1. Digestive System CHAPTER 18 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.

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
2- Secretion
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.
3- Mascularis.
- Responsible for the motility of the GIT,
- has inner circular and outer longitudinal
smooth muscle,
- also contains nerve plexuses.
4- Serosa.
- Outermost layer serves supportive and
protective functions.

5. Innervation of the G. I. T. 1- Autonomic nerve system (ANS) supply.
a) Sympathetic
b) Parasympathetic
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.

6. Interaction of ANS with ENS

7. 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).
Upper Esophageal
Sphincter

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

10. 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).
Bolus
Direction of
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.

11. Structure and Function of the Stomach
divided into:
1- Fundus
2- Body
3- Antrum
Storage
Mixing &
Emptying
Fuction:
1- Storage of food
2- Mixing of food
3- Emptying of food into small intestine
Retropulsion

12. Control of Intragastric Pressure
Empty Stomach
Food Entering Stomach
Distension R R
Laplace law:
Pressure =
2 x Surface Tension
Radius
- To keep the intragastric pressure constant, after food enters the
stomach the peristalsis is inhibited for about 1 hour.

13. Structure and Function of the Stomach
Gastric glands have several types of cells that secrete different materials which
collectively are called gastric juice:
EXOCRINE
1- Gobblet cells- secret mucus. 3- Chief cells – sceret enzyme (pepsinogen)
2- Parietal (oxyntic) cells – sceret HCl 4- G cells – secret hormone gastrin
5- Enterochromaffin-like (ECL) cells –secret histamine.
6- D Cells – secret somatostatin
ENDOCRINE
PARACRINE

14. 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).

15. Activation of Pepsin

16. Small Intestine

17. 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.

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

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

20. 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.

21. Regulation of Intestinal Function
The GI has its own nervous system (enteric nervous system), it can sense and
form reflex actions due to the presence of food.
In the intestine the enteric plexus is very extensive and this system helped by the autonomic
nervous system are involved in several reflexes:
1- Gastroilial reflex =
increase gastric activity causes increased motility of ileum
and movement of chyme (food mixed with digestive juice)
through the ileocecal valve.
2- Ileogastric reflex =
when ileum is distended this decreases the motility and emptying
of the stomach.
3- Intestino-intestinal reflex =
over-distension of one part of the intestine causes reflex relaxation
of the rest of the intestine.

22. 5-Secretion of Bile LIVER LIVER Small intestine
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.
* Bile secretion is the principal digestive function of the liver.
Bile contains:
Bile salts
Urobilinogen
Cholestrol
Lecithin
Bilirubin
Electrolytes
Bicarbonate
5-Secretion of Bile
LIVER
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
function
Metabolic
function
Islets of Langerhans
Secret insulin and glucagon
Acini: secret pancreatic juice

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

27. Regulation of the Digestive System
The activities of the G.I. Tract is regulated by:
Neural mechanisms:
Automomic nervous system
Sympathetic
Parasympathetic
Enteric nervous system
myenteric
Submucosal
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
From stomach