ACEM Primary
Gastrointestinal Physiology
Table of Contents
Enteric nervous system
- Myenteric plexus (Auerbach’s) within muscle layer for motor control
- Submucous plexus (Meissner’s) between muscle and mucosa for secretions
- Autonomic nervous system
Gastrointestinal hormones:
| HORMONE | SYNTHESISED BY | ACTION |
| Gastrin | G cells of antrum Pancreas | gastric acid and pepsin secretion motility Growth of mucosa Stimulates insulin secretion Increased by: luminal peptides/ amino acids/ distension, vagal discharge via GRP, Ca2+ and epinephrine Decreased by: antral acid, somatostatin, secretin/ VIP/ GIP/ glucagon/ calcitonin. |
| Cholecystokinin | I cells in SI Pancreas | Gallbladder contraction Secretion of pancreatic juice rich in enzymes ¯ gastric emptying |
| Secretin | S cells in SI | bicarb secretion from pancreas and biliary tract (pancreatic juice watery/ alkaline) |
| GIP | K cells of SI | Stimulates insulin secretion |
| VIP | Nerves of GIT | Stimulates secretion of electrolytes and water |
| Peptide YY | Jejunum | Inhibits gastric acid secretion and motility |
| Ghrelin | Stomach | Role in central control of food intake |
| Motilin | Mo/ enterochromaffin cells | Contraction of smooth muscle |
| Somatostatin | D cells in pancreatic islets | Inhibits secretion of gastrin/ VIP/ GIP/ secretin and motilin Inhibits pancreatic secretion, gallbladder contraction and absorption |
Mouth/ Oesophagus
- Mastication + saliva from parotid/ submandibular/ sublingual glands -> containing enzymes, mucin, glycoproteins, IgA
- Swallowing
- Reflex response triggered by afferent impulses in trigeminal, glossopharyngeal and vagus nerves
- Integrated into nucleus of tractus solitarius/ ambiguous
- Efferent via same to pharyngeal muscles
- Inhibition of respiration and glottic closure are part of reflex
Stomach
- Fundus cells secrete mucous
- Body = parietal cells (HCl and IF) and chief cells (pepsinogen)
- Surface epithelium secretes mucus/ HCO3- as barrier against acidic environment
- Stimulated by PG
- Gastric juice (pH 1.0) contains cations/ anions/ pepsins, lipase, mucous and IF
- Parietal cells pump H+ into lumen via H/K ATPase, stimulated by histamine/ gastrin
- PUD: breakdown of mucosal barrier secondary to H Pylori, NSAID, ETOH or excess acid secretion in Zollinger Ellison Syndrome (gastrinomas)
Exocrine pancreas
- Pancreatic juice is alkaline (pH ~8.0)
- Raise pH of duodenal contents to 6.0-7.0
- Contains cations/anions/ digestive enzymes/ proteins
- Trypsin, chymotrypsin, elastase, carboxypeptidase
- Secretin -> secretion of enzymes
- CCK -> secretion of enzymes
Liver and Biliary System
- Functions of liver:
- Formation of bile
- Nutrient metabolism (glucose, AA, fats, vitamins)
- Inactivation of toxins, steroids, hormones
- Synthesis of plasma proteins
- Acute phase
- Albumin
- Clotting factors
- Steroid or hormone binding proteins
- Immunity via Kupffer cells
- Bile: salts + pigments + alkaline electrolyte solution
- 500mL secreted daily
- Some components reabsorbed via enterohepatic circulation
- Bile salts = sodium/ potassium salts of bile acids, from cholesterol
- Cholic acid, chenodeoxycholic acid, deoxycholic acid, lithocholic acid
- Emulsification of fats prior to digestion
- CCK causes contraction
Small Intestine
- Duodenum -> (ligament of Trietz) jejunum -> (no sharp anatomic boundary) ileum
- High surface area for absorption
- Mucosa contain lymphatic nodules (Peyer’s patches) + glands (isotonic fluid) + villi
- Plicae circularis
Colon
- Absorption of water, removal 90% fluid that enters
- Active transport of Na+ and water follows osmotic gradient
- Teniae coli (longitudinal muscle) and haustra (outpouchings)
- No villi on mucosa
- Gastro-ileal reflex = vagal, when food leaves stomach, caecum relaxes and allows passage of chyme through ICV
- Faeces = 30% bacteria in solid part
- Defecation = distension of rectum by faeces initiates reflex contraction + SNS/PNS tone to IAS + skeletal (voluntary) to EAS. First urge occurs 18mmHg, at 55mmHg reflex expulsion occurs. Straining leads to abdominal muscle contraction + lowering of pelvic floor + relaxation of puborectalis (anorectal angle reduced 15 deg or less) + relaxation of sphincters.
Digestion and Absorption:
- Mucosal cells of small intestine are called enterocytes = brush border with microvilli + enzymes
| SUBSTANCE | DIGESTION |
| Carbohydrate | Polysaccharides = glycogen, starch, Disaccharides = lactose, sucrose, maltose Monosaccharides = fructose, glucose, galactose (hexose molecule) Saliva: a amylase (optimal pH 6.7) -> breaks polysaccharides into disaccharides. Inhibited by gastric juices. SI: salivary and pancreatic amylase + maltase/ lactase/ sucrase/ a dextrinase/ trehalase -> break down into monosaccharides. Oligosaccharidases deficiency leads to osmotic diarrhoea. Maximal at mid SI. Glucose/ galactose absorbed via secondary active transport by glucose/ Na+ cotransport (SGLT1/2) into cell. Transported from cell into interstitium via GLUT 2. Fructose enters via facilitated diffusion by GLUT 5. Insulin has little effect on intestinal transport of glucose. Maximal rate 120g/ hour. |
| Protein | Stomach: pepsins cleave peptide linkages (pepsinogen activated by HCl, optimal pH 1.6-3.2). Inhibited by pancreatic alkaline secretions in duodenum. Duodenal cap pH 2.0-4.0 -> 6.5 rest of duodenum. SI: Trypsin, chymotrypsin, elastase (endopeptidases) + carboxypeptidases of pancreas (exopeptidases) + brush border peptidases + intracellular peptidases. Thus occurs at 3 sites = intestinal lumen, brush border and enterocyte. Mostly require co-transport with Na+. Maximal at mid SI. |
| Fat | Lingual lipase – active in stomach, digest ~30% dietary TG Gastric lipase of little importance. Duodenum: Pancreatic lipase via colipase, bile-salt acid lipase Insoluble fats are emulsified by detergent action of bile salts to form micelles -> FFA absorbed via active transport |
| Water, Sodium, Potassium and Chloride | Water moves in response to osmotic gradients – maximal reabs in jejunum Na+ via active transport Na/K ATPase K+ secreted as component of mucous and active transport H/K cotransporters in distal colon. Loss of ileal or colonic fluids -> hypokalaemia. Cl- via Na/K/Cl cotransporters in basolateral membrane then into lumen via carriers. Cholera -> increases cAMP concentration thus protein kinase -> Cl- secretion. |
| Vitamins | Fat soluble vitamins ADEK Absorbed in upper SI Water soluble vitamins = Na+ cotransporters Except, B12 and folate are Na+ independent. **B12 – binds to intrinsic factor from stomach parietal cells, absorbed in terminal ileum, deficiency leads to pernicious anaemia. Required for RBC and CNS sensory pathways. |
| Calcium | 30-80% absorbed Requires 1,25-dihydroxycholecalciferol, adjusted to needs |
| Iron | 3-6% absorbed Iron in diet is ferric form (Fe3+) however absorbable iron is ferrous (Fe2+) via ferrous reductase at brush border. Stomach: forms complexes with ascorbic acid to permit solubility Duodenum: absorbed into enterocyte via DMT1 transporter -> some stored as ferritin, most transported to plasma by ferroportin 1, where it forms ferric form, bound to transferrin. 70% body iron is in Hb. Intestinal absorption regulated by 3 factors = dietary intake, iron stores and erythropoiesis, signalling mechanisms unknown. |
Endocrine Pancreas
Insulin
- Anabolic
- Secreted by B cells
- Polypeptide containing 2x chains AA linked by disulfide bridges
- Synthesised from prehormone
- Half life = 5 min
- Degraded by endosomal proteases intracellularly
- Actions
- Binds to insulin R (a/b subunit -> tyrosine kinase -> phosphoinositol 3-kinase -> translocation GLUT 4 to cell membrane.
- Uptake of glucose, AA, K+ into cells
- Protein and fatty acid synthesis
- Cell growth
- Regulation
- Stimulation
- Glucose
- AA
- GIP/ GLP-1/ gastrin/ secretin, CCK
- b keto-acids
- ACh
- Glucagon
- cAMP
- b R agonist
- Theophylline
- Sulfonylurea
- Inhibition
- SST
- a R agonist
- NE/E
- b R antagonist
- Thiazides
- K+ depletion
- Phenytoin
- Insulin
- Stimulation
Glucagon
- Catabolic
- Secreted by A cells
- Half life = 5-10 min
- Degraded mainly by liver
- Actions
- Acts via Gs -> activate adenylyl cyclase and cAMP -> protein kinase A -> activation of phosphorylase.
- Glycogenolysis, gluconeogenesis, lipolysis and ketogenesis
- High doses positive inotropic effect on heart
- Stimulates secretion of GH, SST and insulin
- Stimulation
- AA
- CCK, gastrin
- Cortisol
- Exercise
- Infection
- Stress
- b R agonist
- Theophylline
- ACh
- Inhibition
- Glucose
- SST
- Secretin
- FFA
- Ketones
- Insulin
- Phenytoin
- a R agonist
- GABA
Somatostatin
- Regulatory hormone
- Secreted by D cells
- Actions
- Inhibit secretion of insulin, glucagon and pancreatic polypeptides
Last Updated on September 24, 2021 by Andrew Crofton
Andrew Crofton
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