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• At the end of lecture student should be able to know, • Digestive system

• Digestive system secretions • Small intestine

• Component of small intestine • Intestinal secretions

• Large intestine secretion & functions • Neural control of digestive system

• Integration of neural & hormonal control


• Body cells require a continuous supply of nutrients that come from ingested food.

• Food molecules are too large to pass directly into blood and must be broken into smaller molecules.

• Digestion of food & absorption of nutrients are the major functions of the digestive system.


The Digestive System

• Is a long tube from the mouth to the anus


• Consist of a long tube & accessory organs associated with it.

• The shape of each part is modified according to its function.

• Parts: - mouth, pharynx, esophagus, stomach, small intestine, large intestine and anus.

• Accessory organs: - teeth, salivary glands, liver and pancreas.

Secretions of Digestion

The five organs which produce secretions during digestion: • Salivary glands

• Stomach • Pancreas

• Liver (via gallbladder) • Small intestine


Small Intestine

• Connects the stomach with the large intestine

• It is the major site of digestion • It is also the major site of absorption • Specialized structures (villi, microvilli)

increase the surface area of the small intestine, aiding absorption.

• The small intestine has three parts (duodenum, jejunum, and ileum)

• The bile duct (from liver) and pancreatic duct (digestive juices) empty into the duodenum.

The Small Intestine

• Most digestion and absorption happens here • About 5-7 meters

• Duodenum

– ~10 inches in length – Primary site of digestion • Jejunum – ~4 feet in length – Some digestion • Ileum – ~5 feet in length – Little digestion


Small Intestine

• Composed of 3 segments (proximal to distal) – Duodenum

• Releases bile and pancreatic secretions • Active site of digestion

– Jejunum

• Active site of nutrient absorption – Ileum

• Active site of nutrient absorption

– Most water, vitamins & minerals • Some bacterial presence

– Fermentation

The pH of the small intestine increases towards 7.0 as food moves from the duodenum to the ileum

The Small Intestine

• Folded walls with villi projections • Absorptive cells are located on the villi • Increases intestinal surface area 600x • Rapid cell turnover


Intestinal Epithelial Cell

Specialized Cells Lining Villi

• Absorptive epithelial cell

– Contain brush border on lumen/apical side – Brush border:

• Enzymes

• Nutrient transport molecules

• Goblet cell


Intestinal Mucosa

Absorptive cells

• Produced in crypts

• Migration and maturation from the crypts to the tips of the villi • Degradation of cells at the tips of the villi by digestive enzymes

• Newly formed cells constantly migrate to replace dying ones (< 6 days) • High turnover causes the cells to deteriorate during nutrient deficiency

Secretions Entering SI

• Intestinal mucus • Brush border enzymes • Pancreatic juices

– Produced & stored in pancreas • Bile

– Produced in liver – Stored in gallbladder

Intestinal Secretions

A-Mucus most likely serves a protective role preventing HCL and chyme from damaging the intestinal wall. Acts as lubricant and buffer to protect duodenal wall. Mucus is secreted by :

1-Brunner's glands, which are located within the duodenum

2-Goblet cells located along the length of intestinal epithelium and in the intestinal crypts, called the crypts of Lieberkuhn


Intestinal Secretions


• Enterokinase

Digestive enzymes – not secreted, but present on brush border of enterocytes – includes peptidases, monosaccharidases (sucrase, maltase, isomaltase, lactase), & lipases

• Capable of breaking down small peptides and disaccharides are associated with the microvilli of the epithelial cells lining the intestine.

• Although these enzymes are not secreted into the intestine, they are able to digest small peptides and disaccharide during the absorptive process.

• Function : Completing the digestion of peptides, carbohydrates & fats.

C-Water & Electrolytes: are secreted by all the epithelial cells of the intestine.

1- The watery secretion provides a solvent into which the products of digestion are dissolved.

2- If excessive amounts of fluid are produced, potentially life threatening watery diarrhea can result.

In the Small Intestine

• Bile acid from the liver via the gallbladder • Bicarbonate ions from the pancreas

• Muscle contractions to mix the food with digestive juices • Food remains 3-10 hours in the small intestine



Receive juices from pancreas, liver and its own wall • Secretion from the duodenum: They finish

off the last step of digestion.

• Peptidases (or dipeptidases) break off the bond between dipeptides to free 2 amino acids • Disaccharidase (maltase, sucrase, lactase)

break off disaccharides into 2 monosaccharides (mostly glucose)

• Intestinal lipase breaks off diglycerides into monoglycerides and fatty acids.

• Nutrients are completely degraded into forms that can be absorbed by cell


• Nutrients will be reabsorbed along the jejunum-ileum • Brush border contains villi which increase the surface of



Gastrointestinal Hormones

• Gastrin

– Origin: Stomach

– Stimulus: Food in stomach

– Function: stimulate gastric glands secretion.

– Stimulates HCl & pepsinogen secretion, increases stomach motility

• Secretin

– Origin: Duodenum – Stimulus: Acid

– Function: Inhibit gastric glands secretion & stimulate pancreatic juice secretion.

– Slows stomach motility and acid production

• Cholecystokinin (CCK) – Origin: Duodenum

– Stimulus: Fat & protein in duodenum

– Function: inhibit gastric glands secretion, stimulate pancreatic secretion & gallbladder contraction.

– Also Stimulates bile secretions

• regulates appetite and feed intake

• Gastric Inhibitory Protein (GIP) – Origin: Duodenum

– Stimulus: Fats and bile


Hormones (Motilin)

• Released from stomach and small and large intestine in presence of biliary and pancreatic secretions; promotes gastric emptying and increases GI motility

• motilin secreted from the upper portions of small intestine by specialized cells.

Vasoactive Intestinal Peptide (VIP)

Secretion: Enteroendocrine cells in the small intestine mucosa Crypts of Lieberkuhn

Stimulus: Chyme entering the small intestine.


– Stimulates buffer secretion – Inhibits gastric secretion – Dilates intestinal capillaries


• MALTASE digest maltose to glucose

• SUCRASE digest sucrose to glucose and fructose. • LACTASE digest lactose to glucose and galactose. • LIPASE digest fats to fatty acids.



• Alpha- aminobutyric acid: relaxes LES

• Nor epinephrine: decreases motility, increases contractions of sphincters, inhibits secretions

• Acetylcholine: increases motility, relaxes sphincters, stimulates secretions • Neurotensin: inhibits release of gastric emptying and acid secretion

• Neuropeptide -Y: stimulates feeding behavior


• Last portion of the digestive tract.

• Extend from the ileum to the anus and is made of three segments:- • CECUM – COLON – RECTUM.

• FUNCTION for absorption of water, minerals and vitamins.

• ENTRANCE guarded by the ileocecal sphincter while the anal exist is guarded by internal and external anal sphincters.

Large Intestine

• No digestion occurs in the large intestine.

• In the large intestine, there is absorption of water (about 1 liter/day) and salts from feces

(undigested, unabsorbed food).

• Bacteria produce vitamin K, B vitamins. • Secretion of mucus (lubrication of feces)

• Contractions move feces along large intestine and rectum, to be expelled out of the anal canal.


Large intestine

Mucus secretion

Large intestine have crypts of Lieberkuhn with lots of goblet cells, secreting lot of mucus with moderate amount of bicarbonate ions.

• It act as an adherent medium for holding fecal matter together.


• Reabsorb water from food and digestive juices


• Muscular area of large intestine used for storage of feces and ultimately for defecation

– Feces includes sloughed cells, undigested food and microbial matter


Accessory Organs

• Salivary glands • Pancreas

• Gallbladder • Liver

Neural Control of Digestion

• Neural control of digestion is largely by the parasympathetic nervous system, and local (enteric) reflexes.

• Activation of the parasympathetic system results in secretion of digestive juices, increased motility of the stomach, and slowing down movement of food from the stomach to small intestine.

• Stimuli: Thought, sight, taste of smell of food; distension of GI tract; chemoreceptors detecting nutrients, pH.


Integration of Neural and Endocrine Functions: Central Effects

• CNS: Thoughts, taste, smell of food; chewing – activates parasympathetic nervous system (neurotransmitter: acetylcholine).

• ACh acts directly on parietal cells to increase acid secretion.

• Ach increases gastrin release, inhibits somatostatin release (increased gastric secretion and motility).

• Sympathetic input (activity, stress): increased somatostatin release (inhibiting gastrin secretion – decreased gastric secretion and motility)


Integration of Neural and Endocrine Functions

• Mechanoreceptors in the walls of the GI tract detect movement of food into an organ

– Example: In the stomach distension causes activation of the

parasympathetic system, increasing gastrin secretion and acid release, and decreasing somatostatin secretion.

• Chemoreceptors detect nutrients and pH.

– Example: Presence of amino acids, alcohol, or caffeine in the stomach increases gastrin release.

– Presence of fatty acids in the duodenum causes release of CCK.

Control pathways

• Both hormonal and neural

• Short pathways: involves automatic regulation within the enteric system itself