Week 9 - Gastrointestinal physiology

main purposes of digestion - summary (2)

1. Mechanically and chemically break down food into the smallest absorbable components

2. Absorb nutrients into the body

mechanical breakdown - purpose and methods overview (4)

makes food more accessible

Chewing and masticating food breaks it into smaller pieces -> increased surface area of food exposed to gastric acids and enzymes

Stomach peristalsis = smooth muscle contraction for mechanical churning of food

pH -> acidic environment denatures proteins

chemical breakdown - purpose and methods overview (5)

makes nutrients absorbable

Enzymes secreted into lumen of gut breaks down macromolecules into smaller subunits -> enzymes secreted by accessory organs of digestive tract and epithelial cells lining digestive tract

Proteins -> amino acids

Carbohydrates -> sugars

Fats -> free fatty acids and monoglycerides

organs of digestion - list in order from mouth to anus (1@)

1. Mouth (oral cavity)

2. Salivary glands

3. Saliva has alkaline pH

4. Pharynx

5. Oesophagus

6. Liver

7. Gallbladder

8. Stomach

9. Pancreas

10. Small intestine -> duodenum, jejunum, ilium

11. Large intestine

12. Anus

gastric functions - summary list (7)

1. Ingestion

2. Mechanical processing -> increase surface area of food being digested

3. Digestion -> chemical breakdown of food

4. Secretion -> epithelial secretions into GI tract

5. Propulsion -> peristalsis and segmentation

6. Absorption -> movement across digestive epithelium

7. Excretion

mechanical digesiton in the mouth

breaks into smaller pieces and mixes with saliva to form bolus

chemical digestion in the mouth - enzymes (2)

Amylase = enzyme found in mouth -> begins starch digestion at pH of 6.5 or 7.0 and ceases when pH reaches 2.5

Lingual lipase = enzyme secreted by glands in tongue -> begins breakdown of triglycerides into fatty acids and glycerol

mechanical digestion in the stomach

motility turns food bolus into chyme

chemical digestion in the stomach - overview

cells secrete HCl to create highly acidic content → pH ~2-4 with food inside

gastric lipase continues fat digestion → triglycerides split into milk fats most effectively at pH 5-6 (infant stomach)

roles of HCl in chemical digestion int he stomach (7)

• Denatures proteins

• Activates lingual lipase and pepsin

• Transforms pepsinogen into pepsin -> breaks peptide bonds between certain amino acids

• Breaks up connective tissues and plant cell walls

• Liquifies food to form chyme

• Converts ingesting ferric ions to ferrous ions -> can be absorbed and utilized for Hb synthesis

• Destroys ingested bacteria and pathogens -> antimicrobial properties

strcuture of gastric pits

invaginations with mucosa lined with mucous secreting endothelial cells, parietal cells and chief cells

Mucous cells produce 1-3mm thick layer of mucous -> protect stomach walls form being digested

gasric pits - chief cells (5)

protein digestion

secrete pepsinogen (inactive zymogen) → HCl converts to pepsin (active form) at more anterior regions of gastric pit which is lined by mucous cells

pepsin actiates more pepsinogen → positie feedback loop

secretes gastric lipase → digests butterfat of milk in infants

secretes chymosin → curdles milk by coagulating its proteins

gastric pits - parietal cells (3)

secretes intrinsic gactor (gastric enzyme) essential for absorption of B12 by small intestine

necessary for RBC production → pernicious anaemia

more superficial to chief cells in put and secrete HCl

gastric puts - D cells

secrete regulatory protein (somatostatin) and ECL cells secrete histamine -> both somatostatin and histamine regulate HCl secretion

chemical and mechanical digestion in small intestine - summary

chemical digestion via enzymes found at microvilli cell surface turns macromolecules into monomers

wealk peristalsis compared to stomach → chyme remains for 3-5 hours

segmentation allows for local mixing of chyme with itnestinal juices

small intestine - environment and strcuture

alkaline environment

circular folds, villi and microvilli increase surface area for digestion

role of brush border enzymes in small intestine (2)

Trypsin chymotrypsin and carboxypeptidases = break down proteins in chyme into smaller polypeptide residues -> digested into smaller di- and tri- peptides that can be absorbed at brush border

Lactase, maltase and sucrase = break down carbohydrates into disaccharides via pancreatic amylase

• Lactose -> galactose and glucose via lactase

• Maltose -> glucose and glucose via maltase

• Sucrose -> glucose an fructose via sucrase

accessory organs of digetion - list (2)

panceas

liver

role of pancreas in digestion (2)

endocrine → release insulin and glucagon

exocrine → releases enzymes that aid digestion directly into lumen of small itnestine duodenum + neutralises acid by secreting bicarbonate into chyme

role of liver in digestion

Bile emulsifies fat

Fat in chyme will form large droplets in aqueous chyme -> bile breaks up droplets into small micelles

Micelles increase surface area of triglycerides dissolved in chyme -> allows for lipases dissolved in chyme to target fat glycerol bonds

main function of large intestine

Recovers remaining water that is secreted into the gut via osmosis and into vascular capillaries inside villi -> small intestine absorbs majority

Forms solid faeces from liquid chime -> diarrhoea when too little water is absorbed

motility in large intestine - summary

peristaltic waves at 3-12 contractions/ minute

motility in large intestine - elevator movement process (3)

Haustral churning = elevator movement where relaxed pouches are filled from below by muscular contractions

Gastrolienal reflex = relaxation of ileocecal sphincter when stomach is full (gastric hormone) so small intestine will empty and make room

Gastrocolic reflex = strong peristaltic wave that moves contracts of transverse colon into rectum when stomach fills

chemical digestion in the large intestine

does not involve secretion of enzymes → only mucous

large intestine - gut microflora roles (5)

• Ferment undigested carbohydrates -> CO2 and methane gas

• Ferment undigested proteins -> simpler substances (indoles)

• Turn bilirubin into simpler substances that produce colour

• Produce vit K and B in colon

• Produce gas

neural control of digestion - components and difference (2)

enteric nervous system (ENS) = intrinsic to walls of gastric itnerstinal system

ANS = extrinsic

meaning of ENS control of digestion being intrinsic

neural control is more or less independent of what's going on in the rest of the body -> has its own complete set of motor, sensory and inter neurons located within wall of digestive tract

Has intrinsic sensory neurons -> IPAN

role of ENS inr esponse to arrival of bolus (3)

localiused response to stimuli → therefore:

Regulation of smooth muscle activity → contraction of smooth muscle superior to bolus, relaxation of muscle inferior to bolus

Regulation of absorption and secretions

Regulation of blood vessels through the gut

PNS and SNS effects on digestion

PNS = increase muscle tone, increase gland secretion

SNS = muscle relaxation, decreased gastric blood flow

HCl prouction via parietal cells of gastric pits - process (3)

H+: Carbonic anhydrase inside cell mediates production of H+ via CO2 and water → produces carbonic acid which rapidly dissociates into bicarbonate and H+

Cl-: Anion exchanger on basal membrane exchanges bicarbonate for chloride ions → bicarboante into interstitial

chloride and hydrogen ions released into lumen → react to form HCl

PNS stimulation of ECL activity on HCl production by parietal cells (2)

1. PNS stimulation upregulates ECL cell activity by releasing acetylcholine that binds to muscarinic receptors

2. ECL cells secretes histamines which binds to H2 receptors on basal membrane of parietal cells -> promotes HCl production

antihistimines on inhibiting HCl production by parietal cells

Antihistamines that target H2 receptors can be used to suppress HCl production → impair ECL cell stimualtion and thus PNS innervation

PNS stimulation of D cell activity on HCl production by parietal cells (4)

1. PNS stimulates D cell activity by releasing acetylcholine that binds to muscarinic receptors

2. D cell produces somatostatins

3. Somatostatins bind to somatostatin receptors on ECL cell and inhibit histamine release

4. Reduced HCl secretion

HCl production in antrum of stomach (3)

has G cells that release gastrin → increase pH

1. Gastrin secretion = stimulated by presence of amino acids in lumen of gut and by increase in pH

2. Gastrin binds to gastrin receptor on D cells to promote H+ secretion and ECL cells to promote HCl production

3. Somatostatin produced by D cell can bind to somatostatin receptors on G cell to inhibit gastrin production

stages of gastric secretions - list (3)

1. cephalic → from head and in anticipation of eating

2. gastric → bolus of food entering stomach engages local neural relfexes to secrete gastric juices into stomach

3. intestinal →regualtion of chyme release from stomach into small intestine

cephalic stage of gastric secretions - steps (4)

1. Receptors for sight, smell and taste of food activated + thoughts of food

2. Stimulatory neural effects integrated in cerebral cortex and hypothalamus

3. Hypothalamus to Medulla -> stimulates parasympathetic impulses along Vagus nerves

4. Vagus nerve stimulates gastric secretion by gastric glands -> reduces pH

   • Non-cholinergic nerve fibres stimulate G cells -> gastrin production -> gastrin stimulates gastrin glands

   • Cholinergic nerve fibres stimulate gastric glands

gastric stage of gastric secretions - steps (4)

1. Stretch receptors detect distension of stomach

2. vasovagal reflexes and local reflexes increase stomach secretory activity via acetylcholine stimulating G cell, ECL cell and parietal cells

• Vasovagal reflex also stimulates D cells

3 Chemoreceptors detect increased pH of gastric juices and stimulate G cells to release gastrin into blood

4. increased stomach secretory activity

intestinal stage of gastric secretions

Presence of low pH, partially digested food, fats, or hypertonic solution in duodenum as stomach begins to empty stimulates enteric gastrin released into blood

brief increase in stomach secretory activity

enzymes involved in digestion of carbohdyrates - primary enzyme (2)

salivary and pancreatic amylases

enzymatic digestion of carbohydrates - by organs (4)

Mouth = salivary amylase

Oesophagus and stomach -> nothing happens

Duodenum = pancreatic amylase

Small intestin brush border enzymes act on disaccharides -> maltase, sucrase, lactase

• Produces monosaccharides -> fructose, glucose, galactose

enzymatic digestion of proteins - stomach

HCl denatures and pepsin turns proteins in peptides

enzymatic digestion of proteins - pancreas (3)

digestive enzymes (trypsin, chymotrypsin, carboxypeptidase) split bonds between different amino acids

Proteolytic enzymes = secreted as zymogens -> can digest host tissues so only activated within lumen of gastrointestinal tract

Inactive forms = trypsinogen, procarboxypeptidase, chymotrypsinogen -> zymogens

enzymatic digestion of preotins. - small intestine

Brush border enzymes = aminopeptidase or dipeptidase

cleave amino acid at amino end of molecule to split dipeptide

enzymatic digestion of lipids - mouth

lingual lipase

enzymatic digestion of lipids - stomach

gastric lipase breaks large fat droplets into smaller ones

enzymatic digestion of lipids - small intestine

Bile emulsifies fats -> pancreatic lipase splits into fat droplets into fatty acids and monoglycerides in duodenum

No enzymes in brush border

defecation reflex (3)

1. Stretch in wall of rectum detected by sensory nerve fibres

2. Spinal cord mediated parasympathetic NS reflex

   • Walls of sigmoid colon and rectum contract and internal anal sphincter relaxes → allows for faeces pushed into anal canal

3. External anal sphincter = skeletal muscle -> under voluntary control