The document outlines the key structures and functions of the digestive system, including the roles of the esophagus, stomach, small intestine, liver, gallbladder and pancreas. It describes the processes of digestion and absorption that occur along the gastrointestinal tract. Regulatory mechanisms involving nerves and hormones that control digestive secretions and motility are also summarized.

1. CHAPTER 18
Digestive System
• Functions of the digestive system.
• Structure and innervation of the digestive system.
• Swallowing, peristalsis, Esophagus and LES.
• Structure and secretions of the stomach.
• Mechanism of HCl secretion, GI protection, & Ulcers.
• Intestine, liver, gallbladder, and pancreas.
• Regulation of digestive system.
• Digestion and absorption of major food groups.
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2. Gastrointestinal
Tract and
its Accessory
Organs

3. Functions of the Digestive System
• The major functions of digestive system are to digest food
and absorb the final products into blood.
• Question) How the digestive system achieves the above goal?
• Answer) by the following activities:
1- Motility = Movements (contractions)
2- Secretion = Exocrine + Endocrine
3- Digestion & absorption
• The digestive system also must get ride of the undigested waste materials
this is achieved through the process of defecation.

4. Layers of the G. I. T.
1- Mucosa.
- Direct contact with food,
- directly involved in secretion and absorption,
- contains lymph nodes, goblet cells, and other
secretory cells,
- muscularis mucosa, thin muscular layer
changes the surface area of mucosa.
2- Submucosa.
- Tissue below mucosa, which is supportive to
- mucosa, contains blood vessels, glands,
nerve plexus. Submucosal plexus
3- Mascularis.
- Responsible for the motility of the GIT,
- has inner circular and outer longitudinal
smooth muscle,
- also contains nerve plexuses.
Myenteric plexus
4- Serosa.
- Outermost layer serves supportive and
protective functions.

6. Innervation of the G. I. T.
1- Autonomic nerve system (ANS) supply.
a) Sympathetic Decrease GI activity
b) Parasympathetic Increase GI activity
2- Enteric nervous system (ENS).
“intrinsic nervous system of the gut”
a) Myenteric plexus.
Its stimulation causes increase in motor activity
(motility) of the gut.
b) Submucosal plexus.
Mainly controls secretion and blood flow of the GIT,
also serves many sensory functions.

7. Interaction of ANS with ENS

8. Swallowing
- Swallowing reflex is initiated when pressure receptors in walls of pharynx are
stimulated by food or drink.
- Swallowing is co-ordinated by the swallowing center in brain stem (medulla oblongata).
During swallowing both
nasopharynx and epiglottis
close to prevent food moving
Upper Esophageal into the nose and trachea,
Sphincter respectively.

9. Swallowing
The food passes down the esophagus
by peristalsis and enters into the
stomach via the Lower Esophageal
Sphincter (L.E.S.)
LES

11. Peristalsis
- Aboral propulsion of contractions at a velocity of 0.5 to 2 cm per sec. (much faster
in the proximal compared to distal part of intestine).
-Peristalsis waves are weak and die out after about 10 cm therefore giving a slow
rate of movement of chyme (1 cm/min).
Mouth Anus
Direction of
Bolus Aboral propulsion
propulsion
Circular contract
Longitudinal relax Circular relax
Longitudinal contract
Presence of bolus causes local distention small intestine, and this activates
myenteric plexus between circular and longitudinal muscle layers of intestine.

12. Structure and Function of the Stomach
Fundus
Structure:
divided into: Body
1- Fundus
2- Body Storage
3- Antrum
Mixing &
Emptying
Fuction:
1- Storage of food Retropulsion
2- Mixing of food
3- Emptying of food into small intestine Antrum
The process of mixing food in the stomach is called “retropulsion”

13. Control of Intragastric Pressure
Empty Stomach Distension
R Food Entering Stomach
R
Laplace law:
Both surface tension and radius
2 x Surface Tension increase proportionally therefore
Pressure =
Radius the pressure stays constant
- To keep the intragastric pressure constant, after food enters the
stomach the peristalsis is inhibited for about 1 hour.

14. Structure and Function of the Stomach
Gastric glands have several types of cells that secrete different materials which
collectively are called gastric juice:
1-Mucus = lubrication & protection
1- Gobblet cells- secret mucus.
2- HCl = Help digestion
2- Parietal (oxyntic) cells – sceret HCl
3- Chief cells – sceret enzyme (pepsinogen) 3- Pepsiongen = Digestion
4- G cells – secret hormone gastrin 4- Gastrin = Stimulate HCl secretion
5- D Cells – secret somatostatin 5- Somatostatin = Inhibit gastrin secretion
6- Enterochromaffin-like (ECL) cells
6- Histamine = Stimulate HCl secretion
–secret histamine.

15. How the Mucosa of GI is
Protected against Digestion
Acid (HCl) and enzyme (pepsin) help in digestion of food substances. By this action
the food is broken into smaller molecules which can be absorbed into blood.
The stomach protects itself from digestion by:
1- The chief cells of the gastric mucosa are extremely impermeable to HCl.
2- A layer of alkaline (HCO3-) mucus covers the mucosa.
3- The cells of mucosa are tightly fused to each other so that the HCl does not
leak to the submucosa.
4- Rapid rate of cell division in the mucosa (entire epithelium is replaced
every 3 days).
5- The enzyme pepsin is secreted in inactive form (pepsinogen).

16. Activation of Pepsin
Inactive enzyme
Active enzyme

17. Small Intestine
Both Pilace circularis
and villi increase
surface area

18. A Villus
Intestine folds pilacae circularis
and on these folds there are villi
which have a layer of epithelial
cells which their membrane is
also has folds called microvilli.
These arrangement increase the
Surface area of the small intestine.
Microvilli

19. Intestinal Motility
Types of contractions in small intestine:
1- Segmentation: (mixing movement)

20. Intestinal Motility
2- Peristalsis: (propulsive movement)
Direction of
Bolus propulsion
Circular contract
Longitudinal relax
Circular relax
Longitudinal contract

21. Structure and Motility of the Colon
1- Mixing movements:
Haustrations = occur in cecum, entire colon, and rectum.
Are characterized by contractions of
inner circular muscles.
2- Propulsive movement:
Mass peristalsis = strong peristalsis
which directed aborally, occur
several times a day in cecum and
entire colon.
Reverse peristalsis = orally directed
which gives more time to the fecal
matter to be expose of water
absorption.

22. LIVER
- Is the largest internal organ in the body.
- Has many functions:
1- Detoxification of blood
2- Regulating metabolism
3- Protein synthesis
4- Storage site for vitamins and iron
5-Secretion of Bile
5- Secretion of bile.
* Bile secretion is the principal digestive function of the liver.
- Bile contains:
- Bile salts
LIVER
- Urobilinogen
- Cholestrol
- Lecithin
- Bilirubin
- Electrolytes
- Bicarbonate
Small intestine

23. LIVER
Enterohepatic Circulation
Entero = intestine
Hepatic = liver

24. Gallbladder
The bile which is secreted by the liver is
stored and concentrated in gallbladder.
When bile is needed for digestion, the
walls of gallbladder contract and this
ejects the bile through the bile duct
into duodenum.

25. Pancreas
Weighs about 100g, and produces about 1L of juice/day.
Secretions are both endocrine and exocrine.
Digestive
Metabolic function
function
Islets of Langerhans Acini: secret pancreatic juice
Secret insulin and glucagon

26. Pacreatic Juice
• Pancreatic juice contains:
– Water and electrolytes (Na+, K+, Cl+, Ca++)
– Bicarbonate (HCO3-)
– Digestive enzymes: Sodium Bicarbonate
For neutralization of acid
• Amylase
• Trypsin
• Lipase
• Ribonuclease Enzymes for digestion
• Deoxyribonuclease
• Etc.
of food

27. Regulation of the Digestive System
• The activities of the G.I. Tract is regulated by:
– Neural mechanisms:
• Automomic nervous system
– Sympathetic
Inhibit activity of GI tract
– Parasympathetic Stimulate activity of GI tract
• Enteric nervous system
– myenteric Control contractions
– Submucosal
Control Secretions & Blood flow
– Action of hormones (endocrine):
• Hormones secreted from the glands located in the G.I. Tract act on effector
cells of the G.I. Itself. Examples of such hormones are: secretin, gastrin, CCK
(cholecystokinin)
From small intestine stomach
From

28. Regulation of Gastric Secretion
• Contractions and secretions of the stomach to some extent are automatic. For
example: the presence of food causes distension in the stomach wall and this
will lead to contractions. Presence of some food materials in the stomach
stimulate its glands to secret gastric juice.
• However, when ANS or hormones are active their effects dominate the
function of the stomach.
• There are 3 phases in the control of gastric secretion:
1- Cephalic phase: sight, smell, taste, even thoughts of food can initiate this phase.
2- Gastric phase: when food enters the stomach this phase becomes active.
3- Intestinal phase: when food enters small intestine this phase becomes active.

29. Cephalic Phase
Sight
Smell
Sound Cortex
Thoughts
Taste
Vagal Appetite Hypothalamus
Touch
Mouth Temp. Nuclei center
Etc.
Pancreas
Small intestine
Vagus
Parietal cells HCl
Chief cells Enzyme
ACh
Mucus cells Mucus
G cells Gastrin
Gastric Gland
- Cephalic phase is abolished by vagotomy.

30. Gastric Phase
Vagal
Nuclei
Vago-Vagal
Pancreas Vagus
Small intestine
Food entering stomach
ACh Distension, protein
ACh digestion products
Local
Parietal cells HCl
Chief cells Enzyme
Enteric
Mucus cells Mucus
G cells Gastrin Plexus
Gastric Gland Blocker of ACh
- Gastric secretion is abolished after administration of atropine

31. STRONG Intestinal Phase
1- Inhibition: At low duodenal pH and presence of food
Food entering small intestine Gastric
gland
Decrease acid
Somatostatin
Secretin
S cell Secretion
CCK
Secretin is release in response
Blood
to acid and CCK is released
in response Fat.
WEAK
2- Stimulation: Occurs when duodenal pH is relatively high.
Gastric
gland
Food entering small intestine
G cell
Increased acid
Secretion
Gastrin
Blood