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.

_

XXXX

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.

+

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.

-

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.

-

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.