SMALL AND LARGE INTESTINE SECRETIONS

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SMALL AND LARGE INTESTINE

SECRETIONS

Objectives

• 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

THE DIGESTIVE SYSTEM

• 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.

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The Digestive System

• Is a long tube from the mouth to the anus

DIGESTIVE SYSTEM STRUCTURE

• 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

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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

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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

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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

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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

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Intestinal Secretions

B-Enzymes:

• 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

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Duodenum

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

Jejunum-Ileum

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

absorption

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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

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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.

Actions:

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

SMALL INTESTINE ENZYMES

• 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.

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Neurotransmitters

• 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

THE LARGE INTESTINE

• 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.

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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.

Colon

• Reabsorb water from food and digestive juices

Rectum

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

– Feces includes sloughed cells, undigested food and microbial matter

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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.

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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)

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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

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Figure

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References

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