Intestinal anatomy and physiology

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).
HCO
3
-
: 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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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
HCO
3
-
:
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
HCO
3
-
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 B
12
).
•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)
HepaticGall 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
Ca
2+
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 HCO
3
-
–CCK: secreted in response to fatty acids in
duodenum, causes gall bladder to contract and
sphincter of Oddi to relax.

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GIT physiology 2nd
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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