Intestinal anatomy and physiology

1. 1
Dr. MohanadDr. Mohanad

2. I- Introduction to GIT Physiology
Learning Objectives
 Describe the general functions of the GIT.
 Describe the general organization of the
GIT.
 Describe the common layers of the
digestive tract wall.
 Describe the nervous and hormonal
regulation of GI function.
 Describe the different movements of the
GIT.
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3. Functions of the GIT
 Ingestion
 Motility: mixing and propulsion
 Secretion
 Digestion
 Absorption
 Excretion
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4. Control of GI functions
(secretions and motility)
 Nervous
 Hormonal
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5. Nervous supply to the Gut
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Extrinsic
(autonomic)
 Sympathetic (decreases
motor and secretory
activity, contraction of
sphincters)
 Parasympathetic
(stimulatory)
Intrinsic
 Submucous or Meissner’s
plexus (controls secretory
function)
 myenteric or Auerbach’s
plexus (controls motor
activity)

6. Hormonal control of GIT function
1. Gastrin (from the stomach): stimulates gastric
motility and secretion.
2. CCK (from small intestines) stimulates pancreatic
enzyme secretion, and gall bladder contraction.
Inhibits gastric emptying.
3. Secretin (from small intestines): stimulates
pancreatic and biliary bicarbonate secretion.
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7. Types of movements in the GIT
 Propulsive: move bolus forward.
 Segmentation: mixing in the small
intestine.
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8. Small & Large Intestines
Learning Objectives
 Describe the intestinal juice (volume, pH,
composition, function and its regulation).
 Name the different digestive enzymes and
their substrates.
 Describe the reabsorptive processes of
different substances.
 Describe the different intestinal movements
(motility).
 Describe the different functions of the
colon.
 Describe the defecation reflex and its
regulation.
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9. Small intestine
Structure
 Long tube (about 6m) extending from pyloric
sphincter in stomach and joins large intestine at
the ileocecal sphincter
 duodenum (20 cm), jejunum (2.5 m) and ileum
(3.5 m).
 Surface area greatly increased by intestinal
mucosal foldings, villi and microvilli.
 Structure of a villus (see figure).
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10. SMALL INTESTINE
The small
intestine is
divided
 duodenum
jejunum
ileum.

11. THE DUODENUM
 The duodenum is the
first section of the small
intestine and has a
thicker layer of tissue
than the other areas of
the small intestine.
 It neutralizes stomach
acids and breaks down
carbohydrates and fats.
The duodenum is about
2 feet long.

12. JEJUNUM
 The jejunum is the main
section of the small
intestine. It covers about
15 feet and is responsible
for the absorption of
almost all nutrients except
water.

13. ILEUM
 The ileum is the last
section of the small
intestine and spans
about 6 feet. Its
function is to absorb
water and vitamins.

15. PLICAE CIRCULARES
 (valves of Kerkering) are
macroscopically visible, crescent-
shaped folds of the mucosa and
submucosa.
 permanent structures, i.e. their
presence does not depend on the
state of distension of the small intestine.
 are absent from the first few
centimetres of the duodenum and the
distal part of the ileum.
 well developed in the jejunum.
 increase the surface area of the
mucosa

16. MUCOSA

18. INTESTINAL VILLI
The entire intestinal
mucosa forms intestinal
villi (about one mm long),
which increase the
surface area by a factor
of ten. The surface of the
villi is formed by a simple
columnar epithelium.

20. MICROVILLI

21. INTESTINAL GLANDS

22. INTESTINAL CELLS

23. ENTEROCYTES

24. GOBLET CELL

25. GOBLET CELLS
 The apical end of each
goblet cell is occupied
by a large mass of
mucus, which
compresses adjacent
cells.
 The nucleus toward the
basal end of the cell.
 Attached by junctional
complexes (evidenced
in light microscopy as
the "terminal bar") to
adjacent absorptive
cells .

26. PANETH CELLS
 Paneth cells are
secretory epithelial cells
located at the ends of
intestinal crypts. The
function for these cells is
secretion of anti-
bacterial proteins into
the crypt lumen, thereby
providing protection for
the stem cells which line
the crypt walls.

27. PANETH CELLS
 Paneth cells have
typical serous-
secretory appearance,
with basophilic basal
cytoplasm (containing
protein-synthetic rough
endoplasmic
reticulum) and apical
secretory vesicles
(zymogen granules).

28. ENTEROENDOCRINE CELLS

29. ENTEROENDOCRINE CELLS
Concentrated in
lower portion of
intestinal gland
Produce a lot of
peptide hormones

30. COMPARISON

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35. Small intestine
Function
 Digestion : Enzymes are intracellular
(Disaccharidases and aminopeptidases).
 Absorption: completed in s. intestine, large
surface area (as large as a tennis court area).
(Site where most digestion and absorption take
place).
 Secretion
 Motility
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36. Digestion
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41. Absorption
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44. Exocrine intestinal secretions
 Intestinal juice: alkaline fluid (water and
electrolytes) with few enzymes from
desquamated cells (1000-1500 ml/day).
 HCO3
-
: neutralize acid.
 Mucus: protection and lubrication.
 No digestive enzymes.
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45. Regulation of intestinal
secretion
Local: mechanical stimulation of intestinal
mucosa by the presence of chyme.
Nervous: vagal nerve stimulation causes
secretion of intestinal glands.
Hormonal: ? secretin
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46. Intestinal motility
 Segmentation: ring-like contractions. Mix
chyme and expose it to absorptive
surface.
 Peristaltic: a wave of contraction
preceded by relaxation. Propels the
gastrointestinal contents toward the large
intestine.
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49. Large Intestine
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51. Large intestine
 No digestive function.
 Absorption of water, Na+
and other minerals.
 Secretion of mucus for lubrication.
 Storage of feces (undigested food).
 Bacteria in colon synthesize vitamin K and a
number of B complex vitamins.
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52. Large Intestine:
 Extends from ileocecal junction to anus
 Consists of cecum, colon, rectum, anal canal
 Movements sluggish (18-24 hours)

53. Large Intestine
 Cecum
 Blind sac, vermiform appendix attached
 Colon
 Ascending, transverse, descending, sigmoid
 Rectum
 Straight muscular tube
 Anal canal
 Internal anal sphincter (smooth muscle)
 External anal sphincter (skeletal muscle)
 Hemorrhoids: Vein enlargement or inflammation

54. Secretions of Large Intestine
 Mucus provides protection
 Parasympathetic stimulation increases rate
of goblet cell secretion
 Pumps
 Exchange of bicarbonate ions for chloride
ions
 Exchange of sodium ions for hydrogen ions
 Bacterial actions produce gases called
flatus

55. Histology of Large Intestine

56. Movement in Large Intestine
 Mass movements
 Common after meals
 Local reflexes in enteric plexus
 Gastrocolic: Initiated by stomach
 Duodenocolic: Initiated by duodenum
 Defecation reflex
 Distension of the rectal wall by feces
 Defecation
 Usually accompanied by voluntary movements to
expel feces through abdominal cavity pressure caused
by inspiration

57. Water and Ions:
Water
 Can move in either
direction across wall of
small intestine
depending on osmotic
gradients
Ions
 Sodium, potassium,
calcium, magnesium,
phosphate are actively
transported

58. Defecation reflex
 Stimulus: undigested material distending
rectum.
 Receptor: stretch receptor in rectum.
 Afferent: sensory to sacral segment of spinal
cord
 Centre: sacral segment of spinal cord
 Efferent: parasympathetic nerves to smooth
muscle of rectum and internal anal
sphincter.
 External anal sphincter is under voluntary
control.
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59. Defecation reflex
 Causes the internal anal sphincter to relax
and the rectum and sigmoid colon to
contract.
 If external anal sphincter is relaxed
defecation occurs.
 Defecation can be inhibited by voluntary
tightening of external anal sphincter.
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year Lab & Nutrition
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61. Exocrine Pancreas
Learning Objectives
 Describe the composition and function of
pancreatic secretions.
 Describe the hormonal and nervous
regulation of pancreatic secretion.
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63. Composition and function of
pancreatic secretion
 1.5 L/day, alkaline (pH 8)
 Composition and function:
 Digestive enzymes: peptidases (trypsin and
chymotrypsin), lipases, and amylase.
 Water
 HCO3
-
:
 neutralize the gastric acid
 Provides optimum medium for action of
pancreatic enzymes.
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64. Regulation of pancreatic
secretion
Mainly hormonal:
Secretin: acid chyme in duodenum
stimulates secretions rich in water and
HCO3
-
but poor in enzymes
CCK: digestive products of proteins and
fat in duodenum stimulates secretions of
pancreatic juice rich in enzymes.
Both hormones are secreted by upper intestinal
cells.
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Nervous control of pancreatic
secretion
 Parasympathetic impulses along vagus nerves
stimulate secretion of pancreatic enzymes.

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IV- The Biliary System
Learning Objectives
• List the different functions of the liver.
• Describe the composition of bile and its functions.
• Describe the functions of bile salts.
• Describe the different functions of the gall
bladder.
• Describe the different mechanisms that regulate
bile secretion & gall bladder emptying.
• Describe the types of jaundice according to the
cause.

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The biliary system includes:
• The liver
• The gall bladder
• Associated ducts:
– Hepatic ducts (right, left and common)
– Cystic duct
– Common bile duct

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Functions of liver
• Synthesis & Secretion of bile.
• Metabolic processes (e.g. gluconeogenesis,
glycogenolysis).
• Detoxification and degradation (e.g. drugs and hormones).
• Synthesis of plasma proteins (e.g. albumin and clotting
factors).
• Storage (e.g. iron and Vit B12).
• Activation of vitamin D.
• Removal of bacteria and old RBC.
• Excretion of cholesterol and bilirubin.

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Bile secretion
• Bile is secreted by hepatocytes.
• About 500 ml (250-1200ml) is secreted per
day.
• pH 8
• Enters duodenum during digestion of meals.
• Stored in gall bladder and concentrated
between meals.

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Composition of human bile (gm/dl)
Hepatic Gall bladder
Water 98% 89%
Bile salts 1.1 6
Bilirubin 0.04 0.3
Cholesterol 0.1 0.3-0.9
Fatty acids 0.12 0.3-1.2
Lecithin 0.04 0.3

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Electrolyte content of bile (mmol/L)
Hepatic Gall bladder
Na+
145 130
K+
5 12
Ca2+
5 23
Cl-
100 25
HCO3
-
28 10
pH 8.3 7.3

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Functions of bile
• Bile salts play an important role in fat
digestion and absorption.
• Excretion of waste products (e.g. bilirubin).
• Bicarbonate in bile neutralizes acid in
duodenum.

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Bile salts
• Most important component of the bile.
• They are Na+
and K +
salts of bile acids.
• They are derivatives of cholesterol.
• Recycled through the enterohepatic
circulation.

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Function of bile salts
• Emulsify large fat particles into smaller
ones that can be attacked by lipase
(detergent action).
• Help in the transport and absorption of fat
(micellar formation).

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• Prevent precipitation of cholesterol by
keeping them in solution (prevent gall
stones).
• Stimulate bile secretion by liver cells.

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Gall Bladder
• No digestive role.
• Stores bile.
• Concentrates bile.
• Empties during meals.
• Secretes mucus.

82. 82Regulation of bile secretion and gall
bladder emptying
• Chemical:
– Bile salts: most important stimulant of bile
secretion by liver cells.
• Hormonal:
– Secretin: secreted in response to acid chyme,
causes secretion of bile rich in water and HCO3
-
– CCK: secreted in response to fatty acids in
duodenum, causes gall bladder to contract and
sphincter of Oddi to relax.

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year Lab & Nutrition
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84. Neural
 Vagal stimulation:
Increases bile secretion
Weak contraction of gall bladder
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Jaundice
• Jaundice: yellowish discoloration of the
skin, sclera & mucous membranes due to a
↑ blood bilirubin level.
Causes:
• Pre-hepatic: due to excess production of
bilirubin e.g. haemolytic anaemia.
• Hepatic: liver disease e.g. hepatitis
• Post-hepatic: obstruction to bile flow e.g.
gall stones.

86. Thank you
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