The digestive system breaks down food into nutrients that can be absorbed and used by the body. It consists of the gastrointestinal tract and accessory organs. The gastrointestinal tract includes the mouth, esophagus, stomach, small intestine, and large intestine. Accessory organs that contribute to digestion include the teeth, tongue, salivary glands, liver, gallbladder and pancreas. Each organ has a unique role in the multi-step digestive process which includes ingestion, digestion, absorption and elimination.

1. ALEX SANGA MVM, MBA
ST. JOHN’S UNIVERSITY OF
TANZANIA
SOPH JUNE 2018
The Digestive System

2. The Digestive System

3. Overview of the Digestive Tract
The Components of the Digestive System and
Their Functions

4. The Organs of the Digestive
System
The Mouth
The Salivary Glands
The Esophagus
The Stomach
The Liver
The Gallbladder
The Pancreas
The Small intestines
The Large intestines

5. Digestive System
• 2 parts
– Gastrointestinal (GI) tract (alimentary canal)
– Accessory digestive organs
• 6 functions
– Ingestion: taking in food/liquids
– Secretion: water, enzymes, buffers
– Mixing & propulsion: muscular contraction & peristalsis
– Digestion: chemical=enzymes; mechanical=teeth & churning
– Absorption: small nutrients enter cells
– Defecation: output of undigested material (feces); mainly cellulose

6. Functions
• Motility
• Secretion
• Digestion
• Absorption

7. Basic GI Functions
• Secondary functions
– Mass balance
• Ensuring daily fluid input
and output are equal
– Protection
• GI tract provides a huge
external surface for
pathogens to gain entrance
into the internal
environment

8. Regulation of GI Function
• What is regulated?
– All aspects of the GI processes
• Regulated by
– In general the signals are:
• Neural
• Hormonal
• Paracrine
– Specifically the controls and systems are:
• The Long & Short Reflexes
• GI peptide reflexes
• The autonomous function of the enteric nervous system (ENS)

9. Regulation of GI Function
Long & Short Reflexes
• Long Reflexes
– Integrated within the CNS
• May originate in or outside of the GI tract
– Feed forward & emotional reflexes are initiated and integrated
entirely outside the GI tract
» Called cephalic reflexes
• Short Reflexes
– Integrated in the enteric nervous system
• Initiated by changes in pH, distension, osmolarity, products of
digestion
• Submucosal plexus contains the sensory neurons
• Afferent information to ganglia
• Efferent information to submucosal and myenteric plexuses for
control of secretion, motility and growth

10. Digestive
responses
Enteric Nervous System
Regulation of GI Function
Long & Short Reflexes
external
stimuli
sensory
receptors
sensory
receptors
and
neurons
inter-
neurons
the
cephalic
brain
Effectors
local
stimuli
neurons of
submucosal
and
myenteric
plexuses
muscle
contraction
and/or
relaxation,
exocrine
secretion,
paracrine
release,
endocrine
secretion,
defecation
smooth
muscles
or
secretory
cells
long reflex pathway
short reflex pathway

11. Regulation of GI Function
GI Peptide Reflexes
• Peptides released by the GI tract may act
– As hormones
• Secreted into the blood
• Act on accessory organs, other parts of the GI tract or the brain
– As paracrine signals
• Secreted into the lumen or extracellular fluid
– Lumenal signals bind to apical epithelial receptors
– ECF signals act in the immediate vicinity of secretion
– Effect
• Peptides alter secretion and motility
• Alter behavior related to eating

12. Regulation of GI Function
GI Peptide Reflexes
Gastrin
family
Secretin
family
Peptide
family
Secretin
family

13. Regulation of GI Function
Enteric Nervous System
• Allows for the autonomous behavior of the digestive
system
– CNS control is not required for digestive functioning
– Commonalities between ENS and CNS
• Intrinsic neurons – similar to interneurons of CNS
• Extrinsic neurons – composed of autonomic neurons
• Neurotransmitters and neuropeptides
– Nonadrenergic and noncholinergic receptors
» Same as adrenergic and cholinergic in CNS
• Glial support cells – similar to astrocytes in CNS
• Diffusion barrier – cells around capillaries in the ganglia are tight,
just as the capillaries in the brain, forming the BBB
• ENS acts as its own integrating center, just as the CNS does

14. GI Anatomy

15. Functions of
Digestive System
food
Intestines
Absorbable
nutrients
lumen
blood
wall
wall

16. digestion
food
Intestines
Absorbable
nutrients
lumen
blood
wall
wall

17. digestion
food
Absorbable
nutrients
blood
absorption

18. Forms of Nutrients
In Food Absorbable by Intestine
Protein Amino acids
Carbohydrates Monosaccharides (glucose)
Fat Fatty acids, glycerol
DNA, RNA Bases + monosaccharides
Vit B12 B12+intrinsic factor
Other vitamins Original form
Cholesterol Original form
Electrolytes Original form
Water Original form

19. digestion
food
blood
absorption
Amino acids
Glucose
Fatty acids,
Glycerol
Bases +
monosaccharides
vitamins
Cholesterol
Electrolytes,
Water

20. The Mouth
has the following
functions :
1) breaking food
2) sense of taste
3) swallowing
4) speech

21. Saliva and the Salivary Glands
1) moisten food and the
mouth
2) clean the teeth, inhibit
bacterial growth,
3) dissolve molecules so
they can stimulate taste
buds
4) digest a small amount of
starch and fat

22. Esophagus

23. Esophagus

24. The Stomach

25. The Stomach
Functions
- food storage
- host defense
- minor role in digestion and
absorption of nutrients
- The above functions are
performed via gastric secretion
and motility
Gastric pits

26. G cells:
produce
hormones
that regulate
digestion
chief cells:
produce
pepsinogen
parietal cells:
produce HCl
and intrinsic
factor
mucous cells:
produce
mucus
The bottom part of
the pits is the
gastric gland.

27. The gastric glands produce 2-3 L
of gastric juice daily, containing:
pepsinogen
HCl
intrinsic factor

28. Pepsinogen
Pepsinogen
- is the inactive precursor of
the active enzyme pepsin.
- activated by HCl or pepsin.
Pepsin: chop proteins into
small fragments

29. Hydrochloric
Acid ( HCl )
- reduces pH of
gastric juice to as low
as pH 0.8.
HCl

30. HCl
Functions of HCl:
(1) activates pepsinogen
into pepsin.
(2) breaks up connective
tissues and plant cell
walls.
(3) converts ferric ions to
ferrous ions.
(4) destroys ingested
pathogens.

31. 1) a highly alkaline mucous
coat.
2) rapid replacement of
epithelial cells (3-6 d)
3) tight junctions between
epithelial cells
HCl
Self protection of the stomach
from HCl and pepsin by

32. Intrinsic Factor
- is essential to the absorption
of vitamin B12
- is the only indispensable
function of the stomach.
Intrinsic
Factor

33. Gastrin
secreted by G cells in
gastric gland not into
gastric juice but blood
- is a hormone
- stimulates
1) the secretion of HCl
and pepsinogen
2) motility of the large
intestine

34. - relax during swallowing
- stretch further when
food enters the stomach
(stress-relaxation
response)
Gastric Motility
- Next, pacemaker cells in
the greater curvature initiate
peristaltic contractions.
- Each peristaltic wave pushs ~ 3 mL of chyme into the
duodenum.

35. Regulation of Gastric Function
Gastric secretion and motility is divided
into three stages:
1) Cephalic
2) Gastric
3) Intestinal phases

36. The cephalic phase is
stimulated by the sight,
smell, taste, or mere
thought of food.
1) The Cephalic Phase

37. - is stimulated by food in the
stomach
- accounts for two-thirds of
gastric secretion.
2) The Gastric Phase

38. Phases of Digestion
Gastric Phase
2. Digests the food into chyme
– By continuation of salivary amylase until denatured
– By action of secretions
• Parietal cells secrete HCl (gastric acid) and intrinsic factor
– HCl dissociates into H+ and Cl-
– Intrinsic factor required for B12 absorption in the intestine
• Chief cells secrete pepsinogen & gastric lipase
– Pepsinogen is converted to pepsin by the action of H+
– Pepsin is an endopeptidase
• Mucous neck cells
– Secretes mucous for protection
– Secretes bicarbonate for protection
• Enterochromaffin-like cells
– Secretes histamine in response to parasympathetic activity and gastrin and increases
parietal cell
• D cells
– Secretes somatostatin when pH drops to inhibit further parietal cell secretions
• G cells
– Secrete gastrin to stimulate parietal cells, also relaxes ileocecal sphincter, increases
pyloric sphincter activity and lower stomach motility

39. a) the enterogastric reflex.
b) local hormones
Secretin
Cholecystokinin
gastric inhibitory peptide
3) The Intestinal Phase
After entering
small intestines, chyme inhibit
gastric secretion and mobility
via:

40. Phases of Digestion
Intestinal Phase
3. Hormonal and neural aspects of the intestinal phase
– entrance of chyme into duodenum gets the enteric nervous system
going, secreting:
• Secretin
– slows gastric emptying & gastric acid production
– Stimulates bicarbonate (HCO3
-) production from pancreas to buffer
acidic chyme
• cholecystokinin (CCK)
– Secreted in response to lipids and slows gastric motility and gastric acid
secretion
– Acts hormonally on the hypothalamus,
• Incretin hormones (GIP and GLP-1)
– GIP (gastric inhibitory peptide)
– GLP-1 (glucagon-like peptide1)
» Slow gastric acid and emptying
» stimulate insulin release from pancreas

41. Phases of Digestion
Intestinal Phase
3. Major processes occurring in the intestinal phase
– Buffering
• Via pancreatic exocrine secretion
– Digestion
• By pancreatic exocrine secretion
– Trypsinogen, chymotrypsinogen, procarboxypeptidase, procolipase and
prophospholipase
• By bile release from gallbladder (stimulated by CCK)
– Bile emulsifies the lipids, increasing surface area for pancreatic lipases
• By intestinal mucosal enzymes (brush border enzymes) that are
“anchored” to apical surface
– Peptidases, disaccharidases, enteropeptidase
– Absorption
• Most of the water & nutrients are absorbed in the small intestine

42. Phases of Digestion
Intestinal Phase
3. Activation of
pancreatic
proteolytic
enzymes

43. The small intestine receives not
only chyme from the stomach but
also secretions from the liver and
pancreas.

44. The Liver
has tremendous
variety of functions,
including the
secretion of bile
for digestive
purposes.

45. Hepatocytes secret bile into the bile canaliculi and
bile ductules.
bile ductule

46. Bile ductules  hepatic ducts 
common
hepatic
duct
cystic
duct
common hepatic duct
gallbladder
common bile duct
hepatopancreatic
sphincter
pancreas
duodenum

47. When no chyme is in the small intestine, the
hepatopancreatic sphincter is closed. Bile flows
into gallbladder.
cystic
duct
common hepatic duct
gallbladder
common bile duct
hepatopancreatic
sphincter
pancreas
duodenum

48. Function of Gallbladder
- absorbs water and stores the bile for
later use.
gallbladder
hepatopancreatic
sphincter
duodenum

49. - is constantly produced by the liver (500-1,000
mL of bile per day).
- is a yellow-green fluid containing :
minerals
bile pigments
bile acids
phospholipids
cholesterol
neutral fats
Bile
facilitate fat
digestion and
absorption

50. - reabsorbed in the ileum and returned to the liver
via enterohepatic circulation.
Bile acids
phospholipids
Recycle of Bile acids and Phospholipids

51. The Pancreas

52. Endocrine:
- secretes insulin, glucagon, somatostatin into
the blood.
The Pancreas
Exocrine:
- secretes pancreatic
juice into the lumen of the
pancreatic duct

53. Exocrine
secretes 1,200-1,500 mL of pancreatic juice per
day into the main pancreatic duct.
It empties into
duodenum
when hepato-
pancreatic
sphincter
opens.

54. 1) sodium bicarbonate:
neutralize HCl
2) inactive digestive enzymes and zymogens which
are activated after secreted into duodenum.
The activated enzymes digest carbohydrates,
lipids, RNA, DNA, and proteins.
Pancreatic juice is an
alkaline mixture
containing:

55. - secreted in response to
similar stimuli.
-Cephalic and Gastric
phases
The vagus nerves
stimulate pancreatic
secretion.
Regulation of Bile and Pancreatic
Secretion

56. 1) relaxation of the hepatopancreatic sphincter,
2) the contraction of the gallbladder
3) secretion of pancreatic juice and enzymes.
The Intestinal Phase
- Chyme with acid and fat,
stimulate the duodenal
mucosa to secrete
cholecystokinin (CCK).

57. secretion of bicarbonate
by both the hepatic and pancreatic ducts
The Intestinal Phase
- Acidic chyme also
stimulates the duodenum
to release secretin.

58. Phases of Digestion
Intestinal Phase
3. The large intestines main processes are
– Concentrating waste
• Removal of water
– Only about .1L of water lost daily through feces
– Movement & defecation
• Ileocecal valve controls chyme entrance into colon
– Relaxes in sequence with intestinal peristalsis as well as when gastric
emptying starts (gastrocolic reflex)
» CCK, serotonin and gastrin are potential initiators of the
gastrocolic reflex
• Defecation reflex
– Increases abdominal pressure, relaxes anal sphincters
– Digestion and absorption
• Digestion mainly through bacterial action which produces
– Lactate and fatty acids which are absorbable by simple diffusion
– Bacterial action also produces vitamin K
– By product of bacterial fermentation is gas (CO2, methane & HS)

59. The Small Intestine

60. Overview
- Nearly all chemical
digestion and nutrient
absorption occur in the
small intestines.

61. 1) The duodenum
- ~ 25 cm
2) The jejunum
- comprises the next 2.5 m.
3) The ileum
- forms the last 3.6 m.

62. - The surface area inside the small intestine is
greatly increased by circular folds, villi, and
microvilli.
villi

63. Microvilli (brush border)
brush border enzymes

64. brush border enzymes
- activates zymogens
- complete digestion of
carbohydrates and proteins

65. - 1-2 L of intestinal juice per day.
- The duodenum endocrine cells
secret cholecystokinin (CCK) and
secretin. (Both are hormones.)
Secretion by the small intestines

66. Phases of Digestion
Intestinal Phase
• The large intestines main processes are
– Concentrating waste
• Removal of water
– Only about .1L of water lost daily through feces
– Movement & defecation
• Ileocecal valve controls chyme entrance into colon
– Relaxes in sequence with intestinal peristalsis as well as when gastric
emptying starts (gastrocolic reflex)
» CCK, serotonin and gastrin are potential initiators of the
gastrocolic reflex
• Defecation reflex
– Increases abdominal pressure, relaxes anal sphincters
– Digestion and absorption
• Digestion mainly through bacterial action which produces
– Lactate and fatty acids which are absorbable by simple diffusion
– Bacterial action also produces vitamin K
– By product of bacterial fermentation is gas (CO2, methane & HS)

67. Chemical Digestion and Absorption
of Nutrients

68. Chemical Digestion and Absorption of Nutrients
Carbohydrates
Proteins
Lipids
Nucleic Acids
Vitamins
Minerals
Water

69. Digestion and Absorption of
Carbohydrate
- Most digestible dietary
carbohydrate is starch.
- The starch digestion begins in
the mouth by salivary amylase.
- But fully digestion of starch
occurs in the small intestines.
-By brush border enzymes –
dextrinase, glucoamylase,
sucrase, lactase, maltase

70. - Starch is digested to oligosaccharides (3-8 glucose
residues), disaccharide maltose, and glucose.

71. starch
oligosaccharides glucose
pancreatic amylase
Brush border
enzymes
Intestinal lumen Intestinal
epithelial
cells
blood
glucose glucose

72. Glucose and galactose are absorbed by:
- sodium-dependent glucose transporter
(SGLT).
-solvent drag
Fructose is by facilitated diffusion

73. Absorption
• Carbohydrate
absorption

74. Digestion and Absorption of
Proteins

75. - Proteins are digested by proteases and
peptidases.
- Protein digestion starts in the stomach.

76. Protein digestion continues in the small intestine by
pancreatic enzymes trypsin and chymotrypsin.

77. Protein digestion is completed in the small intestine
by brush border enzymes carboxypeptidase,
aminopeptidase, and dipeptidase.
Amino acid absorption is similar to that of
monosaccharides, via several sodium-dependent amino
acid cotransporters.

78. proteins short peptides amino acids
Gastric pepsin &
pancreatic proteases
Brush border
enzymes
Intestinal lumen Intestinal
epithelial
cells
blood

79. Absorption
• Protein
absorption

80. Digestion and Absorption of
Lipids

81. Digestion and Absorption of Lipids
- Lipids are digested by enzymes called lipases.
- Most fat digestion occurs in the small intestine via
several steps.

82. 1) Fats are first broken up into smaller
emulsification droplets by lecithin and bile
salts (acids) in the bile.

83. 2) When lipase digests fats, the products are
two fatty acids (FFAs) and a monoglyceride.

84. 3) Bile salts coat these and other lipids and
form droplets called micelles.

85. Micelles release
their lipids,
which diffuse
freely across the
plasma
membrane.

86. Resynthesis of triglycerides.
Coating with protein, forming droplets called
chylomicrons.
Intestinal absorptive cell

87. Chylomicrons are too large to enter blood capillaries
and must be first transported in the lymphatic lacteal.

88. Fat globule
emulsification
droplets micelles
chylomicrons
Intestinal lumen Intestinal
epithelial
cells
lacteal blood
Bile
salts
lipase &
bile salts

89. Absorption
• Lipid digestion
& absorption

90. Digestion and Absorption of
Nucleic Acids

91. Digestion and Absorption of Nucleic Acids
- The pancreatic nucleases hydrolyze nucleic
acids to their component nucleotides.
- The brush border nucleosidases and
phosphatases further break them down, and
the products are transported across the
intestinal epithelium by membrane carriers.

92. Absorption of Vitamins

93. Absorption of Vitamins
- Vitamins are absorbed without digestion.
- The fat-soluble vitamins are absorbed with other
lipids.
- Water soluble vitamins are absorbed by simple
diffusion, with the exception of vitamin B12.
- Vitamin B12 is an unusually large molecule that
can only be absorbed with the help of intrinsic
factor.

94. Absorption of Minerals

95. Absorption of Minerals
- Minerals (electrolytes) are absorbed without
digestion.
- Iron and calcium are unusual in that they are
absorbed in proportion to the body's need.
- Other minerals are absorbed at fairly constant
rates regardless of need.

96. Absorption of Water

97. Absorption of Water
- The digestive tract receives about 9 L of water per
day
0.7 L in food
1.6 L in drink
6.7 L in gastrointestinal secretions
- ~ 8 L absorbed by the small intestine via osmosis;

98. Absorption
• Absorbed
nutrients and
water are
returned via the
hepatic portal
system

99. Intestinal Motility
serve three functions:
1) to mix chyme with intestinal
juice, bile, and pancreatic juice
(segmentation)
2) to churn chyme and bring it in
contact with the brush border for
digestion and absorption;
(segmentation)
3) to move residue toward the
large intestine. (peristalsis)

100. - Segmentation is the most
common type of movement of
the small intestine.
- When nutrients have been
absorbed, segmentation slows
and peristalsis begins.
- The intensity of the
contractions is modified by
nervous and hormonal
influences.

101. The Large Intestine

102. Large Intestine
• Includes the cecum, the colon, the rectum
and the anal canal
• It has larger diameter than the small
intestine (6.5 cm by 2.5 cm)

104. - No further chemical
digestion
- Water (~ 1L) is further
absorbed.
- The feces consists of:
75% water
25% solid matter, of which
30% is bacteria, and 30%
undigested fiber.

105. Bacterial Flora
- refer to several species of
useful bacteria in large intestine.
- ferment cellulose and other
undigested carbohydrates
- synthesize B vitamins and
vitamin K, which are absorbed by
the colon.

106. Strong contractions
called mass
movements occur
one to three times a
day, last about 15
minutes each, and
occur especially an
hour after breakfast.

107. Neural Control of Defecation
1. Filling of the rectum
2. Reflex contraction of
rectum & relaxation of
internal anal sphincter
3. Voluntary relaxation of
external sphincter

108. Diarrhea
too little water absorbed
Constipation
too much water absorbed, causing
difficulty in defecation

109. In the absence of bile, a fat-rich
diet causes diarrhea.
Accumulation of fat molecules
osmolarity of intestinal content
retain water
diarrhea

110. People lacking lactase have
diarrhea after drinking milk.
Lactose (a dimer)
glucose galactose
lactase

111. People lacking lactase have
diarrhea after drinking milk.
Accumulation of Lactose
osmolarity of intestinal content
retain water
diarrhea

112. The Stomach
The Liver
The Gallbladder
The Pancreas
The Small intestine
The Large intestine
Digestion and
Absorption of Nutrients
Carbohydrates
Proteins
Lipids
Nucleic Acids
Vitamins
Minerals
Water
The Organs
SUMMARY

113. THE END