1. Lesson # 16 The Digestive System 2 Chapter 24 Objectives:
1- To describe the macroscopic and microscopic anatomy of the stomach. 2- To describe the function of the stomach.
3- To discuss the significance of the low pH in the stomach.
4- To discuss the regulation of the gastric activity.
5- To describe the macroscopic and microscopic anatomy of the intestines.
6- To describe the macroscopic and microscopic anatomy of the pancreas.

2. Functions of Stomach 1- Storage of ingested food.
2- To produce the chyme by:
Mechanical breakdown of ingested food.
Disruption of chemical bonds in food material by acid and enzymes.
3- Production of intrinsic factor, a glycoprotein required for absorption of vitamin B12 in small intestine.

3. Anatomy of the Stomach Body Greater curvature Fundus Cardia Serose
Muscularis Externa:
Lesser curvature
Longitudinal layer
Pylorus:
Body
Antrum
Circular layer
Pyloric canal
Oblique layer
Pyloric sphincter
It regulates the release of chyme to the duodenum.
Mucosa
Rugae
Duodenum
They let the gastric lumen expand.

4. Histology of the Stomach
Simple columnar epithelium
Mucosa
It secrets alkaline protective mucus
Gastric pit
Lamina propria
Muscularis mucosae
Submucosa
Muscularis externa
Oblique muscle
Circular muscle
Longitudinal muscle
Serosa (visceral peritoneum)

5. Gastric Glands Mucous cells Gastric pit Parietal cells Chief cells
They produce alkaline protective mucus.
Parietal cells
They produce HCl and Intrinsic Factor.
Gastric gland
Chief cells
They produce Pepsinogen
(In new born and infants produce rennin and gastric lipase).
G cells
They are enteroendocrine cells that produce Gastrin, Somatostatin, Histamine and Serotonin.

7. Functions of Hydrochloric Acid
Alkaline Tide:
It is the increase in the blood pH produced by the influx of bicarbonate ions from the interstitial fluid to the blood stream, during the formation of HCl acid in the stomach.
Functions of Hydrochloric Acid
1- The acidity of gastric juice kills most of the micro-organisms ingested with food
2- The acidity denatures proteins and inactivates most of the enzymes in food.
3- The acidity helps break down plant cell walls in food and the connective tissue in meat..
4- An acidic environment is essential for the activation and function of pepsin, a protein-digesting enzyme secreted by chief cells.

8. Partially digested proteins The Production and Action of Pepsin
HCl
Pepsinogen (zymogen)
Pepsin (active enzyme)
Removed peptide
Dietary proteins
Partially digested proteins
The Production and Action of Pepsin
Parietal cells
Chief cells
Zymogens: They are digestive enzymes secreted as inactive proteins. They are converted to active enzymes by removing some of their amino acids.
Pepsinogen: It is a zymogen secreted by the chief cells. Hydrochloric acid removes some of its amino acids and forms pepsin that digests proteins.
Pepsin digests dietary proteins into shorter peptide chains. Protein digestion is completed in the small intestine.

9. Regulation of Gastric Activity Parasympathetic division
The nervous and endocrine systems gastric activity.
Parasympathetic Division:
Sympathetic Division:
It increases secretions and motility in the digestive system.
It decreases secretions and motility in the digestive system.
The fibers of the parasympathetic division release acetylcholine (Ach). They are called cholinergic fibers.
The fibers of the sympathetic division release epinephrine (adrenaline). They are called adrenergic fibers.
Mucous cells
Chief cells
Parietal cells
G cells
Mucus
Pepsinogen
HCl
Gastrin
CNS
CNS
Parasympathetic division
Sympathetic division _ +
ACh
Adrenaline X
Mucous cells
Mucus
Chief cells
Pepsinogen
Parietal cells
HCl
Gastrin
G cells

10. Regulation of Gastric Activity
The nervous and endocrine systems collaborate to increase gastric secretion and motility when food is eaten and to suppress them when the stomach empties.
Gastric activity is divided into three phases:
Stimuli:
Sight, smell, taste, or thought of food
CNS
1- The Cephalic Phase: The stomach is being controlled by brain.
Vagus nerve
2- The Gastric Phase: The stomach is controlling itself.
Parasympathetic division +
ACh
3- The Intestinal Phase: The stomach is being controlled by the small intestine.
Mucous cells
Mucus
Chief cells
Pepsinogen
Parietal cells
HCl 1
Cephalic Phase
Gastrin
G cells
Vagus nerve stimulates gastric secretion even before food is swallowed.
The Cephalic Phase is directed by the CNS and prepares the stomach to receive food.

11. Sight, smell, taste, or thoughts of food
CEPHALIC PHASE
Food
Sight, smell, taste, or thoughts of food
Central nervous system
Vagus nerve (N X)
Submucosal plexus
Mucous cells
Mucus
Chief cells
Pepsinogen
Parietal cells
Gastrin
HCl
G cells
KEY
Stimulation 11

12. Parasympathetic division2
Gastric Phase
It begins with arrival of food in the stomach.
Stimuli
Ingested food produces:
Hypothalamus
Distention of the stomach.
Sensory fibers
An increase in the pH of the gastric content.
Motor fibers
Presence of undigestive materials in the stomach.
Vagus nerve
Responses:
Parasympathetic division
1- Neural Response +
ACh
Long reflex
Gastrin +
2- Hormonal Response
Gastrin – a hormone produced by the enteroendocrine G cells of the pyloric antrum. +
ACh pH
Long reflex
3- Local Response
Stretching activates short reflex mediated through myenteric nerve plexus.
Short reflex
Short reflex

13. GASTRIC PHASE Neural Response Stretch receptors
Submucosal and myenteric plexuses
Distension
Elevated pH
Chemoreceptors
via bloodstream
Mucous cells
Mucus
Chief cells
Pepsinogen
Mixing waves
Parietal cells
Gastrin
HCl
G cells
Partly digested peptides 13

14. - X Intestinal Phase Stimuli Responses: 3
It begins when chyme first enters the duodenum.
The function of the intestinal phase is controlling the rate of gastric emptying to ensure the secretory, digestive and absorptive functions of the small intestine.
Stimuli
Distention of the duodenum by the chyme.
Decrease in the pH of the duodenum by the chyme.
Medulla oblongata
Responses:
1- Neural Response
Vagus nerve
Stretch receptors and chemoreceptors in the duodenum trigger the Enterogastric Reflex. -
Sympathetic nerve
The medulla oblongata inhibits vagal nuclei, reducing vagal stimulation of the stomach. +
The medulla oblongata stimulates sympathetic neurons that send inhibitory signals to the stomach. X
Mucous cells
Mucus
Chief cells
Pepsinogen
The net result is that immediately after the chyme enters the duodenum, gastric contractions decrease, and further discharge of chyme is prevented, giving the duodenum time to neutralize and digest the acidic chyme.
Parietal cells
HCl
Gastrin
G cells

15. Lipids & Carbohydrates3
Intestinal Phase
2- Hormonal Responses
1- The arrival of lipids and carbohydrates stimulates the duodenal enteroendocrine cells to release cholecystokinin (CCK) and Gastric Inhibitory Peptide (GIP), which inhibit gastric secretion.
Medulla oblongata
Vagus nerve
As a result, a meal high in fats stays in the stomach longer to allow more time for lipids to be digested and absorbed in the small intestine
Sympathetic nerve
2- A drop in pH below 4.5 stimulates the secretion of the hormone Secretin by the duodenal enteroendocrine cells:
CCK & GIP -
Secretin:
It inhibits parietal and chief cells.
Mucous cells
Mucus
It stimulates the pancreas to produce bicarbonate to neutralize the acid in the chyme. X
Chief cells
Pepsinogen
Parietal cells
HCl
Lipids & Carbohydrates
G cells
The pyloric sphincter contracts tightly to limit chyme entering duodenum in order to give duodenum time to work on chyme. The duodenum receives secretions from liver and pancreas pH
Secretin
Pancreas
Bicarbonate
Neutralizes the pH

16. INTESTINAL PHASE Neural Responses KEY Enterogastric reflex
Myenteric plexus
via bloodstream
Chief cells
Parietal cells
Duodenal stretch and chemoreceptors
Peristalsis
CCK
Presence of lipids and carbohydrates
GIP
KEY
Secretin
Inhibition
Decreased pH 16

17. The Small Intestine
It is the major digestive and absorptive organ of the body. It extends from the pyloric sphincter to the ileocecal valve (19.7 ft )
Duodenum (10 inches)
It receives the pancreas and liver secretions that mix with the chyme.
Jejunum (8.2 ft )
It is where most chemical digestion and absorption take place.
Ileum (11.48 ft )
It is where digestion is completed. It ends at the ileocecal valve, which control the passage of undigestive materials to the large intestine.
Ileocecal valve
Cecum
Appendix
Functions:
1- It is where most chemical digestion takes place.
2- To absorb 99% of the digested nutrients. 17

18. Histology of the Small Intestine
They increase the surface area for absorption.
They increase the surface area for absorption.
Plica circulares
They slow movement to allow time for absorption.
They make chyme spiral through the lumen to mix with intestinal juice .
Villi
Columnar epithelial cells with microvilli
Villus
Mucous cell
Intestinal crypt
Lacteal
Capillary network
Mucosa
Highly folded lining where absorption occurs.
Nerve
Muscularis
mucosae
Submucosa
Connective tissue containing blood vessels and nerves.
Arteriole
Muscularis externa
Lymphatic vessel
Venule
Circular and longitudinal muscle layers used for peristalsis
Serosa

19. The Large Intestine Functions: 1- Reabsorb water.
Transverse colon
Haustra
Supported by the transverse mesocolon.
Taenia coli
Ascending colon
Descending colon
Ileum
Ileocecal valve
Cecum
Appendix
Sigmoid colon
Functions:
1- Reabsorb water.
Supported by the sigmoid mesocolon.
Rectum
2- Compact undigested food stuff into feces.
Anus
3- Eliminate feces.
4- Absorb bacterial vitamins (vitamin K, biotin, and vitamin B5).
5- Store fecal matter until defecation. 19

20. The Pancreas
It is a spongy retroperitoneal gland posterior to the greater curvature of the stomach.
The head of the pancreas is encircled by the duodenum.
It is both an endocrine and exocrine gland.
The endocrine portion consists of the pancreatic islets that secrete insulin and glucagon.
The exocrine portion consists of the pancreatic acini, it is about 99% of pancreas and secretes 1200 to 1500 mL of pancreatic juice per day.
Accessory
pancreatic duct
Pancreatic duct
Lobules
Common bile duct
Body
Tail
Minor duodenal papillae
Major duodenal papillae
Head
Duodenum

21. Endocrine pancreas Exocrine pancreas Islets of Langerhans
Pancreatic acini
Acinar cells and epithelial cells of duct system secrete pancreatic juice.
1- Beta cells:
2- Alpha cells:
3- Delta cells:
4- F cells:
Insulin
Glucagon
Somatostatin
Pancreatic polypeptide
They secret 1000 mL pancreatic juice per day, controlled by hormones from duodenum.
The pancreatic juice contains the pancreatic enzymes:
Pancreatic alpha-amylase: It is a carbohydrase that breaks down starches.
It is similar to salivary amylase.
Pancreatic lipase: It breaks down complex lipids and releases products (e.g., fatty acids) that are easily absorbed.
Nucleases: They break down nucleic acids.
Proteolytic enzymes: They break certain proteins apart.
Proteases: They break large protein complexes.
Peptidases: They break small peptides into amino acids.
They are the 70% of all pancreatic enzyme production, are secreted as inactive proenzymes and are activated after reaching small intestine.