This document provides an overview of the physiology of the digestive system. It describes the main functions and organization of the gastrointestinal tract, including ingestion, digestion, absorption and excretion. It details the layers of the digestive tract wall and discusses the roles of the various glands involved in digestion. Furthermore, it examines the motility, secretions and functions of different parts of the GI tract such as the mouth, esophagus, stomach, pancreas, liver, gallbladder, small intestine and large intestine.

1. PHYSIOLOGY OF
DIGESTIVE SYSTEM
Dr. BIKILA R.(MD)

2. Functions of the GIT
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 Ingestion
 Motility: mixing and propulsion
 Secretion
 Digestion
 Absorption
 Excretion

3. Organization of the GIT
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 The alimentary canal: mouth, pharynx,
esophagus, stomach, small intestine, large
intestine, rectum and anal canal.
 Associated glands: salivary glands,
pancreas, liver and gall bladder.

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5. Layers of digestive tract
wall
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 Mucosa
 Submucosa
 Muscularis externa
 Serosa

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7. Mucosa
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 This layer is concerned with secretion of both
the digestive juices and certain hormones as
well as the absorption of the various nutrients.
 It contains blood capillaries, lymph vessels
and a layer of smooth muscle called the
muscularis mucosa.

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 Submucosa: This is a dense connective
tissue layer that contains larger blood and
lymph vessels as well as a network of
neurons called submucous or Meissner’s
plexus.
 Muscularis externa: an outer longitudinal
layer and inner circular layer of smooth
muscle. In between myenteric or Aurbach’s
plexus.
 Serosa: an outer fibrous coating

9. Control of GI functions
(Nervous and Hormonal)
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)
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10. Hormonal control of GIT function
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1. Gastrin (from the stomach): stimulates
gastric motility and secretion.
2. CCK (small intestine) stimulates pancreatic
enzyme secretion, and gall bladder
contraction. Inhibits gastric emptying.
3. Secretin (from small intestines): stimulates
pancreatic and biliary bicarbonate secretion.

11. Types of movements in the
GIT
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 Propulsive: move bolus forward.
 Segmentation: mixing in the small intestine.

12. The Salivary glands
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 Parotid glands
 Submandibular or submaxillary glands
 Sublingual glands

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14. Composition of Saliva
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 Mostly water 99.5%
 Ionic content: low in Na+ and Cl-, high in K+
and HCO3
-
 Enzymes: lingual lipase and -amylase.
 Mucins
 Lysozymes: proteolytic enzyme
 Immunoglobulin A (IgA)
 Digestive functions, Lubrication, Protection

15. Esophagus
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 Function:
 swallowing or deglutition, which is the process
of moving food from the mouth through the
esophagus into the stomach
 No digestion or absorption
 Secretions: mucus

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17. Stomach
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18. Stomach
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Functions of the stomach:
 Motor:
 Storage
 Mixing
 Emptying
 Secretory:
 Exocrine (HCL, mucus, intrinsic factor, pepsinogen)
 Endocrine (Gastrin)
 Digestive (proteins and lipids)
 Absorption: water, ions, alcohol and aspirin

19. Gastric secretion
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 3 L/day, highly acidic (pH 1-2).
 Composition and function:
 Mucus (from mucus cells): protection
 Enzymes (from chief cells): pepsin, lipase:
digestion.
 Intrinsic factor & HCL (from parietal cells):
Intrinsic factor is important for vit B12
absorption from small intestine.

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Functions of HCL
 Kills ingested bacteria.
 Aids protein digestion (activates pepsinogen
into pepsin).
 Provides the optimum pH for pepsin action.
 Stimulates the secretion of hormones that
promote the flow of bile and pancreatic
juice.

21. Exocrine Pancreas
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22. Composition and function of
pancreatic secretion
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 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.

23. Regulation of pancreatic
secretion
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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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The biliary system
• 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)
• 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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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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Gall Bladder
• No digestive role.
• Stores bile.
• Concentrates bile.
• Empties during meals.
• Secretes mucus.

30. Small intestine
Structure
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 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.

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32. Small intestine
Function
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 Digestion : Enzymes are intracellular
(Disaccharidases and aminopeptidases).
 Absorption: completed in s. intestine, large
surface area.
(Site where most digestion and absorption
take place).
 Secretion
 Motility

33. Exocrine intestinal
secretions
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 Intestinal juice: alkaline fluid (water and
electrolytes) with few enzymes (1000-1500
ml/day).
 HCO3
-: neutralize acid.
 Mucus: protection and lubrication.
 No digestive enzymes.

34. Intestinal motility
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 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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37. Large Intestine
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38. Large intestine
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 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.

39. THANK YOU!