fcg

1. Chapter Ten
Gastrointestinal physiology
1

2. General chapter outline
• Introduction
• Functional anatomy of GIT
• General functions of the digestive system
• Innervation of GIT
• Hormonal Control of GI Function
• Mouth, pharynx and esophagus
• Obesity and metabolic rate
• Stomach and gastric secretion
• Liver, gall bladder and pancreas
• Digestion and absorption
• Small intestine
• Large intestine
2

3. Introduction
• Food is required by the body for the production of energy and
for the growth and repair of tissues
• Each day an average adult consumes around 1 kg of solid food
and 1- 2 liters of fluid
• Majority of this material is in a form that cannot be used
immediately by the body for cellular metabolism
• It must be broken down into simple molecules which can
be absorbed into the bloodstream for distribution to the
tissues.
 The gastrointestinal system performs this task
3

4. Gastrointestinal system/ Digestive System
• The digestive system consisting of the digestive tract and
accessory organs
• Alimentary canal or gastrointestinal tract (GIT):
• A muscular tube extending from the mouth to the anus
• It measures about 10 m in adult cadaver
• It is significantly shorter in a living individual (why?)
• 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, exocrine pancreas and
biliary system (liver and gallbladder)
• Assist in the breakdown of food
4

5. 5
The Components of the digestive System

6. 6
The Components of the digestive System

7. Functions of gastrointestinal tract
• Ingestion: taking materials into the digestive tract
• Motility: the muscular contractions that mix and move
forward (propulsion) the contents within the tract
• Digestion: the breakdown of ingested food into smaller
components
– Mechanical digestion: ingested material  smaller pieces (without
changing its chemical structure)
– Chemical digestion: complex molecules  smaller
molecules(absorbable units) 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
by epithelium of digestive tract and glandular organs
• Defecation: elimination of indigestible solid wastes
7

8. GIT and its natural defense
• Its lumen is continuous with external environment (mouth -
anus) - extension of the external world
• Therefore, can possibly harbor microorganisms in its luminal
surfaces.
• However, the GI-system can protect itself from such hazards by
some defense mechanisms found in:
a. Mouth: Saliva contains lysozymes, IgA etc
b. Stomach: HCl, pepsin etc. Have bactericidal effect
c. SI:
• Immuno-competent cells (Peyer's patches) attack microbes
in the SI wall
• Macrophages: developed from monocytes destroy microbes
by phagocytosis
8

9. Layers of GIT and their function
• From esophagus to the anal canal the walls of the GIT have
four tunics
• From the lumen outward they are:
1. Mucosa - consists of:
• Epithelium (mucous membrane)
• Lamina propria
• Muscularis mucosa
2. Submucosa
3. Muscularis externa - consists of:
• Circular muscle layer -inner
• Longitudinal muscle layer - outer
4. Serosa / adventitia:
9

10. 1. Mucosa
• Innermost layer (faces lumen) or in contact with GIT content
• Moist epithelial layer
• Its major functions are:
• Secretion: mucus, enzymes and hormones
• Absorption: end products of digestion
• Consists of 3 sublayers: epithelium, lamina propria, and
muscularis mucosa
10

11. Epithelium:
• In contact with the GIT content
• Contains:
a. Exocrine gland cells: secretion of digestive juices
b. Endocrine gland cells: secretion of blood-borne GI hormones
c. Epithelial cells:
 Simple columnar epithelium (in stomach, small intestine, large
intestine): allows for secretion and absorption
• Goblet cells:
–Protect digestive organs from self digesting
–Ease the passage of food along the tract
 Stratified squamous epithelium (in mouth, surface of tongue, pharynx
and esophagus): withstand abrasion
11

12. Lamina propria:
• Thin middle layer
• Houses the gut associated lymphoid tissue (e.g. Peyers patches
produce B-lymphocytes), which is important in defense
against disease-causing intestinal bacteria
• Nourishes the epithelium and contains capillaries for
absorption of nutrients
12

13. Muscularis mucosa:
• Outermost mucosal layer of smooth muscle
Note:
• In the small intestine (the main site of digestion and
absorption), the mucosal surface is highly folded  greatly
increase the surface area  maximize nutrient, water, and
electrolyte absorption
• The esophagus exhibits little mucosal folding because it
functions primarily as a transit tube
• The pattern of surface folding can be modified by contraction
of the muscularis mucosa
• Importance: exposing different areas of the absorptive
surface to the luminal contents
13

14. 2. Submucosa
• Thick layer of connective tissue
• Provides the digestive tract with its distensibility and elasticity
• Contains :
• Blood vessels, lymphatic vessels
• Glands
• Submucosal nerve plexus (Meissner’s plexus) at its juncture
with the circular muscle layer
14

15. 3. Muscularis Externa
• The major smooth muscle coat of the digestive tube, surrounds
the submucosa
• Consists of two layers: an inner circular layer and an outer
longitudinal layer
• Inner circular layer :
• Contractions narrow the diameter of lumen
• Thicker and has more gap junction than the longitudinal
• More powerful in exerting contractile forces on the contents
of GIT
15

16. • Outer longitudinal layer:
• Contractions shorten a particular segment of GIT
• Another nerve network, the myenteric plexus/Auerbach
plexus, lies between the two muscle layers
• Together, contractile activity of these smooth muscle layers
produces the propulsive and mixing movements
• The submucosal and myenteric plexuses help regulate local gut
activity
16

17. 17
Motility

18. 4. Serosa
• Outer connective tissue covering of the digestive tract
• Helps in the attachment of gut to the surrounding structures
• Secretes a watery, slippery fluid (serous fluid) that lubricates
and prevents friction between the digestive organs and the
surrounding viscera
• It is continuous with the mesentery
• Mesentery: suspends the digestive organs from the inner wall
of the abdominal cavity
18

19. 19

20. GI Smooth Muscles & Their Electrical Activity
• The GI smooth muscle acts as a functional syncytium
Q. What is a functional syncytium?
• Muscle fibers arranged in bundles or sheets
• Cells are electrically linked by gap junctions
• An electrical signal (AP) in one cell spreads rapidly through
the entire sheet of digestive smooth muscle which then
contract as a single
• Such a group of interconnected muscle cells that function
electrically and mechanically as a unit is known as a functional
syncytium (Excited and contracting as a single unit)
20

21. Electrical activity of gastrointestinal smooth muscle
• Two basic types of excitation:
1. Slow waves (basic electrical rhythm (BER))
• Are slow, undulating changes in the RMP (between −50 and
−60 mV)
• Generated by pacemaker cells known as the interstitial cells of
Cajal
– Located throughout the layers of the muscularis externa
• Are not action potentials
• Are unique to GI muscle
• Intensity usually varies between 5 and 15 mv
• Frequency ranges in different parts of the GIT from 3 to 12/min
21

22. • Do not cause calcium ions to enter the smooth muscle fiber
(they only cause entry of sodium ions)
• Therefore, they do not cause muscle contraction (Exception: may
be in the stomach?)
22
Electrical slow-wave frequencies:
• Stomach : 3 waves/ min
• SI (Duodenum): 12 waves/min
• SI (Jejunum): 9–11 waves/min
• SI (Ileum): 8 – 10 waves /min

23. 2. Spike potentials
• Are true action potentials
• Occur when the RMP of the GI smooth muscle becomes more
positive than about −40 millivolts
• The channels responsible for the action potentials are
a. Calcium-sodium channels
– Allow large numbers of calcium ions to enter along with smaller
numbers of sodium ion
– Are much slower to open and close
• Cause significant quantities of Ca2+ to enter the fibers
• Accounts for the long duration of the action potentials
23

24. • The resultant Ca2+ entry has two effects:
1. It is responsible for the rising phase of an AP
– Falling phase : brought about as usual by K+ efflux
2. It triggers a contractile response
• The greater the number of action APs, the higher the cytosolic
Ca2+ concentration, the greater the cross-bridge activity, and the
stronger the contraction
24

25. 25
Membrane potential in intestinal smooth muscle cells

26. Types of the GI smooth muscle contraction:
A. Phasic contractions
• Periodic contractions followed by relaxation evoked by action
potentials
• Frequency/number of APs grade the degree and duration of
contraction
• Dominant in esophagus, gastric antrum, and intestines
B. Tonic contractions
• Sustained contraction without relaxation for prolonged periods
of time
• Often increases or decreases in intensity but continues.
26

27. • In lower esophageal sphincter, orad region of the stomach,
ileocecal and internal anal sphincter
• Not associated with slow waves
• It is sometimes caused by
– Continuous repetitive spike potentials
– Hormones
– Continuous entry of calcium ions into the interior of the cell
27

28. Innervation of the GI tract
1. Extrinsic NS : ANS
• Originates outside the DT and innervate the various digestive
organs
a. Parasympathetic nervous system
• Excitatory on the functions of the GI tract
• Increase smooth muscle motility and promote secretion of
digestive enzymes and hormones
• Promotes digestive & absorptive processes
28

29. • The parasympathetic supply to the gut is divided into cranial
and sacral divisions (Origin: cranio-sacral)
29
, pancreas,
gall bladder
• Site of ganglion cells:
Myenteric plexus & meissner's
plexus
(CN X)
(S2-S4)
Note: Both make synaptic connections
with intramural plexuses

30. 30
The digestive tract is innervated by the parasympathetic division of
the autonomic nervous system

31. • Functions of parasympathetic fibers (cholinergic, Ach):
–Smooth muscle contraction
–Relaxation of sphincters
–Stimulation of gut motility and secretion
• The vagus is mixed nerve in which approximately 80% of the
fibers are afferent
• Receptors in the mucosa and smooth muscle relay information
back to MO via these afferents which, in turn, initiates vagal
reflex signals that return to the GIT via efferent in the vagus
nerve
• Reflexes of this type are called vagovagal reflexes
31

32. b. Sympathetic nervous system
• Is usually inhibitory on the functions of the GI tract
• Slows digestive & absorptive processes
• Origin: thoracolumbar region (T-5 - L-2)
• The preganglionic fibers pass through the lateral sympathetic
chain; they continue as splanchnic nerves and relay in
sympathetic ganglion (prevertebral ganglia in the abdomen:
celiac and mesenteric ganglia)
• Postganglionic fibers end on the intrinsic nervous system
• Sympathetic division supplies all areas of GIT except oral
cavity and anus (truly parasympathetic)
32

33. • Mechanism of action: Norepinephrine (NE) released by
sympathetic nerve endings inhibits smooth muscle and neurons
of intrinsic nervous system
• Functions:
–Smooth muscle relaxation
–Inhibition of gut motility
–Contraction of sphincters
–Reduce intestinal blood flow
33

34. 34
The digestive tract is innervated by the sympathetic division
of the autonomic nervous system

35. 2. Intrinsic NS: Enteric nervous system (Little Brain )
• Lie entirely within the digestive tract wall and run its entire
length (from esophagus to the anus)
• The digestive tract has its own intramural (“within-wall”)
nervous system (=Enteric nervous system )
• The number of neurons is >100 million
• Coordinates and relays information from the parasympathetic
and sympathetic nervous systems to the GI tract
• Is especially important in controlling GI movements and
secretion, even in the absence of extrinsic innervation
• Composed mainly of two plexuses
35

36. 1. Myenteric plexus (Auerbach’s plexus)
–It is concerned with control of peristaltic activity of the GIT
• Principal effects:
–Increased tone
–Increased intensity of contractions
–Increased rate of contractions
–Increased velocity of conduction
• Partially controlled by autonomic nervous system
2. Submucosal plexus (Meissner’s plexus)
–Controls mainly gastrointestinal secretion and local BF
–Receives sensory information from chemoreceptors and
mechanoreceptors in the GI tract
• Partially controlled by autonomic nervous system
36

37. 37
Neural control of the gut wall

38. Neurotransmitters secreted by enteric neurons
• Different neurotransmitter substances that are released by the
nerve endings of different types of enteric neurons
• Common:
–Acetylcholine: Excites gastrointestinal activity
• Stimulates smooth muscle contraction
• Increases intestinal secretions
• Releases enteric hormones (e.g. gastrin, CCK, secretin )
• Dilates the blood vessels
–Norepinephrine & epinephrine: almost always inhibits
gastrointestinal activity
• Derived from extrinsic sympathetic neurons
• Inhibits the GI activity
• Causes vasoconstriction
38

39. – Nitric oxide and vasoactive intestinal peptide: inhibitory
• Other types:
– ATP, serotonin, dopamine, CCK, substance P, somatostatin, leu-
enkephalin, met-enkephalin, gastrin releasing peptide (bombesin)
• Mixture of excitatory and inhibitory agents
39

40. Gastrointestinal Reflexes
• The anatomical arrangement of the ENS and its connections
with the ANS support three types of GI reflexes
• Are essential to GI control
1. Local reflexes :
• Reflexes integrated entirely within the gut wall ENS
• Control much gastrointestinal secretion, peristalsis, mixing
contractions, local inhibitory effects
2. Short reflexes:
• Reflexes from the gut to the prevertebral sympathetic
ganglia and then back to the GIT
• Signals from the stomach to cause evacuation of the colon
= Gastrocolic reflex
40

41. • Signals from the colon and SI to inhibit stomach motility and
stomach secretion
= Enterogastric reflexes
• Reflexes from the colon to inhibit emptying of ileal
contents into the colon
= Colonoileal reflex
3. Long reflexes:
• Reflexes from the gut to the spinal cord or brain stem
and then back to the GIT
• Include:
a. Reflexes from the stomach and duodenum to the brain stem
and back to the stomach by way of the vagus nerve
• Control gastric motor and secretory activity
41

42. b. Defecation reflexes that travel from the colon and rectum to
the spinal cord and back again to produce the powerful
colonic, rectal, and abdominal contractions
Defecation (the defecation reflexes)
42

43. 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
43

44. Mechano- ,osmo- and chemo receptors stimulation
elicit
1. Neural reflexes
• Local reflex
• Short reflex
• Long reflex
or
2. Secretion of hormones
Alter the activity level in the digestive system’s effector
cells
1. Smooth muscle cells: for modifying motility
2. Exocrine gland cells: for controlling secretion of
digestive juices
3. Endocrine gland cells: for varying secretion of GI
hormones
44

45. Hormonal Control of GI Function
• The GI hormones are released into the portal circulation and
exert physiological actions on target cells with specific
receptors for the hormone
1. Gastrin
• Secreted from “G” cells of the antrum of the stomach
• Stimuli:
– Distention of the stomach
– Products of proteins (small peptides)
– Gastrin-releasing peptide (released by the nerves of the gastric
mucosa during vagal stimulation)
• Actions :
– Stimulation of gastric acid secretion
– Stimulation of growth of the gastric mucosa
45

46. 2. Cholecystokinin (CCK)
• Secreted from “I” cells in the mucosa of the duodenum and
jejunum
• Stimuli :
– Fat, fatty acids, and monoglycerides in the intestinal contents
• Actions:
– Gallbladder contraction (expelling bile into the SI)
– Increases pancreatic digestive enzymes into the duodenum
– Moderate inhibition of stomach contractility (slows the emptying of food
from the stomach to give adequate time for digestion of the fats in the
upper intestinal tract)
– Inhibits appetite to prevent overeating during meals
46

47. 3. Secretin
• Secreted from the “S” cells in the mucosa of the duodenum
• Stimulus:
– Acidic chyme in the duodenum
• Action:
– Promote pancreatic secretion of bicarbonate (helps to neutralize the
acid in the SI)
4. Glucose-dependent insulinotropic peptide ( Gastric
inhibitory peptide [GIP])
• Secreted from “K” cells in the mucosa of the upper SI
• Stimuli :
– Fatty acids and amino acids
– Carbohydrate (lesser extent )
47

48. • Action:
– Decreases motor activity of the stomach(slows emptying
of gastric contents into the duodenum)
– Stimulates insulin secretion
5. Motilin
• Secreted by “M” cells in the stomach and upper duodenum
• Released cyclically
• Stimulus: Fasting
• Actions:
– Increase GI motility
– Stimulates interdigestive myoelectric complexes (move through the
stomach and SI every 90 minutes )
• Clear the stomach and SI of residual food, secretions, and
desquamated cells and propel them to the colon
48

49. . Paracrines
• Are released from cells in the GI mucosa
• Diffuse over short distances to act on target cells
1. Somatostatin:
• Is secreted by cells of the GI tract in response to H+
• Inhibits the release of all GI hormones
• Inhibits gastric H+ secretion
• Its secretion is inhibited by vagal stimulation
2. Histamine:
• Is secreted by mast cells of the gastric mucosa
• Increases gastric H+ secretion directly and by potentiating the
effects of gastrin and vagal stimulation
49

50. GIT-blood Supply
• At rest ~25% of the CO flows and reaches the abdominal cavity
(largest share)
• The combined circulation to the stomach, liver, pancreas,
intestine, and spleen is called the splanchnic circulation
Portal circulation:
• Venous blood (poor in o2, but rich in nutrients from the
intestine, spleen, pancreas etc.) join and form the portal vein
that drains into the liver
• Then blood is directed to liver sinusoids, hepatic vein and
lastly to inferior vena cava
• Potentially harmful agents that come with the blood are
destroyed by macrophages of the liver (reticuloendothelial
cells)
50

51. Schematic of the splanchnic circulation under fasting conditions
51

52. • Lymphatic drainage:
• Is important for the transport of lipid-soluble substances
that are absorbed across the GI tract wall
• Lipids and other lipid-soluble molecules (including some
vitamins and drugs) are packaged into particles that are too
large to pass into the capillaries and instead pass into
lymph vessels in the intestinal wall
• These lymph vessels drain into larger lymph ducts, which
finally drain into the thoracic duct
52

53. Factors that control blood flow to the GIT
1. Metabolism:
• Increase in metabolism during or after a meal increases
releases vasoactive substances that increases blood flow by 8-
fold
– Peptide hormones: cholecystokinin, vasoactive intestinal peptide, gastrin,
and secretin
– GI glands release kinins: kallidin and bradykinin
2. Decreased O2-tension:
• Increases blood flow
3. Neural:
• SNS: NE….. decreases blood flow
• ENS: VIP, NO……increase blood flow
53

54. GI Motility
1. Propulsive movements
• Cause food to move forward along the tract at an appropriate
rate to accommodate digestion and absorption
2. Mixing movements
• Keep the intestinal contents thoroughly mixed at all times
• Both are brought about by the ENS, but are influenced by the
extrinsic nervous system, especially the parasympathetic
nervous system
54

55. Propulsive Movements
• The basic propulsive movement of the GIT is peristalsis
• Peristalsis: ring-like contractions of the circular smooth muscle
that move progressively forward, pushing the bolus into a
relaxed area ahead of the contraction
• Stimulus:
– Distension of the gut
– Physical or chemical irritation of the surface epithelium
– Strong stimulation by the PNS
• Controlled by:
– Actual peristalsis is brought about by the myenteric plexus
– But it is influenced by the PNS
55

56. Mechanism of myenteric reflex or the peristaltic reflex
1. Distension of the gut
2. Stimulation of the ENS
3. The gut wall contracts 2-3 cm behind the point of stimulus 
formation of a contractile ring
4. Initiation of a wave of contraction  travels in the anal
direction (=directional movement)
5. Relaxation of the gut wall distal to the point of stimulation -
receptive relaxation (allowing the food to be propelled more
easily toward the anus)
• Therefore, peristalsis moves towards the anus
• The peristaltic reflex + the anal direction of movement of the
peristalsis is called the “law of the gut”
56

57. 57
Peristalsis
• 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

58. Mixing Movements
• Differ in most parts of the gut
–Segmentations- in small intestine
–Haustrations- in large intestine
58
Segmentation
• The contracted segments relax
and the previously relaxed areas
contract
• These oscillating contractions
thoroughly mix the chyme

59. • 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
59

60. 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.
60

61. The Mouth (oral cavity)
• The entrance to the digestive tract is oral cavity
• Parts of the oral cavity: cheeks, lip, tongue, hard and soft
palate, teeth
• Is responsible for mechanical digestion of solid food by
mastication (chewing)
• Most of the muscles of chewing are innervated by the
motor branch of the fifth cranial nerve (trigeminal)
• The chewing process is controlled by nuclei in the brain stem
61

62. Functions of chewing are:
• To grind and break food up into smaller pieces
–Facilitate swallowing
• Prevents excoriation of the GIT
–Increase the food surface area on which salivary enzymes
will act
• Rate of digestion is dependent on the total surface
area exposed to the digestive secretions
• To mix food with saliva
• To stimulate the taste buds (contain the taste receptor cells)
62

63. Saliva
• Three major pairs of salivary glands
a. A pair of parotid
• The largest, just below and in front of the ears
• Produce approximately 25% of saliva
b. A pair of submandibular
• At the posterior corners of mandible
• Produce approximately 75% of saliva
c. A pair of sublingual gland
• Below the tongue
• Produce approximately 5% of saliva
Buccal glands: in the cheeks near the front of the mouth
• Daily saliva production approximates 1 to 1.5 L
63

64. Secretion of ions in saliva
• Saliva contains
– K+ and HCO3
- (in large quantities )
– Na+ and Cl-(several times less than in plasma) Why???
• A typical compound gland (e.g. the submandibular gland) that
contains acini and salivary ducts
1. The acini secrete a primary secretion
– Contains ptyalin and/or mucin in a solution of ions
– Isotonic and has the same Na+, K+, Cl–, and HCO3
– concentrations as
plasma
2. The primary secretion flows through the ducts
– Two major active transport processes take place
– Modify the ionic composition of the fluid in the saliva
64

65. a. Na+ actively reabsorbed and K+ actively secreted in exchange
for the sodium
• [Na+] of the saliva becomes greatly reduced, whereas [K+ ] becomes
increased
• Excess sodium reabsorption compared with potassium
secretion
• Creates electrical negativity of about -70 mV in the salivary ducts
• Causes chloride ions to be reabsorbed passively
• [Cl- ] in the salivary fluid falls
b. Bicarbonate ions
• Secreted by the ductal epithelium into the lumen of the duct
• This secretion is at least partly caused by
• Passive exchange of bicarbonate for chloride ions
• An active secretory process
65

66. • At rest, final salivary secretion is hypotonic
• Reasons for hypotonicity of the saliva, includes:
1. Na+ and Cl- reabsorption from the lumen to the plasma is greater than K+
and HCO3- secretion into the lumen
2. The ducts are relatively impermeable to water (i.e. H2O does not follow
the osmotic gradient of NaCl)
66

67. • The parotid glands secrete almost entirely the serous type of
secretions
• The submandibular and sublingual glands secrete both serous
secretion and mucus
• Saliva has a pH between 6.0 and 7.0 (favorable range for the
digestive action of ptyalin)
• Components:
–H2O (99.5%)
–0.5%= Electrolytes: Na+, K+, HCO3
−, Ca++, Mg++, and Cl− +
Other organic substances include:
• Enzymes (amylase), lingual lipases, lysozymes, thiocyanate,
glycoproteins, IgA, lactoferrin, mucus, etc.)
67

68. 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, lactoferrin & 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 (helping prevent dental
caries), the neutral pH is good for ptyalin action (helping
prevent dental caries)
e. Lubrication: Mucus found in saliva facilitates moistening
and swallowing of food
68

69. e. It serves as a solvent for molecules that stimulate the taste
buds. Only molecules in solution can react with taste bud
receptors
f. It plays an important role in oral hygiene by helping keep
the mouth and teeth clean
69

70. 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.
70

71. 71
Salivary glands and control of salivary secretion

72. 72
Regulation of salivary secretion.

73. 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
Xerostomia:
• Diminished salivary secretion
• Difficulty in chewing and swallowing, inarticulate speech
73

74. 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:
• Initiates the swallowing process
• The tongue rolls and pushes the bolus against the hard and soft
palate in the oral cavity and into 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.
74

75. The pressure of the bolus in pharynx
stimulates pharyngeal pressure receptors
impulses via trigeminal (CN V) and
glossopharyngeal (CN IX) nerves
swallowing center (in the medulla and lower pons )
via 5th, 9th, 10th, 12th (hypoglossal (CN XII))
1. Activates in the appropriate sequence the muscles
involved in swallowing
2. Inhibits respiratory center in the brain stem
75

76. Steps:
a. The soft palate is pulled upward and closes the nasopharynx
b. The palatopharyngeal folds on each side of the pharynx are
pulled medially to approximate each other
• Impede too much any large object to pass into the esophagus
b. The vocal cords are pulled together and narrow the opening of
the larynx (glottis)
c. The epiglottis folds backward down over the closed glottis as
further protection
d. The pharyngeal muscles contract to force the bolus into the
esophagus
76

77. 3. Esophageal phase (involuntary):
Esophagus
• A fairly straight muscular tube that extends between the
pharynx and the stomach
• It is guarded at both ends by sphincters
• Pharyngoesophageal sphincter (Upper esophageal sphincter):
prevents large volumes of air from entering the esophagus
and stomach
• Gastroesophageal sphincter (Lower esophageal sphincter):
prevents reflux of gastric contents
• Musculature of pharyngeal wall and upper 1/3rd of the
esophagus is striated muscle
– Peristaltic waves controlled by skeletal nerve impulses from the
glossopharyngeal and vagus nerves
77

78. • Musculature of lower 2/3rd of the esophagus is smooth muscle
• Controlled by - vagus nerves
- myenteric nervous system
• It exhibits two types of peristaltic movements: primary
peristalsis and secondary peristalsis
• Primary peristalsis:
–Continuation of the peristaltic wave that begins in the
pharynx
–Triggered by swallowing center
• Secondary peristalsis :
–Initiated if the primary peristaltic wave fails to move all the
food into the stomach
–Result from distention of the esophagus itself by the retained
food
–Initiated or modified by intrinsic nerve plexus and strong
enough to push the bolus to the stomach
78

79. Secretion of the esophagus
• Mucous glands that surround the esophagus secrete mucus
• Lubricates passage of food (decreases esophageal damage by
any sharp edges on food)
• Protect the esophagus from damage by acid and enzymes in
gastric juice (if gastric reflux occur)
• Distension of the esophagus also reflexly increases salivary
secretion
79

80. Gastric Motility
• The function of the stomach is threefold:
1. Storage of large quantities of food
2. Mixing
• Involves food blending with digestive juices to form chyme
(a thick liquid mixture)
• The stomach can progressively extend to accommodate 0.8
to 1.5 L
3. Emptying (stomach  SI)
• At a rate appropriate for optimal digestion and absorption
80

81. • Anatomically stomach is divided in to 3 major parts
a. Fundus: lies above the esophageal opening
b. Body: middle or main part
c. Antrum: lower part of the stomach; has heavier musculature
• Physiologically,
1. Orad region:
• Fundus + the proximal body (first two thirds of the body)
• Responsible for accommodating an ingested meal (gastric reservoir )
2. Caudad region:
• Antrum + the distal body
• Responsible for the contractions that lead to mixing and propulsion
into the duodenum
81

82. 82
Physiological anatomy of the stomach
A B

83. • There are two types of gastric motility
1. Mixing movements
• Antrum contracts against closed pylorus
• Chyme is propelled forward and tossed back into the antrum
with each peristaltic contraction, a process called retropulsion
• Retropulsion (pyloric pump) serves to effectively mix food and
gastric secretions, and to grind gastric contents into chyme
2. Peristaltic movement
• Antrum contracts against open pylorus
• Push chyme from stomach into the SI
83

84. Gastric Secretion
1. Mucus-secreting cells:
• Line the entire surface of the stomach
• Secrete large quantities of a very thick & sticky mucus that coat
a gastric mucosa
– Provide a shell of protection for the stomach wall
– Contribute to lubrication of food transport
– It is alkaline – neutralize acid
2. Oxyntic glands (Gastric glands)
• In the body and fundus of the stomach
• Secrete HCl, pepsinogen, intrinsic factor, and mucus
84

85. • Composed of three types of cells:
i. Mucous cells
ii. Chief cells
iii. Parietal cells
3. Pyloric glands :
• Secrete:
–Mucus (mainly) for protection of the pyloric mucosa from
the stomach acid
–Gastrin from G cells
–Somatostatin from D cells
85

86. 86

87. 87
• Parietal cell: contains large branching intracellular canaliculi
• The hydrochloric acid is formed at the villus-like projections
inside these canaliculi

88. Basic mechanism of hydrochloric acid secretion
88

89. 1. H2O dissociated into H+ and OH− in parietal cell cytoplasm
2. The H+ is then actively secreted into the canaliculus in
exchange for K+ (catalyzed by H+-K+ ATPase)
3. OH- combine with CO2 (formed during metabolism in the cell
or while entering the cell from the blood) to form HCO3
-
4. The HCO3- is then transported across the basolateral
membrane into the ECF in exchange for Cl-
5. Cl- are secreted through chloride channels into the canaliculus
and combine with H+ to from HCl
6. Water passes into the canaliculus by osmosis
because of extra ions secreted into the canaliculus
89

90. Stimulation of gastric acid secretion
• Parietal cells bear receptors for 3 stimulators of acid secretion
– Acetylcholine (M3 muscarinic receptor)
– Gastrin (gastrin-cholecystokinin type 2 (CCK2)receptor)
– Histamine (H2 type receptors)
• ACh and gastrin both operate through IP3/Ca2+ second
messenger pathways
• Histamine activates a cAMP second messenger pathway to
bring about its effects
• These messengers all bring about increased secretion of HCl by
promoting the insertion of additional H+–K+ ATPases into the
parietal cells’ plasma membrane
90

91. 91
Receptors and signal transduction pathways in the parietal cell
PLC: Phospholipase C
PIP2: Phosphatidylinositol 4,5-
bisphosphate
IP3: Inositol 1,4,5-trisphosphate
DAGP: Diacylglycerol
PKC: Protein kinase C
AC: Adenylyl cyclase

92. • ACh and gastrin each bind to specific receptors (M3 and CCK2,
respectively) that are coupled to the G protein Gq
• The result is activation of PLC, which ultimately leads to the
activation of PKC and the release of Ca2+
• Ca2+ acts via calmodulin-dependent protein kinase(CaM kinase)
• The histamine binds to an H2 receptor, coupled through Gs to
adenylyl cyclase (AC)
• The result is production of cAMP and activation of PKA
• Two inhibitors of acid secretion also act directly on the parietal
cell
• Somatostatin and prostaglandins bind to separate receptors that are
linked to Gi
• These agents thus oppose the actions of histamine
92

93. • To keep from destroying itself, the stomach has mucosal
barrier
93

94. • 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
94

95. 95
• Once formed, pepsin acts on other pepsinogen molecules to
produce more pepsin, a mechanism called an autocatalytic
process (autocatalytic means “self-activating”)

96. Function of intrinsic factor
• Is secreted by the parietal cells along with the secretion of
HCl
• Essential for absorption of vitamin B12 in the ileum
• Binding of intrinsic factor with vitamin B12 triggers receptor mediated
endocytosis of this complex in the terminal ileum
• Destruction of parietal cells, e.g. chronic gastritis, gastrectomy…
↓
a. Pernicious anemia due to the failure of RBCs maturation
(absence of vitamin B12 stimulation of the bone marrow)
b. Achlorhydria (lack of stomach acid secretion) develop
96

97. 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
• Stimuli: sight, smell, thought, taste of food, chewing, or
swallowing
• The greater the appetite, the more intense is the stimulation
97

98. • This phase of secretion normally accounts for about 20% of the
gastric secretion associated with eating a meal.
98

99. Gastric Phase
99
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

100. 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
100

101. Inhibitory component
• Gastric secretion gradually decreases as food empties
from the stomach into the intestine by three ways.
• Inhibitory component has a very strong influence on
gastric secretion.
101

102. Pancreas
• An elongated gland that lies behind and below the stomach,
above the first loop of the duodenum
• This mixed gland contains both exocrine and endocrine tissue
Major exocrine function of the pancreas:
• Pancreatic exocrine secretion consists of an aqueous or
bicarbonate component and an enzymatic component
a. Aqueous component
• Consists primarily of water and sodium bicarbonate
– Neutralizes duodenal contents (prevents injury to the duodenal
mucosa)
– Bringing the contents within the pH range necessary for optimal
enzymatic digestion of nutrients
b. Enzymatic(protein) component
• Low volume secretion
• Contains enzymes for the digestion of all major foodstuffs
102

103. Location of secretion
• Exocrine tissue is organized into acini and ducts very similar to
that of the salivary glands
1. Digestive enzymes and electrolytes are secreted by acinar
cells of the pancreas
2. HCO3
- and water are secreted by the duct cells lining the
pancreatic ducts
103

104. 104

105. • 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)
105

106. Pancreatic proteases
• Are secreted as inactive zymogens (to prevent from digesting
the proteins of the cells in which they are formed)
• They include trypsinogen, chymotrypsinogen, and
procarboxypolypeptidase
• All the inactive enzymes flow through the pancreatic duct into
the duodenum
• At the beginning, an enzyme called enterokinase
[=enteropeptidase] (located on the wall of the duodenum)
changes trypsinogen to trypsin
• Trypsin then activates the others as follows (look steps b and
c):
106

107. a. Trypsinogen ----enterokinase  Trypsin
b. Chymotrypsinogen---trypsin  Chymotrypsin
b. Procarboxypolypaptidase ---trypsin Carboxypolypeptidase
• 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
107

108. Pancreatic amylase:
• Enzyme for carbohydrates digestion
• Hydrolyzes starches, glycogen, and most other carbohydrates
(except cellulose) to form mostly disaccharides and a few
trisaccharide
Fat digestion:
• Pancreatic lipase: hydrolyzes neutral fat into fatty acids and
monoglycerides
• Cholesterol esterase: causes hydrolysis of cholesterol esters
• Phospholipase : splits fatty acids from phospholipids
108

109. 109
Regulation of Pancreatic Secretion

110. 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
110

111. • 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)
111

112. • 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
112
Receptors of the pancreatic acinar cell and the regulation
of secretion

113. 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. Bile plays an important role in fat digestion and absorption
• Because bile acids in the bile perform two functions:
a. Emulsification of fat in the duodenum
=changing greater fat globules into smaller fat-droplets
b. Aid in absorption of the digested fat end products
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
113

114. • After bile is produced in the liver it will be stored in the gall
bladder
Liver Hepatocytes
↓
Bile canaliculi
↓
Bile ductules
↓
Bile ducts
↓
Hepatic ducts
↓
Common hepatic duct
↓ ↓
Cystic duct Common bile duct
↓ ↓
GB Hepatopancreatic ampulla
↓ Sphincter of Oddi
Duodenum
114

115. 115

116. 116
Liver secretion and gallbladder emptying

117. 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 HCO3-
than plasma) ~1%.
5. H2O (~ 84%) also takes the higher share of bile
117

118. 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
118

119. • 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
119

120. 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 120

121. 121

122. • 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
122

123. 123

124. 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
124

125. 125

126. Microvilli
• Microscopic projections found on the luminal surface of the
absorptive cells - form the brush border ( contains
enterokinase, disaccharidases (maltase, sucrase-isomaltase,
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
126

127. 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
127

128. • 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
128

129. • 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
129

130. 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-isomaltase, and lactase) split
these nutrient molecules into monosaccharides (glucose,
fructose, and galactose)
130

131. • The α-limit dextrins is only broken down by sucrase-
isomaltase
• 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
131

132. 132

133. 133

134. Lactose intolerance
• A deficiency of lactase, the disaccharidase specific for the
digestion of lactose, or milk sugar
• Most children younger than 4 years of age have adequate
lactase, but this may be gradually lost
• When lactose-rich milk or dairy products are consumed by a
person with lactase deficiency
1. Accumulation of undigested lactose creates an osmotic
gradient that draws H2O into the intestinal lumen
2. Bacteria attack the lactose as an energy source and produce
large quantities of CO2 and methane gas in the process
– Distension of the intestine by both fluid and gas produces pain
(cramping) and diarrhea
• Infants with lactose intolerance may also suffer from
malnutrition
134

135. 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
• The aminopeptidases, which hydrolyze most of the small
peptide fragments into their amino acid components, thereby
completing protein digestion
• Amino acids enter the absorptive cells by way of secondary
active transport
135

136. • 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
136

137. 137
(a) Protein digestion

138. 138
(b) Protein absorption
H/oligopeptide cotransporter

139. 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
139

140. • 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)
140

141. • 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
141

142. 142
(a) Fat digestion

143. 143

144. 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
144

145. Malabsorption
• Impairment of absorption
• One of the most common causes is gluten enteropathy, also
known as celiac disease
• Is a complex immunological disorder
• The person’s SI is abnormally sensitive to gluten, a protein
constituent of wheat, barley
• Exposure to gluten erroneously activates a T-cell response that
damages the intestinal villi
• Decreases the surface area available for absorption
• The condition is treated by not eating gluten
145

146. 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
146

147. • 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
• It contains lymphocytes and assists in defense against
bacteria that enter the body through the digestive system
147

148. Appendicitis
• Hardened fecal material lodged in the appendix  obstruct
normal circulation and mucus secretion
• The inflamed appendix often becomes swollen and filled with
pus, and the tissue may die as a result of local circulatory
interference
• If not surgically removed, the diseased appendix may rupture,
spewing its infectious contents into the abdominal cavity
148

149. • Colon:
• Largest portion of the large intestine
• Consists of four regions:
1. Ascending colon (travels upward toward the diaphragm on
the right side of the abdomen)
2. Transverse colon (crosses the abdomen under the
diaphragm)
3. Descending colon (travels downward through the abdomen
on the left side)
4. Sigmoid colon (S-shaped region found in the lower
abdomen)
149

150. 150

151. • 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
151

152. Secretion of the large intestine
• Large intestine does not secrete any digestive enzymes
• Colonic secretion consists of an alkaline (NaHCO3) mucus
solution
• The mucus has the following functions
1. It helps to lubricate feces
2. It neutralizes against any acids present
3. It protects against irritation
4. It provides a binding medium for fecal matter
152

153. Secretion of water & electrolytes
• Stimulus: irritation (e.g. bacterial infection)  the mucosa
secretes large amount of water & electrolytes in addition to the
alkaline mucus
• This dilute the irritating factors and causes rapid movement of
the feces toward the anus
153

154. Absorption in the colon
• Most of absorption in the colon occurs in the proximal half of
the colon (absorptive colon)
• The distal colon function for storage (storage colon)
1. Water absorption, about 0.5- 1.5L/day is absorbed by osmosis
2. Na+ absorption, about 60 mmol/day is actively absorbed in the
presence of Na+-K+ ATPase at the basolateral membrane to
blood
3. K+, Cl- and HCO3
-
– K+ may be absorbed or secreted depending on the remaining
concentration in the lumen
– Cl- is absorbed in exchange for HCO3
- which is secreted
154

155. 4. Folic acid and some AA and short chain FA resulting from
bacterial fermentation of CHO are absorbed
• Folic acid is required for the formation of nucleic acids, the
maturation of RBCs, and growth
5. Certain drugs as steroids and aspirin may be absorbed
• 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
155

156. Bacterial Action in the Colon
• Numerous bacteria, especially colon bacilli, are present even
normally in the absorbing colon
• They are capable of digesting small amounts of cellulose
–Can provide a few calories of extra nutrition for the body
• Other substances formed as a result of bacterial activity
–Vitamin K, vitamin B12, thiamine, riboflavin
–Gases that contribute to flatus – CO2, hydrogen gas, and
methane
–Vitamin K: Important because its amount in the daily
ingested foods is insufficient to maintain adequate blood
coagulation
156

157. Motility of the large intestine
• Normally sluggish
• Two types
a. 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
157

158. b. 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
158

159. • Both reflexes transmitted by autonomic nervous system
• Defecation
• When mass movement forces feces into the rectum
–Immediate desire to defecate
• Reflex contraction of rectum
• Relaxation of anal sphincter
• Approx. 80 to 200 mL of fecal matter expelled daily
159

160. 160

161. 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
161

162. 162

163. • 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
163

164. 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)
164

165. Reading assign.
• Obesity and metabolic rate
• Constipation
• Diarrhea
• Vomiting
165

166. THE END!!
166