Physiology - Gastrointestinal system

Digestive Process

1. Motility (mixing and metering)

2. Secretion (exocrine and endocrine)

3. Digestion (mechanical and chemical breakdown)

4. Absorption

Regultion of GI function

1. Autonomous smooth muscle function

2. Intrinsic nerve plexuses

3. Extrinsic nerves

4. GI hormone

Phases of Digestion

1. Inter-digestive: little activity

2. Cephalic: largely nerves

3. Gastric: nerve and hormones

4. Intenstinal: largely hormones

The Mouth functions

1. Teeth: mastication (smaller pieces) → increases surface area

2. Salivary Glands: secrete saliva → salivary amirayse, antibacterial and mucus.

Function of the stomach - Gastric Filling

• Weak peristaltic waves move food into the body of the stomach

• Receptive relaxation

• Can accomodate about 1 litre of contents

Function of the stomach - Gastric Storage

• storage in the body of the stomach

• Thin smooth muscle layer, so little mixing of food

Function of the stomach - Gastric Mixing

• occurs in the antrum of the stomach

• Strong peristaltic waves mixes ingested food with gastric secretions

• Chyme is pushes towards the pyloric spinchter.

Functions of the stomach - Gastric Motility

• emotions can influence gastric motility

• Irritation to digestive tract can lead to repulsion of food (abdominal smooth muscle contracts).

Triggers of Gastric Emptying

1. Factors in the stomach

• Aount of chyme and degree of fluidity = increased rate of GE

2. Factors in the duodenum

• Fat, acid, hypertonicity and distention decrease rate of GE via enterogastric reflux).

Gastric digestion and absorption

• protein digestion (antrum)

• Little carbohydrate digestion (body)

• Some fat digestion

• Absorption of alcohol and aspirin

Secretory Cells - Endocrine/exocrine

Exocrine

• Mucus neck sells

• Chief cells

• Parietal/oxyntic cells

Endocrine

• enterochromaffin-like cells

• G cells

• D cells

Mucus surface and neck cells

• exocrine

• Mucus and bicarbonate-rich secretion

• Stomach = hostile environment

• Protects surface

Chief ( or zymogen-secreting) cells:

• Exocrine

• Secretes pepsinogen, when reacted with HCl → pepsin

  • Active proteolytic enzyme → breaks down proteins into amino acids/peptides

• Begins protein digestion

Parietal (oxyntic) cells

• Exocrine

• Secretes HCl

  • denature protein

  • Activates pepsinogen

  • Kills most ingested microorganisms

  • Solublizes Fe3+, assisting absorption

• Secretes Intrinsic factor

  • Binds vitamin B12

  • Triggers absorption in ileum (endocytosis)

H+ Generation and Secretion - Parietal cells

• acid is secreted vis a proton pump, K+ and Cl- channels and ATPase

• Gastric juice - 155mM HCl + 15mM KCL

Stimulation of Acid Secretion

Enterochromaffin-like (ECL) cells

• endocrine

• Present in oxyntic gland (body of stomach)

• Releases histamine (paracrine) → stimulates acid secretion (parietal cells)

G cells

• endocrine

• Present in pyloric gland (antrum)

• Secretes gastrin → stimulates parietal, chief and ECL cells

D cells

• endocrine

• Present in the pyloric gland (not rum)

• Secrete somatostatin → negative feedback

Gatric Secretion - Cephalic Phase

• repaired the digestive system for food processing

• 30% of total gastric secretions

• Input: Neural

• Output: “predatory” acid and enzyme secretion → increases efficiency of digestion

GA Secretion - Gastric Phase

• Filling of stomach distends gastric wall

  • increased sensory feedback → increased parasympathetic output to parietal, ECl and G cells

• Presence of amino acids into the antrum stimulates G cells to increase gastrin secretion.

• 60% of total gastric secretions

Protective Barrier

• Mucus Surface cells produce a mucosal barrier

• High levels of bicarbonate, prevents stomach lining from being digested

Gastric Emptying - Order

1. Isotonic saline/water

2. Acidic and caloric fluids

3. Fatty materials

4. <2mm solids

Intestinal Motility - Peristalsis

• Initiared by nervous and hormonal signals

• Spreads out chyme throughout the instestine and propels of towards the ileocecal valve

Intestinal phase of digestion

• Requires further digestion → new enzymes required

Secretions:

1. Pancrease

   • copious enzymes to further digest food

   • Bicarbonate to neutralise acid

2. Liver (biliary system) - secretes bile salts

Pancrease Acinus

• Acinar cells: releases, proteases, lipases, amylases and nuclei acid enzymes

• Intercalated duct cell: bicarbonate

Control of pancreatic Secretion

• Vagus (parasympathetic) nerve (via Ach) - controls both enzyme release and bicarbonate release

• Cholecystoknin (CCK) - enzyme release only

• Secretin - bicarbonate release only

Control of CCK Release

• Stimulated by:

  • digested fat and proteins

  • CCK-releasing factor

  • H+

• Inhibited by:

  • Trypsin in lumen

  • Somatostatin

• CCK-RF and Trypsin are continuously released into the duodenal lumen

Secretion of CCK

• CC-RF binds to enteroendocrine cells which further stimuleast CCK release

• Between meals:

  • Trypsin binds to and break down CCK-RF

  • Digested CCK-RF can’t bind to enteroendocrine cells therefore decreased CCK

• After meal:

  • Trypsin preferentially binds to and digest proteins

  • Therefore no CCK-RF broken down

  • CCK-RF binds to enteroendocrine cells → triggers CCK secretion (pancreatic enzyme secretion)

CCK Actions on Pancrease

• stimulus pancreatic enzyme secretion largely via stimulation of vagal Afferent (sensory)

• No CCK receptors on acinar cells

• Vagotomy or atropineinhibits pancreatic enzyme secretion

Secretion Action on Pancreas

• Secrein released from enterocytes in repsonse to aci d

• Acts via vago-vagal pathway and directly via secretin receptors on duct cells

• Induces ducts cells to produce bicarbonate