Lesson # 16 The Digestive System 2Chapter 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.
Functions of Stomach
1- Storage of ingested food.
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.
2- To produce the chyme by:
Lesser curvature
Greater curvature
Cardia Fundus
Body
Duodenum
Pylorus:
Serose
Longitudinal layer
Circular layer
Oblique layer
Muscularis Externa:
Mucosa
Rugae
Antrum
Pyloric canal
Pyloric sphincter
They let the gastric lumen expand.
It regulates the release of chyme to
the duodenum.
Anatomy of the Stomach
Histology of the StomachSimple
columnar epithelium
It secrets alkaline protective mucus
Gastric pitMucosa
Lamina propria Muscularis mucosae
Submucosa
Oblique muscle
Circular muscle
Longitudinal muscle
Serosa (visceral peritoneum)
Muscularis externa
Gastric pit
Gastric gland
Parietal cellsThey produce HCl and
Intrinsic Factor.
Chief cellsThey produce Pepsinogen
G cellsThey are enteroendocrine cells that produce Gastrin, Somatostatin, Histamine
and Serotonin.
Mucous cells
(In new born and infants produce rennin and gastric
lipase).
They produce alkaline protective mucus.
Gastric Glands
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.
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.
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.
HCl
Mucous cells
Chief cells
Parietal cells
G cells
Mucus
Pepsinogen
HCl
Gastrin
Parasympathetic division
CNS
Regulation of Gastric ActivityThe nervous and endocrine systems gastric activity.
+Mucous cells
Chief cells
Parietal cells
G cells
Mucus
Pepsinogen
HCl
Gastrin
ACh
Parasympathetic Division:It increases secretions and motility in the digestive system.The fibers of the parasympathetic division release acetylcholine (Ach). They are called cholinergic fibers.
Sympathetic division
CNS
Adrenaline
Sympathetic Division:It decreases secretions and motility in the digestive system.The fibers of the sympathetic division release epinephrine (adrenaline). They are called adrenergic fibers.
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Vagus nerve
1 Cephalic Phase
Parasympathetic division
CNS
Regulation of Gastric ActivityThe 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:
1- The Cephalic Phase: The stomach is being controlled by brain.
2- The Gastric Phase: The stomach is controlling itself.
3- The Intestinal Phase: The stomach is being controlled by the small intestine.
+
Stimuli:
Vagus nerve stimulates gastric secretion even before food is swallowed.
Sight, smell, taste, or thought of food
The Cephalic Phase is directed by the CNS and prepares the stomach to receive food.
Mucous cells
Chief cells
Parietal cells
G cells
Mucus
Pepsinogen
HCl
Gastrin
ACh
CEPHALIC PHASE
Central nervous system
Food
Vagus nerve (N X)
Sight, smell, taste,or thoughts of food
Gastrin
MucouscellsChiefcellsParietalcells
G cells
Mucus
Pepsinogen
HCl
Stimulation
KEY
Submucosal plexus
2 Gastric Phase
+
It begins with arrival of food in the stomach.
Stimuli
Sensory fibers
Motor fibersDistention of the stomach.An increase in the pH of the gastric content.
Ingested food produces:
Gastrin – a hormone produced by the enteroendocrine G cells of the pyloric antrum.
1- Neural Response
Stretching activates short reflex mediated through myenteric nerve plexus.
+ACh
+ACh
pH
Hypothalamus
Short reflex
Long reflex
Presence of undigestive materials in the stomach. Vagus nerveParasympathetic
division
Long reflex
2- Hormonal Response
3- Local ResponseShort reflex
Responses:
Gastrin
GASTRIC PHASE
Submucosal andmyenteric plexuses
Stretchreceptors
Chemoreceptors
Gastrin
MucouscellsChiefcellsParietalcells
G cells
Mucus
Pepsinogen
HCl
Distension
Elevated pH
Mixingwaves
Partly digested peptides
viabloodstream
Neural Response
3 Intestinal Phase It begins when chyme first enters the duodenum.
Vagus nerve
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.
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Stretch receptors and chemoreceptors in the duodenum trigger the Enterogastric Reflex.
Sympathetic nerve
0The medulla oblongata inhibits vagal nuclei, reducing vagal stimulation of the stomach.The medulla oblongata stimulates sympathetic neurons that send inhibitory signals to the stomach.
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Medulla oblongata
StimuliDistention of the duodenum by the chyme.Decrease in the pH of the duodenum by the chyme. Responses:1- Neural Response
Mucous cells
Chief cells
Parietal cells
G cells
Mucus
Pepsinogen
HCl
Gastrin
XXXX
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.
Vagus nerve
Sympathetic nerve
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.
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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
Medulla oblongata
2- Hormonal Responses
3 Intestinal Phase
Lipids & Carbohydrates
pH
2- A drop in pH below 4.5 stimulates the secretion of the hormone Secretin by the duodenal enteroendocrine cells:
Mucous cells
Chief cells
Parietal cells
G cells
Mucus
Pepsinogen
HClXX
CCK & GIP
Secretin
Secretin: It inhibits parietal and chief cells.It stimulates the pancreas to produce bicarbonate to neutralize the acid in the chyme.
Pancreas
Bicarbonate
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 Neutralizes the pH
INTESTINAL PHASE
Inhibition
KEY
Decreased pH
Presence of lipids and carbohydrates
Peristalsis
Secretin
GIP
CCK
Duodenalstretch andchemoreceptors
via bloodstream
Enterogastricreflex
Myentericplexus
Chiefcells
Parietalcells
Neural Responses
The Small IntestineIt is the major digestive and absorptive organ of the body. It extends from the pyloric sphincter to the ileocecal valve (19.7 ft )
Functions:1- It is where most chemical digestion takes place.2- To absorb 99% of the digested nutrients.
It receives the pancreas and liver secretions that mix with the chyme. It is where most
chemical digestion and absorption take place.
It is where digestion is completed. It ends at the ileocecal valve, which control the passage of undigestive materials to the large intestine.
Jejunum (8.2 ft )
Duodenum (10 inches)
Ileum (11.48 ft )
Ileocecal valve
Appendix
Cecum
Histology of the Small IntestinePlica circularesVilli
Mucosa
Submucosa
Muscularismucosae
Muscularis externa
Serosa
Highly folded lining where absorption occurs.
Connective tissue containing blood vessels and nerves.
Circular and longitudinal muscle layers used for peristalsis
ArterioleVenuleLymphatic vessel
Nerve
Capillary network
Mucous cell
Lacteal
They increase the surface area for absorption.
They make chyme spiral through the lumen to mix with intestinal juice .
They slow movement to allow time for absorption.They increase the surface area for absorption.
Intestinal crypt
Villus
Columnar epithelial cells with microvilli
The Large Intestine
Ileocecal valve
Anus
Cecum
Functions:1- Reabsorb water.2- Compact undigested food stuff into feces. 3- Eliminate feces.4- Absorb bacterial vitamins (vitamin K, biotin, and vitamin B5).5- Store fecal matter until defecation.
Appendix
Ascending colon
Haustra
Sigmoid colon
Rectum
Descending colon
Taenia coli
Ileum
Transverse colon
Supported by the transverse mesocolon.
Supported by the sigmoid mesocolon.
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.
Accessorypancreatic duct
Head
Body Tail
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.
Pancreatic duct
Major duodenal papillae
Minor duodenal papillae
Common bile duct
Duodenum
Lobules
Endocrine pancreas Exocrine pancreas Pancreatic acini
1- Beta cells:2- Alpha cells:3- Delta cells: 4- F cells:
InsulinGlucagonSomatostatin
Pancreatic polypeptide
Islets of LangerhansAcinar cells and epithelial cells of duct system secrete pancreatic juice.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.
They are the 70% of all pancreatic enzyme production, are secreted as inactive proenzymes and are activated after reaching small intestine.
Proteolytic enzymes: They break certain proteins apart.Proteases: They break large protein complexes.Peptidases: They break small peptides into amino acids.