The document summarizes key aspects of gastrointestinal physiology. It describes the components and functions of the digestive system, including the layers of the gastrointestinal tract and roles of the various organs. It then discusses motility and movement of contents through the tract, the processes of mechanical and chemical digestion, and secretions and functions of saliva, stomach acid, enzymes and other factors. Motility is controlled by the enteric and autonomic nervous systems in response to stretch and chemical receptors.

1. Chapter Ten
Gastrointestinal physiology
1

2. Digestive System
• The digestive system consisting of the digestive tract (GIT)
and accessory organs
• The alimentary canal or gastrointestinal (GI) tract is
composed of the organs mouth, pharynx, esophagus,
stomach, small intestine, large intestine and anus
• Accessory digestive organs – teeth, tongue and glandular
organs, such as the salivary glands, liver, and pancreas
2

3. The Components of the Digestive System
3

4. Functions of gastrointestinal Tract
• Ingestion: taking materials 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 with the
help of enzymes
• Absorption: movement of nutrients from the GI tract lumen
into the blood or lymph
• Secretion: release of water, acids, enzymes, buffers, and salts
• Defecation: elimination of indigestible solid wastes
4

5. Layers of the GIT
1. Mucosa: Inner layer of the luminal surfaces
- protection, secretion & absorption
2. Submucosa: blood & lymph vessels, nerve plexuses
- necessary for motility + secretion
3. Muscularis externa : contains circular & Longitudinal
smooth muscle, mix and propel the chyme
Circular : Contraction causes a decrease in diameter of the
lumen of the GI tract
Longitudinal : Contraction causes shortening of a segment
of the GI tract
4. Serosa: outer most protective layer, consists of CT
• Secretes a slippery fluid that lubricates and prevents
friction
5

6. 6

7. Receptors of the GIT
• Mechano- ,osmo- and chemo receptors respond to stretch and
pH, presence of substrate, and end products of digestion
respectively
• They initiate reflexes that:
–Activate or inhibit digestive glands
–Mix lumen contents and move them along
7

8. Innervation of the GI tract
1. Extrinsic NS
• originate outside the digestive tract and innervate the various
digestive organs
= parasympathetic and sympathetic nervous systems
a. Parasympathetic nervous system
• Excitatory on the functions of the GI tract
• Promotes digestive & absorptive processes
8

9. b. Sympathetic nervous system
• Inhibitory on the functions of the GI tract
• Slows processes
2. Intrinsic NS
• Lie entirely within the digestive tract wall
=enteric nervous system
• Coordinates and relays information from the
parasympathetic and sympathetic nervous systems to the GI
tract
• Controls motility and secretion
• Shows local, intrinsic regulation
9

10. a. Myenteric plexus (Auerbach’s plexus)
• Primarily controls the motility of the GI smooth muscle
b. Submucosal plexus (Meissner’s plexus)
• Primarily controls secretion
• Types of Neurotransmitters: Several e.g., Ach, NE, E, NO
vasoactive intestinal peptide (VIP), etc.
10

11. GI Motility
A. Peristaltic (propulsive) movements:
• Consists of a progressive wave of strong contraction followed
by relaxation
• The main stimulus to cause peristalsis is distension (stretch)
of the gut wall.
Propel or push the contents forward through the digestive tract
• Transit of food through the esophagus is rapid
• In the small intestine—the main site of digestion and
absorption—the contents are moved forward slowly
allowing time for the breakdown and absorption of food
11

12. a. Mixing movements:
• Occurs due to local contractions taking place in SI segments
Mixing food with the digestive juices (promote digestion of
the food)
Facilitate absorption by exposing all parts of the intestinal
contents to the absorbing surfaces of the digestive tract
The law of the gut movement always occurs from mouth-
to- anus unless pathological
12

13. Digestion
• Two parts:
1. Mechanical digestion:
• Is the physical grinding of food to smaller units without
altering their chemical composition.
• Helps to expose and increase the surface area for enzymatic
attacks
2. Chemical digestion:
• Is hydrolytic breakdown of nutrients (CHO, fat, protein etc)
by different chemical reactions into simpler forms.
• This processes changes the original composition of the
nutrient.
13

14. The Mouth (oral cavity)
• The entrance to the digestive tract
• Parts of the oral cavity: cheeks, lip, tongue, hard and soft
palate, teeth
• Is responsible for mechanical digestion of solid food by
mastication
Functions of chewing are:
• To grind and break food up into smaller pieces
–Facilitate swallowing
–Increase the food surface area on which salivary enzymes
will act
• To mix food with saliva
• To stimulate the taste buds
14

15. Saliva
• Three major pairs of salivary glands
a. Parotid glands (25%)
Secrete mainly serous watery fluid rich in ptyalin
b. Submandibular glands (70%)
Produce both serous and mucous fluid
c. Sublingual glands (~5%)
Secrete mainly thick mucous with little serous fluid
• Components:
–H2O (99.5%)
–Electrolytes(principally Na+, Cl-, and HCO3
-), antibodies
(IgA) , lysozyme, amylase, thiocyanate, mucus..(0.5%)
–Total secretion = about 1-1.5 L/day
15

16. Saliva functions
a. Digestion: CHO-digestion begins in saliva . The enzyme
ptyalin breaks starch- to-maltose. Lingual lipase begins
fat digestion in the mouth.
b. Protection: has anti-microbial actions (contains IgA,
lysozyme & thiocyanate that kills microbes).
c. Involved in speech: Clear & fluent articulation is possible
in the presence of saliva.
d. Secretes HCO3
- : neutralize acids in food as well as acids
produced by bacteria in the mouth, the neutral pH is good
for ptyalin action.
e. Lubrication: Mucus found in saliva facilitates moistening
and swallowing of food.
16

17. Control of Saliva Secretions
A. Parasympathetic fibers: causes copious secretion of saliva
(Cholinergic).
B. Sympathetic fibers: Causes small and insignificant secretion
which is viscous (Adrenergic).
Salivary reflex
• Sight, smell, and test or thinking of food  Receptors in oral
cavity or smell  Sensory fibers from the tongue to the
salivatory nuclei in MO  Parasympathetic fibers (act on
salivary glands)  increase salivary secretion.
Salivation can also be controlled by higher centers like
hypothalamus which has nerve connections with salivatory
nuclei in the Medulla oblongata (MO)
Higher centers like appetite area in the hypothalamus are also
involved in reflex control.
17

18. Phases of Salivary secretions
3-phases of salivary secretions include
1. Cephalic (brain) phase: triggered by thought, smell, or sight
of food
2. Oral phase: triggered by food that stimulate touch & test
receptors in the mouth
3. Gastric phase: triggered by substances which stimulate the
gastric mucosa (acids or sour tastes) in the stomach.
18

19. Swallowing reflex (Deglutition)
• Swallowing is an act of orderly propulsion of food from the
mouth to the stomach.
3-phases of swallowing reflex
1. Voluntary phase in the Oral cavity:
- the tongue rolls and pushes the chyme against the hard and
soft palate in the oral cavity and pushes it to the pharynx.
2. Pharyngeal phase (involuntary): Reflexes whose nuclei are
located in swallowing center of the MO, stimulate the closure
of the nasal and tracheal openings and cause inhibition of
respiration.
19

20. Steps:
a. The soft palate is pulled upward and closes the nasopharynx.
b. The vocal cords are pulled together and narrow
c. The epiglottis moves upward and covers the larynx
d. This so called “primary peristalsis” in the pharynx starts and
pushes the bolus downward through the upper esophageal
sphincter
20

21. 3. Esophageal phase (involuntary):
• Upper esophageal sphincter closes and the bolus is pushed
down to the stomach.
• If the peristaltic contraction is not adequate to push the bolus,
then distention of the esophageal walls cause the so called
“secondary peristalsis” that is modified by enteric nerves and
strong enough to push the bolus to the stomach.
Secretion of the esophagus
Mucous glands that surround the esophagus secrete mucus that
provide lubrication and protection from gastric digestion (HCl)
that may arise from reflux of stomach contents.
21

22. Stomach
22

23. The stomach, functions
Storage (1-3 L)
Mixing (soupy chyme)
Secretion of intrinsic factor and other enzymes
Secretes enzymes that begin protein digestion
Has antiseptic actions (HCl) etc
23

24. Production of gastric juices in stomach
Cells Secretions
1. Parietal cells HCl + Intrinsic factor
2. Chief cells Pepsinogen (stimulated by HCl)
3. Mucous cells Mucous
4. G-cells Gastrin (at pyloric antrum)
5. D-cells Somatostatin
6. H-cells Histamine
24

25. • When pepsinogen is first secreted, it has no digestive activity
However, as soon as it comes in contact with HCl, it is
activated to form active pepsin
• Pepsin functions as an active proteolytic enzyme in a highly
acid medium (optimum pH 1.8 to 3.5), but above a pH of
about 5 it has almost no proteolytic activity and becomes
completely inactivated in a short time.
• HCl is as necessary as pepsin for protein digestion in the
stomach
25

26. • The substance intrinsic factor, essential for absorption of
vitamin B12 in the ileum, is secreted by the parietal cells
along with the secretion of HCl
• Enterochromaffin - like cells (ECL cells), secrete histamine.
• Gastrin is secreted by gastrin cells, also called G cells.
26

27. Phases of Gastric Secretion
• Gastric secretion is occur in three phases:
- a cephalic phase, a gastric phase, and an intestinal phase.
Cephalic Phase
• Occurs in feedforward fashion (before food reaches the
stomach), especially while it is being eaten.
• It results from the sight, smell, thought, taste of food,
chewing, or swallowing and the greater the appetite, the
more intense is the stimulation.
27

28. • This phase of secretion normally accounts for about 20% of the
gastric secretion associated with eating a meal.
28

29. Gastric Phase
29
The gastric phase of secretion accounts for about 70% of the
total gastric secretion associated with eating a meal
Begins when food reaches the stomach

30. Intestinal Phase
• The intestinal phase has two components that influence gastric
secretion:
• Excitatory
• Inhibitory
Excitatory component
• Occurs in response to the presence of products of protein
digestion in the duodenum
• Intestinal gastrin travels in the blood to the stomach, where it
enhances the secretion of HCl and pepsinogen
• It accounts only for approximately 10% of the acid secretory
response to a meal
30

31. Inhibitory component
• Has a very strong influence on gastric secretion
31

32. Pancreas
Major function of the pancreas
1. Involved in digestion processes by producing:
a. Digestive enzymes: necessary to digest
CHO, fat, and protein
b. Bicarbonates : to neutralize the gastric juice
c. Water and electrolytes (Na+, K+ etc):
Location of secretion
1. Digestive enzymes and electrolytes are secreted by acinar
cells of the pancreases
2. HCO3
- and water are secreted by the duct cells lining the
pancreatic ducts.
32

33. 33

34. • The inactive proenzymes secreted by cells of the pancreas that
break protein, fat, and CHO include:
• Trypsinogen (protein digestion)
• Chymotrypsinogen (protein digestion)
• Procarboxypolypeptidase (protein digestion)
• Pancreatic lipase (fat digestion)
• Alpha amylase (CHO digestion)
34

35. • All the inactive enzymes (to prevent from digesting the
proteins of the cells in which they are formed) flow through the
pancreatic duct into the duodenum.
• At the beginning, an enzyme called enterokinase
(=enteropeptidase) that is located on the wall of the
duodenum changes trypsinogen to trypsin.
• Trypsin then activates the others as follows (look steps b and
c):
a. Trypsinogen ----enterokinase  Trypsin
b. Chymotrypsinogen----trypsin  Chymotrypsin
c. Procarboxypolypaptidase ---trypsin Carboxypolypeptidase
35

36. • The most important of the pancreatic enzymes for digesting
proteins are trypsin, chymotrypsin, and carboxypolypeptidase.
• By far the most abundant of these is trypsin.
• Trypsin and chymotrypsin split whole and partially digested
proteins into peptides of various sizes but do not cause release
of individual amino acids.
• However, carboxypolypeptidase does split some peptides into
individual amino acids
36

37. • The pancreatic enzyme for digesting carbohydrates is
pancreatic amylase, which hydrolyzes starches, glycogen, and
most other carbohydrates to form mostly disaccharide maltose
• The main enzyme for fat digestion is:
Pancreatic lipase hydrolyzes dietary triglycerides into
monoglycerides and free fatty acids, which are the absorbable
units of fat
37

38. 38
Hormonal control of pancreatic exocrine secretion

39. Phases of Pancreatic Secretion
• Pancreatic secretion occurs in three phases, the same as for
gastric secretion: the cephalic phase, the gastric phase, and the
intestinal phase
Cephalic and Gastric Phases
• The same nervous signals from the brain that cause secretion in
the stomach also cause acetylcholine release by the vagus
nerve endings in the pancreas
• This causes moderate amounts of enzymes to be secreted in the
pancreatic acini
39

40. • Cephalic phase: approximately 20% of pancreatic secretory
response to a meal
• Gastric phase: 5 to 10% of pancreatic secretory response to a
meal
Intestinal phase
• Most pancreatic secretion takes place
• Secretin(intestinal hormone) stimulates release of a large
volume of pancreatic juice with a high [HCO3-]
• Secretin is released in response to acidic chyme in the
duodenum (maximal release at pH ~ 3.0)
40

41. • Cholecystokinin(intestinal hormone) stimulates the release of
digestive enzymes from the pancreas
• It is released in response to the presence of the products of
protein and lipid digestion
41

42. Secretion of Bile by the Liver
• One of the many functions of the liver is to secrete bile,
normally between 600 and 1000 ml/day.
• Bile serves two important functions:
1. Emulsification of fat in the duodenum
=changing greater fat globules into smaller fat-droplets called
micelles
2. Means for excretion of several important waste products from
the blood.
These include especially bilirubin, an end product of
hemoglobin destruction, and excesses of cholesterol
42

43. • After bile is produced in the liver it will be stored in the gall
bladder
• It is transported toward the small intestine by the hepatic duct
(from the liver) and the cystic duct (from the gallbladder),
which join to form the common bile duct
• The common bile duct and the pancreatic duct join to form the
hepatopancreatic ampulla, which empties into the duodenum
• The entrance to the duodenum is surrounded by the Sphincter
of Oddi
• It is closed between meals and relaxes in response to the
intestinal hormone cholecystokinin
43

44. 44

45. 45

46. Constituents of bile
• Bile contains the following constituents:
1. Bile salts (bile acids), ~11%
2. Bile pigment (bilirubin), ~1%
3. Others organic constituents like: ( ~3%)
Cholesterol, Lecithin, protein etc
4. Electrolytes (Na+, K+, Ca2+, Cl-, and greater HC03-
than plasma) ~1%.
5. H2O (~ 84%) also takes the higher share of bile
46

47. Bile pigments (Bilirubin):
• Does not play a role in digestion at all but instead is a waste
product excreted in the bile
• Is the primary bile pigment derived from the breakdown of
worn-out red blood cells
• The end product from degradation of the heme (iron
containing) part of the hemoglobin
• In the intestine, bacterial actions change bilirubin into
urobilinogen. They are further oxidized and excreted in feces
or urine as stercobilin and urobilin.
• Gives faces its brown color
• Excess level of bilirubin in the blood (>18 mg/L) causes
jaundice, that is, yellow coloration of the sclera's, skin etc
occurs.
47

48. • Gallbladder (GB) is a storage sac for bile.
• GB is stimulated by CCK to contract and release bile.
• Bile emulsifies fat in the duodenum.
• Cholesterol and other substances precipitate in the GB and this
effect favors gall-stone formation.
• 75% of gallstones are derived from cholesterol, other 25%
from bilirubin abnormal precipitation
48

49. The Small intestine (SI)
• SI is specialized for completion of digestion and absorption
of nutrients.
a. Duodenum: ~25 cm
It receives chyme from the stomach and digestive secretions
from the pancreas and liver and neutralize its acids
b. Jejunum : ~1.5m
Bulk of chemical digestion and nutrient absorption occurs
c. Ileum : ~1.7m
Mainly absorptive
Ileocecal valve: prevent backflow of fecal contents from the
colon into the SI
Ileocecal sphincter: slows emptying of ileal contents into the
cecum 49

50. 50

51. • The mucosa of SI is well adapted for digestion and absorption
with certain anatomical modifications:
• Plicae circulares (circular folds)
• Villi
• Microvilli
Plicae circulares (circular folds)
• Form internal rings around the circumference of the SI
• Are formed from inward foldings of the mucosal and
submucosal layers of the intestinal wall
• Well developed in the duodenum and jejunum
• Increase the absorptive surface area of the mucosa about
threefold
51

52. 52

53. Villi
• Millions of smaller projections of mucosa that cover
plica
• Two types of epithelial cells cover the villi:
–Goblet cells (produce mucus)
–Absorptive cells (most abundant)
• Taken together, the villi increase the absorptive surface
area another 10-fold
53

54. 54

55. Microvilli
• Microscopic projections found on the luminal surface of the
absorptive cells - form the brush border ( contains
enterokinase, disaccharidases (maltase, sucrase,and lactase),
aminopeptidases)
• Increase the surface area for absorption another 20-fold
• Together, these three anatomical adaptations of the intestinal
mucosa — plicae circulares, villi, and microvilli — increase
the surface area as much as 600-fold
55

56. Motility of the small intestine
• Segmentation and peristalsis take place in the small intestine
• Segmentation: ring-like contractions along the length of the
SI
• Mixes chyme with digestive juices and exposes it to the
intestinal mucosa for absorption
• Causes only a small degree of forward movement of the
chyme along the small intestine
56

57. • Peristalsis:
• The wave-like form of muscle contraction
• Primarily moves chyme along the intestine and causes only
a small amount of mixing
• Weak and slow in the small intestine so that time is
sufficient for complete digestion and absorption of the
chyme
57

58. • The motility of the SI may be enhanced during a meal by:
• Distension of the small intestine
–Distension of the duodenum elicit segmentation
contractions in this segment
• Gastrin - causes segmentation of the empty ileum
• Extrinsic nerve stimulation
–Parasympathetic stimulation, by way of the vagus
nerve, further enhances segmentation.
–Sympathetic stimulation inhibits this activity
58

59. Digestion and absorption in the small intestine
Carbohydrates
• Starch is initially acted upon by amylase
• Salivary amylase breaks down starch molecules in the mouth
and stomach.
• Pancreatic amylase carries on this activity in the small intestine
• Amylase fragments polysaccharides into disaccharides (maltose,
composed of two glucose molecules)
• The disaccharide molecules, primarily maltose, are presented to
the brush border of the absorptive cells.
• As the disaccharides (maltose, sucrose & lactose) are absorbed,
disaccharidases (maltase, sucrase, and lactase) split these
nutrient molecules into monosaccharides (glucose, fructose, and
galactose)
59

60. • Glucose and galactose enter the absorptive cells by way of
secondary active transport
• Fructose enters the absorptive cells by way of facilitated
diffusion.
• All monosaccharide molecules exit the absorptive cells by
way of facilitated diffusion and enter the blood capillaries
60

61. 61

62. 62

63. Proteins
• Protein digestion begins in the stomach by the action of the
gastric enzyme pepsin.
–This enzyme fragments large protein molecules into smaller
peptide chains.
• Digestion is continued in the small intestine by the pancreatic
enzymes trypsin, chymotrypsin, and carboxypeptidase,
which hydrolyze the peptide chains into amino acids,
dipeptides, and tripeptides
• Amino acids enter the absorptive cells by way of secondary
active transport
63

64. • As the nutrient molecules are absorbed, aminopeptidases split
dipeptides and tripeptides into their constituent amino acids
• The amino acid molecules then exit the absorptive cells by way
of facilitated diffusion and enter the blood capillaries
64

65. 65
(a) Protein digestion

66. 66
(b) Protein absorption

67. Lipids
• Fat digestion begins in the mouth and stomach by the action of
the salivary enzyme lingual lipase
• However, the role of this enzyme is minor
• Lipids are digested primarily in the small intestine
1. Bile salts cause emulsification, which is the dispersal of large
fat droplets into a suspension of smaller droplets
2. Pancreatic lipase acts on the lipid droplets to hydrolyze the
triglyceride molecules into monoglycerides and free fatty
acids- these are water insoluble
3. Micelles formed by the amphipathic bile salts
• Monoglycerides and free fatty acids are carried in this interior
region of the micelle
67

68. • Upon reaching the brush border of the absorptive cells, they
leave the micelles and enter the cells by simple diffusion
–This process takes place primarily in the jejunum and
proximal ileum
• The bile salts are absorbed in the distal ileum by way of
secondary active transport
• Within the absorptive cells:
• Monoglycerides + FFAS triglycerides (in endoplasmic
reticulum)
• Triglycerides packaged in a lipoprotein coat (in Golgi
apparatus)
• These protein-coated lipid globules, referred to as
chylomicrons (water soluble)
68

69. • Chylomicrons leave the absorptive cell by way of exocytosis
• They enter the lacteals, which are part of the lymphatic system
B/c they unable to cross the basement membrane of the
blood capillaries
69

70. 70
(a) Fat digestion

71. 71

72. Water and electrolytes
• The absorption of nutrient molecules primarily takes place in
the duodenum and jejunum, creates an osmotic gradient for
the passive absorption of water
• Sodium absorption:
• Passive diffusion (through “leaky” tight junctions)
• Na+– Cl- cotransport
• Na+–glucose cotransport
• Na+–amino acid cotransport
• Na+- K+ - 2 Cl–cotransport
72

73. Large intestine
• From the ileocecal valve (juncture between the ileum
• and the large intestine) to the anus
• Has a larger diameter than the small intestine
• Mucosa:
• Composed of absorptive cells and mucus-secreting goblet cells
• Does not form villi
• Consists of the following structures:
- Cecum - Colon
- Appendix - Rectum
73

74. • Cecum:
• The most proximal portion of the large intestine
• Receives chyme from the ileum of the SI through the
ileocecal valve
• Appendix:
• a small projection at the bottom of the cecum
• is a lymphoid tissue. This tissue contains lymphocytes and
assists in defense against bacteria that enter the body through
the digestive system
74

75. • Colon:
• Largest portion of the large intestine
• Consists of four regions:
• Ascending colon (travels upward toward the diaphragm on
the right side of the abdomen)
• Transverse colon (crosses the abdomen under the diaphragm)
• Descending colon (travels downward through the abdomen
on the left side)
• Sigmoid colon (S-shaped region found in the lower abdomen)
75

76. 76

77. • Rectum:
• Last portion of the digestive tract
• Leads to the external surface of the body through the anus
• Internal and external anal sphincters
• The large intestine typically receives 500 to 1500 ml of chyme
per day from the small intestine
• Chyme consists of indigestible food residues (e.g., cellulose),
unabsorbed biliary components, and any remaining fluid (b/c
most digestion and absorption in SI).
• Therefore, the two major functions of the large intestine are:
• Drying
• Storage
77

78. • The colon extracts more H2O and salt from the contents. What
remains to be eliminated is known as feces
• The primary function of the large intestine is to store feces
before defecation
Motility of the large intestine
• Normally sluggish
• Two types
–Mixing movements (Haustrations)
• Ring-like contractions (about 2.5 cm) of the circular muscle
divide the colon into pockets called haustra
• Haustrations = bulging of the large intestine into baglike
sacs as a result of circular and longitudinal muscle
contraction
• Serve primarily to move the contents slowly back and
forth, exposing them to the absorptive surface
78

79. –Propulsive movements (Mass movements)
• Propel chyme from the cecum to the sigmoid colon
• When a mass of feces is forced into the rectum, there is a
desire to defecate
Reflexes Affecting Mass Movements
• Gastrocolic reflex – stimulatory
–Distention of the stomach
• Duodenocolic reflex - stimulatory
–Distention of the small intestine
• Both push the colonic contents into the rectum, triggering the
defecation reflex
79

80. • Both reflexes transmitted by autonomic nervous system
• Defecation
• When mass movement forces feces into the rectrum
–Immediate desire to defecate
• Reflex contraction of rectrum
• Relaxation of anal sphincter
• Approx. 80 to 200 mL of fecal matter expelled daily
80

81. 81

82. Defecation Reflex
1. Distension of the rectum.
2. Stimulation of the stretch receptors in the rectum.
3. A. Short reflex: Stimulation of myenteric plexus in sigmoid
colon and rectum.
B. Long reflex: stimulation of parasympathetic motor neurons
in sacral spinal cord.
C. stimulation of somatic motor neurons.
4. Increased local peristalsis, relaxation of internal anal
sphincter and contraction of external anal sphincter.
82

83. 83

84. • From the spinal cord, defecation signals also have the
following effects
• Taking deep breath
• Closure of glottis
• Contraction of abdominal wall muscles
• Relaxation and movement of pelvis floor downward
84

85. Secretion of the large intestine
• Large intestine does not secrete any digestive enzymes
• Colonic secretion consists of an alkaline (NaHCO3) mucus
solution
• The mucus provides lubrication to facilitate passage of the
feces
• The NaHCO3 neutralizes irritating acids produced by local
bacterial fermentation
• No digestion takes place within the large intestine because there
are no digestive enzymes. However, the colonic bacteria do
digest some of the cellulose for their use
85

86. Composition of Feces
• Three-fourths water
• One-fourth solid matter
–30% dead bacteria
–10-20% fat
–10-20% inorganic matter
–2-3% protein
–30% undigested roughage (e.g. bile pigment, sloughed
epithelial cells)
86

87. Reading assignment
Function of Gastrointestinal Hormones
• Gastrin
• Secretin
• Cholecystokinin (CCK)
• Gastric inhibitory peptide(GIP)
87