior to the ear between the
masseter muscle and skin. Parotid duct opens into
the vestibule next to second upper molar
• Submandibular – lies along the medial aspect
of the mandibular body. Its ducts open at the base of the
lingual frenulum
• Sublingual – lies anterior to the submandibular
gland under the tongue
• # Study : Mumps

16. Salivary Glands
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Parotid
gland Parotid duct
Tongue
Sublingual
ducts
Figure 25.9
Masseter
muscle
Submandibular
duct
Lingual
Submandibular frenulum
gland
Sublingual Opening of
gland Mandible submandibular
duct 25-16

17. Salivary Glands
Figure 23.9a

18. Saliva: Source and Composition
• Secreted from serous and mucous cells of
salivary glands
• 97-99.5% water, hypo-osmotic, slightly
acidic solution containing
– Electrolytes – Na+, K+, Cl–, PO42–, HCO3–
– Digestive enzyme – salivary amylase
– Proteins – mucin, lysozyme, defensins, and IgA
– Metabolic wastes – urea and uric acid

19. Teeth
• Primary and permanent dentitions have formed
by age 21
• Primary – 20 deciduous teeth that erupt at
intervals between 6 and 24 months
• Permanent – enlarge and develop causing the
root of deciduous teeth to be resorbed and fall
out between the ages of 6 and 12 years
– All but the third molars have erupted by the end of
adolescence
– Usually 32 permanent teeth

20. Classification of Teeth
• Teeth are classified according to their shape and
function
– Incisors – chisel-shaped teeth for cutting
or nipping
– Canines – fanglike teeth that tear or pierce
– Premolars (bicuspids) and molars – have
broad crowns with rounded tips; best
suited for grinding or crushing
• During chewing, upper and lower molars lock
together generating crushing force

21. Deciduous Teeth

22. Permanent Teeth
Figure 23.10.2

23. Dental Formula: Permanent Teeth
• A shorthand way of indicating the number and
relative position of teeth
– Written as ratio of upper to lower teeth for the
mouth
– Primary: 2I (incisors), 1C (canine), 2M (molars)
– Permanent: 2I, 1C, 2PM (premolars), 3M
2I 1C 2PM 3M
X 2 (32 teeth)
2I 1C 2PM 3M

24. Tooth Structure
• Two main regions – crown and the root
• Crown – exposed part of the tooth above the
gingiva
• Enamel – acellular, brittle material composed
of calcium salts and hydroxyapatite crystals;
the hardest substance in the body
– Encapsules the crown of the tooth
• Root – portion of the tooth embedded in the
jawbone

25. Tooth Structure

26. Esophagus
• – a straight muscular tube 25-30 cm long
– begins at level between C6 and the cricoid cartilage
– extends from pharynx to cardiac orifice of stomach passing
through esophageal hiatus in diaphragm
– lower esophageal sphincter – food pauses at this point
because of this constriction
• prevents stomach contents from regurgitating into the
esophagus
• protects esophageal mucosa from erosive effect of the
stomach acid
• heartburn – burning sensation produced by acid reflux
into the esophagus
• #Study: Deglutition

27. Esophagus

28. Stomach
• Chemical breakdown of proteins begins and food is
converted to chyme.it has the following parts:-
• Cardiac region – surrounds the cardiac
orifice
• Fundus – dome-shaped region beneath
the diaphragm
• Body – midportion of the stomach
• Pyloric region – made up of the antrum
and canal which terminates at the pylorus
• The pylorus is continuous with the
duodenum through the pyloric sphincter

29. Gross Anatomy of Stomach
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Esophagus
Fundic region
Lesser curvature Cardiac orifice
Cardiac region
Duodenum
Body
Pyloric region:
Gastric rugae
Pylorus
Pyloric
sphincter
Pyloric
canal
Antrum Greater curvature
(b)
© The McGraw-Hill Companies, Inc./Rebecca Gray, photographer/Don Kincaid, dissections
Figure 25.12b
longitudinal wrinkles called rugae can be seen in empty stomach wall.
25-29

31. Microscopic Anatomy of the Stomach

32. Glands of the Stomach Fundus and Body
• Gastric glands of the fundus and body have a
variety of secretory cells
– Mucous neck cells – secrete acid mucus
– Parietal cells – secrete HCl and intrinsic
factor
– Chief cells – produce pepsinogen and
prorenin
– Enteroendocrine cells – secrete gastrin,
histamine, endorphins, serotonin, cholecystokinin
(CCK), and somatostatin into the lamina propria

33. Functions of Hydrochloric Acid
• activates pepsin and lingual lipase
• breaks up connective tissues and plant cell walls
– helps liquefy food to form chyme
• converts ingested ferric ions (Fe3+) to ferrous ions
(Fe2+)
– Fe2+ absorbed and used for hemoglobin synthesis
• contributes to nonspecific disease resistance by
destroying most ingested pathogens
• #Study: Ulcer
25-33

34. Pepsin
• zymogens – digestive enzymes secreted as inactive
proteins
– converted to active enzymes by removing some of their
amino acids
• pepsinogen – zymogen secreted by the chief cells
– hydrochloric acid removes some of its amino acids and forms
pepsin that digests proteins
– autocatalytic effect – as some pepsin is formed, it converts
more pepsinogen into more pepsin
• pepsin digests dietary proteins into shorter peptide
chains
– protein digestion is completed in the small intestine
25-34

35. Intrinsic Factor
• intrinsic factor – a glycoprotein secreted by parietal
cells
• essential to absorption of vitamin B12 by the small
intestine
– binds vitamin B12 and intestinal cells absorb this complex by
receptor-mediated endocytosis
• vitamin B12 is needed to synthesize hemoglobin
– prevents pernicious anemia
• secretion of intrinsic factor is the only indispensable
function of the stomach
– digestion can continue if stomach is removed
25-35
(gastrectomy), but B12 supplements will be needed

36. Small Intestine
• Runs from pyloric sphincter to the ileocecal valve
• Has three subdivisions:
• Duodenum, Jejunum, And Ileum
• Structural modifications of the small intestine wall
increase surface area
– Plicae circulares: deep circular folds of
the mucosa and submucosa
– Villi – fingerlike extensions of the mucosa
– Microvilli – tiny projections of absorptive
mucosal cells’ plasma membranes

37. Small Intestine

38. Duodenum and Related Organs

39. Liver
• The largest gland in the body
• Superficially has four lobes – right, left,
caudate, and quadrate
• The gallbladder rests in a recess on the
inferior surface of the right lobe

40. Liver:functions
• Hepatocytes’ functions include:
– Production of bile
– Processing bloodborne nutrients
– Storage of fat-soluble vitamins
– Detoxification
– Deamination
– Gluconeogenesis, glucogenesis etc.

41. Liver: Associated Structures
• Bile produced by the liver leaves it via:
–Bile ducts, which is formed by fusion
of cystic duct of gall bladder and
hepatic duct of liver
–This bile duct then fuses with the
pancreatic duct to form
hepatopancreatic
duct/ampulla(sphincter of Oddi)

42. Liver: Microscopic Anatomy
• Hexagonal-shaped liver lobules are the
structural and functional units of the liver
– Composed of hepatocyte (liver cell) plates
radiating outward from a central vein
Figure 23.24c

43. Microscopic Anatomy of Liver
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Hepatocytes
Bile
canaliculi
Stroma
Central vein
Hepatic triad:
Hepatic
Branch of sinusoid
hepatic
portal vein
Branch of
proper hepatic Stroma
artery
Bile ductule
(a)
Figure 25.20a 25-43

44. Liver: Microscopic Anatomy
• Liver sinusoids – enlarged, leaky
capillaries located between hepatic
plates
• Kupffer cells – hepatic macrophages
found in liver sinusoids

45. Histology of Liver - Hepatic Triad
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Stroma
Central vein
Hepatic
lobule
Branch of
hepatic
portal vein
Bile ductule
Lymphatic
vessel
Branch
of proper
hepatic artery
(b) 0.5 mm
© The McGraw-Hill Companies, Inc./Dennis Strete, photographer
Figure 25.20b 25-45

46. Gross Anatomy of the
Gallbladder, Pancreas, and Bile Passages
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Gallbladder: Hepatic ducts
Neck
Body
Head
Common hepatic duct
Cystic duct
Bile duct
Accessory Pancreatic duct
pancreatic duct
Duodenum
Minor duodenal Pancreas:
papilla Tail
Body
Circular folds Head
Hepatopancreatic Duodenojejunal
sphincter flexure
Major duodenal
papilla
Jejunum
Hepatopancreatic
ampulla
Figure 25.21
25-46

47. Composition of Bile
• A yellow-green, alkaline solution containing bile
salts, bile pigments, cholesterol, neutral
fats, phospholipids, and electrolytes
• Bile salts are cholesterol derivatives that:
–Emulsify fat
–Facilitate fat and cholesterol
absorption
–Help solubilize cholesterol
• The chief bile pigment is bilirubin, a waste
product of heme

48. The Gallbladder
• Thin-walled, green muscular sac on the ventral
surface of the liver
• Stores and concentrates bile by absorbing its
water and ions
• Releases bile via the cystic duct, which flows
into the bile duct
• #Study: Gallstones and Lithotripsy

49. Pancreas
• Location
– Lies deep to the greater curvature of the stomach
– The head is encircled by the duodenum and the tail abuts
the spleen.
– Pancreas is a heterocrine gland that has
both endocrine and exocrine functions:-
• Exocrine function
– Secretes pancreatic juice which breaks down all
categories of foodstuff. This pancreatic juice is
secreted in acini.
– Acini (clusters of secretory cells) contain zymogen
granules with digestive enzymes
• Endocrine function – release of insulin and glucagon

50. Pancreatic Acinar Cells
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Acinar cells
Zymogen
granules
(a)
Stroma
Ducts
Figure 25.22 a-b
Exocrine
acinar cells
Vein
(b) 50 µm
25-50
© The McGraw-Hill Companies, Inc./Dennis Strete, photographer

51. Large Intestine
• Is subdivided into the
cecum, appendix, colon, rectum, and
anal canal
• The saclike cecum:
– Lies below the ileocecal valve in the right iliac
fossa
– Contains a wormlike vermiform appendix

53. Anatomy of Large Intestine
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Greater
Right colic omentum
flexure (retracted)
Left colic
flexure
Transverse
colon
Superior Taenia coli
mesenteric
artery
Mesocolon
Haustrum
Figure 25.31a
Ascending Descending
colon colon
Ileocecal Omental
valve appendages
Ileum
Cecum
Appendix
Sigmoid
colon
Rectum
Anal canal
External anal
sphincter
25-53
(a) Gross anatomy

54. Colon
• Has distinct regions:
• ascending colon, hepatic flexure,
transverse colon, splenic flexure,
descending colon, and sigmoid colon
• The sigmoid colon joins the rectum
• The anal canal, the last segment of the
large intestine, opens to the exterior at the
anus

55. Large intestine
• Has three unique features:
– Teniae coli – three bands of longitudinal smooth
muscle in its muscularis
– Haustra – pocketlike sacs caused by the tone of
the teniae coli
– Epiploic appendages – fat-filled pouches of
visceral peritoneum

56. Functions of the Large Intestine
• Other than digestion of enteric bacteria, no
further digestion takes place
• Vitamins, water, and electrolytes are reclaimed
• Its major function is propulsion of fecal
material toward the anus
• Though essential for comfort, the colon is not
essential for life
• #Study :- Diarrhea and Constipation.

57. Physiology of
digestion.
1.Digestion of carbohydrates.
2.Digestion of proteins.
3.Digestion of fats.

58. Digestion of carbohydrates
• Digestion of carbohydrates starts in the oral
cavity.
• Salivary amylase present in the saliva causes
the breakdown of complex carbohydrates into
smaller parts.
• Polysaccharides salivary amylase oligosaccharides,
disaccharides,
monosaccharides.

59. Carbohydrate digestion in stomach
• Stomach does not have any enzymes for
digestion of carbohydrates.

60. Carbohydrate digestion in small intestine
• Small intestine receives two juices:-
intestinal juice and the pancreatic juice
• Pancreatic juice have one enzymes for
carbohydrate digestion:- pancreatic
amylase
• Pancreatic amylase will act upon
complex carbohydrates and break them
into small carbohydrates Eg. Starch can
be broken down into
maltose, isomaltose, etc.

61. Carbohydrate digestion in small intestine
• Intestine contains many enzymes for
carbohydrate digestion:- intestinal
amylase, maltase, isomaltase, lactase, s
ucrase.
• Thus all these enzymes will ultimately
convert complex carbohydrates into
monosaccharide like glucose, fructose
and galactose

62. Digestion of proteins
• The final product of protein digestion are the
amino acids.
• There are no protein digesting enzymes in the
mouth and thus Protein digestion starts in the
stomach . Enzyme in stomach is pepsinogen
– Chief cells of stomach – produce pepsinogen
• Pepsinogen is activated to pepsin by:
– HCl in the stomach
– Pepsin itself via a positive feedback mechanism
–Pepsin will then act on large protein and
will break them into small peptides and
amino acids

63. Digestion of proteins in small intestine
• Enzymes acting in the small intestine
–Pancreatic enzymes – trypsinogen,
chymotrypsinogen, and
procarboxypeptidase
Pancreatic enzymes are in inactive form
and they need to be activated to
become functional
- Intestinal enzymes – aminopeptidases,
carboxypeptidases, and dipeptidases

64. Activation of Pancreatic Enzymes in the
Small Intestine
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Chymotrypsinogen Chymotrypsin
Procarboxypeptidase Carboxypeptidase
Trypsinogen
Trypsin
Enterokinase
Figure 25.23 25-64

66. Digestion of fats
• Fat (triglycerides) digestion starts in the stomach
and is completed in the intestine. Saliva do not
have any fat digesting enzyme.
• Enzymes for fat digestion are :-
• Lingual lipase
• Gastric lipase
• Pancreatic lipase
• Intestinal lipase
• Bile plays a very important role of emulsifying
the fats.
• All these enzymes will break fats into fatty
acids and glycerol.

67. Action of bile on fat

68. Absorption
• Absorption refers to transport of digested
products of food from the intestinal lumen into
the blood circulation.
• Carbohydrate absorption: glucose and galactose
are carried by active transport from the intestinal
lumen into the epithelial cells of intestine. Then
they move out of these cells into the blood
capillaries by simple diffusion
• Protein absorption: it is very similar to
absorption of carbohydrates. amino acids are
carried from lumen to epithelial cells by active
transport and then they leave these cells by
diffusion.

69. Figure 23.34

70. Fatty Acid Absorption
• Fatty acids and monoglycerides combine with
bile salt and cholesterol to form micelles
• Micelles act as ‘ferry’ and help fatty acids to
enter the intestinal cells by diffusion
• Once inside the intestinal cells the free fatty
acids and glycerol are resynthesized into
trigylceride. Triglycerides combine with
protein to form chylomicron. Resulting
chylomicrons are extruded and they enter
lacteals and are transported to the circulation
via lymph

71. Fatty Acid Absorption Fatty acids and
Lumen of monoglycerides
intestine associated with
micelles in
lumen of intestine
1 Fatty acids and
monoglycerides
resulting from fat
Absorptive digestion leave
epithelial cell micelles and enter
cytoplasm epithelial cell by
diffusion.
2 Fatty acids are
ER used to synthesize
triglycerides in
smooth endo-
plasmic reticulum.
Golgi
apparatus 3 Fatty globules are
combined with
proteins to form
chylomicrons
(within Golgi
apparatus).
4 Vesicles containing
chylomicrons
migrate to the
basal membrane,
are extruded from
the epithelial cell,
and enter a lacteal
(lymphatic capillary).
5 Lymph in the
Chylomicron
lacteal transports
Lacteal
chylomicrons away
from intestine.
Figure 23.36

72. Fatty Acid Absorption Fatty acids and
Lumen of monoglycerides
intestine associated with
micelles in
lumen of intestine
Absorptive
epithelial cell
cytoplasm
ER
Golgi
apparatus
Lacteal
Figure 23.36

73. Fatty Acid Absorption Fatty acids and
Lumen of monoglycerides
intestine associated with
micelles in
lumen of intestine
1 Fatty acids and
monoglycerides
resulting from fat
Absorptive digestion leave
epithelial cell micelles and enter
cytoplasm epithelial cell by
diffusion.
ER
Golgi
apparatus
Lacteal
Figure 23.36

74. Fatty Acid Absorption Fatty acids and
Lumen of monoglycerides
intestine associated with
micelles in
lumen of intestine
1 Fatty acids and
monoglycerides
resulting from fat
Absorptive digestion leave
epithelial cell micelles and enter
cytoplasm epithelial cell by
diffusion.
2 Fatty acids are
ER used to synthesize
triglycerides in
smooth endo-
plasmic reticulum.
Golgi
apparatus
Lacteal
Figure 23.36

75. Fatty Acid Absorption Fatty acids and
Lumen of monoglycerides
intestine associated with
micelles in
lumen of intestine
1 Fatty acids and
monoglycerides
resulting from fat
Absorptive digestion leave
epithelial cell micelles and enter
cytoplasm epithelial cell by
diffusion.
2 Fatty acids are
ER used to synthesize
triglycerides in
smooth endo-
plasmic reticulum.
Golgi
apparatus 3 Fatty globules are
combined with
proteins to form
chylomicrons
(within Golgi
apparatus).
Lacteal
Figure 23.36

76. Fatty Acid Absorption Fatty acids and
Lumen of monoglycerides
intestine associated with
micelles in
lumen of intestine
1 Fatty acids and
monoglycerides
resulting from fat
Absorptive digestion leave
epithelial cell micelles and enter
cytoplasm epithelial cell by
diffusion.
2 Fatty acids are
ER used to synthesize
triglycerides in
smooth endo-
plasmic reticulum.
Golgi
apparatus 3 Fatty globules are
combined with
proteins to form
chylomicrons
(within Golgi
apparatus).
4 Vesicles containing
chylomicrons
migrate to the
basal membrane,
are extruded from
the epithelial cell,
and enter a lacteal
(lymphatic capillary).
Chylomicron
Lacteal
Figure 23.36

77. Fatty Acid Absorption Fatty acids and
Lumen of monoglycerides
intestine associated with
micelles in
lumen of intestine
1 Fatty acids and
monoglycerides
resulting from fat
Absorptive digestion leave
epithelial cell micelles and enter
cytoplasm epithelial cell by
diffusion.
2 Fatty acids are
ER used to synthesize
triglycerides in
smooth endo-
plasmic reticulum.
Golgi
apparatus 3 Fatty globules are
combined with
proteins to form
chylomicrons
(within Golgi
apparatus).
4 Vesicles containing
chylomicrons
migrate to the
basal membrane,
are extruded from
the epithelial cell,
and enter a lacteal
(lymphatic capillary).
5 Lymph in the
Chylomicron
lacteal transports
Lacteal
chylomicrons away
from intestine.
Figure 23.36

78. Hormonal regulation of digestion
• The release of digestive secretions is under control
of various hormones:-
• Gastrin: stimulate secretion of gastric
juice
• Enterogastrone:stops secretion of
gastric juice
• Cholecystokinin:release of bile from
gall bladder and release of enzymes of
pancreatic juice.

79. Hormonal regulation of digestion
• Secretin: speeds up release of bile and
cause release of sodium bicarbonate in
pancreatic juice
• Villikinin: accelerates the movement of
villi
• Duocrinin: release of mucus from
brunner’s glands
• Enterocrinin :release of enzymes from
crypts of leiberkuhn