Physiology of the Gastrointestinal Tract

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Physiology of the Gastrointestinal Tract

• Dr. Sherwan Rahman Sulaiman
• www.doctorsherwan.com

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What are the Functions of the Gut?

• Digestion Process by which large molecules in
  diet are broken down into smaller ones, which are
  acceptable to the enterocytes for absorption.

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What are the Functions of the Gut?

• AbsorptionProcess by which contents of the
  small bowel enter the mucosal epithelial cells,
  and eventually the portal vein or lymphatics.

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What are the Functions of the Gut?

• Protection Barrier to entry of pathogens and
  toxins

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Anatomy and Physiology of Digestive System - Mouth

• Mouth - Digestive Action
• Food mechanically broken down
• Saliva - normally about 25 ml/hr is secreted,
  increases up to 300 ml per hour with eating
• Salivary a-amylase degrades starch, a-amylase
  is deactivated by gastric acid

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Anatomy and Physiology of Digestive System - Mouth

• Mouth - Protective Action
• Preventive bacteria in the mouth are important
  for defense against invading microorganisms
• Contains specific antimicrobial proteins like
  lysozyme, lactoferrin and lactoperoxidase, but
  also mucin, IgA, and nitric oxide-donating
  substances such as nitrates
• Mucus covers food and follows it to colon. It can
  attach to mucosal surfaces and forms protective
  barrier

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Anatomy and Physiology of Digestive System - Mouth

• Inhibition of Saliva
• Anticholinergics, analgesics, antispasmodics,
  antidiarrheals, antidepressants, antihistamines,
  antihypertensives, antipsychotics, and diuretics
• Stimulation of Saliva
• Pilocarpine 5mg po tid

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Anatomy and Physiology of Digestive System -
Esophagus

• Esophagus
• Transports food to stomach
• Pharyngeal lipase hydrolyzes triglycerides to
  diglycerides and fatty acids

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Anatomy and Physiology of Digestive System -
Stomach
Stomach Stores, mixes, and grinds food to form
an emulsion
Fundus
Gastroesophageal Junction
Cardia
Body
Incisura
Antrum
Pylorus
Duodenal Bulb
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Anatomy and Physiology of Digestive System -
Stomach
Hydrochloric acid from parietal cells denatures
protein Pepsinogen (Pepsin) from zymogen
(chief) cells begins proteolysis
Surface Epithelial Cell
Mucous Cell
Parietal Cell
Zymogen Cell
Lymph Nodule
Argentaffine Cell
Muscularis Mucosae
Submucosa
Pyloric Glands
Gastric or Fundic Glands
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Anatomy and Physiology of Digestive System -
Stomach
Pepsin
Acid
Gastric lumen
pH 1-3
Mucous gel layer
Bicarbonate / Mucus
pH 7
Gastric mucosa
Zymogen Cell
Mucous Cell
Parietal Cell
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Anatomy and Physiology of Digestive System -
Small Bowel

• Small Intestine - averages around 5 meters in
  length
• Duodenum 10 long, 2 in diameter
• Jejunum 2/5 length of rest of small bowel, 1 ½
  to 1 ¼ in diameter, thick, many blood vessels.
• Ileum 3/5 length, 1 ¼ to 1 diameter, thin
  wall, large Peyers patches

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Anatomy and Physiology of Digestive System -
Small Bowel

• Small Intestine
• Major organ for nutrient absorption
• Absorptive surface enhanced by plicae circulares,
  foldings called villi with surface projections
  called microvilli
• Final surface area about 1.7 m cm2
• About 800 cm2 to absorb 1 Kcal (100-200 cm2/Kcal
  minimum)

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Anatomy and Physiology of Digestive System -
Small Bowel
Plica circularis or Valve of Kerckring
Villus
Jejunum Ileum
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Anatomy and Physiology of Digestive System - Colon

• Absorbs water and electrolytes
• Stores waste
• Organ within an Organ

• Colonic microflora ferment malabsorbed nutrients
  and soluble fiber to a form the colonic mucosa
  can absorb

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Absorption of Water

• 8 L/day fluid reaches the small intestine of
  which about 2 L is dietary in origin
• Small bowel absorbs about 7 L/d, mainly in
  jejunum, colon absorbs 1 to 1.5 L/d (can increase
  absorption up to 4 L/d)

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Absorption of Water

• Water absorption is entirely passive - follows
  absorption of solutes (can move in either
  direction, depending on osmotic gradients)
• Particularly true of the solute Na
• High sodium enteral diets enhance water
  absorption
• Low sodium diets predispose to diarrhea

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Absorption of Sodium and Chloride - Small Bowel

• Sodium absorption is directly coupled to
  absorption of organic solutes such as glucose,
  amino acids, water-soluble vitamins, and bile
  salts

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Absorption of Sodium and Chloride - Small Bowel

• Once inside cell, sodium is extruded against
  chemical and electrical gradient via a
  basolateral membrane-associated Na-K-ATPase

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Absorption of Sodium and Chloride - Small Bowel

• Chloride passively follows absorption of sodium

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Absorption of Sodium and Chloride Distal Ileum
and Colon

• Neutral NaCl co-transport, Na for H and Cl for
  HC03

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Absorption of Potassiumin Small Bowel

• Overall K movement is result of solvent drag and
  is potential-dependent
• K actively secreted in colon
• Rectosigmoid colon has active K absorption -
  exchanges K for H

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Absorption of Calcium

• Passive - throughout the small intestine
• Predominates at concentrations ³ 10 mmol/L
• Active - primarily in the duodenum
• Below 10 mmol/L, active transport occurs with
  1,25 dihydroxy vitamin D3

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Absorption of Calcium
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Fiber Digestion and Absorption

• Non-starch carbohydrate of plant origin that
  escapes enzymatic digestion in the small
  intestine
• Two types
• Cellulosic high molecular weight, non-soluble
  (cellulose, wheat bran)
• Noncellulosic soluble (hemicelluloses, pectin,
  gums, mucilages)

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Triglycerides

• Majority of triglycerides contain long-chain
  fatty acids (16 -18 C)
• A few dietary triglycerides contain medium-chain
  fatty acids (8 -12 C)

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Lipid Absorption

• Lipids in diet are emulsified in the stomach by
  mechanical grinding
• Pancreatic lipase in duodenum digests to free
  fatty acids and 2-monoglycerides
• Bile acids form water soluble micelles which
  diffuse easily across the unstirred water layer

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Fat Digestion and Absorption

• Medium-chain triglycerides, which are more water
  soluble, may be absorbed intact with direct
  transport to the portal system as free fatty acids

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Protein Digestion and Absorption

• Derived from animal and vegetable sources and
  make up to 11 to 14 of average caloric intake
  (70 to 100 gm/day)
• Primarily absorbed in the duodenum and proximal
  jejunum, yet some does pass into and is absorbed
  by the colon