Overview of GI function Flashcards
Functions of GI system
- Digestion - chemical and mechanical breakdown of large particles into those small enough to be absorbed
- Absorption: movement of organic substrates, electrolytes, vitamins and water across digestive epithelium into blood/lymph
- Motility: smooth muscle contractions - movement and mixing
- Motility can be peristalsis (oesophagus and stomach), segmentation contractions (small intestine), migrating motility complex (small intestine whilst fasting), haustral contractions (large intestine) and mass movements (large intestine)
- Gastrocolic reflex: 30 mins after meal - promote removal of material from large intestine to rectum
- Secretion: water, mucus, acids, enzymes, buffers and salts
- Excretion: waste material
What is the mesentery?
Attaches to posterior abdominal wall
Gastrocolic reflex
30 mins after meal - promote removal of material from large intestine to rectum
GI tract wall
- Mucosa: mucous membrane (epithelial layer), lamina propria (connective tissue and GALT), muscularis mucosa (smooth muscle)
- Submucosa: connective tissue containing nerves, blood vessels and glands
- Muscularis externa: circular and longitudinal layers of smooth muscle, nerves
- Serosa: lubricating outer layer of connective tissue connected to mesentery
Where are lymphocytes found?
Appendix
Enteric nervous system
Myenteric and submucosal plexus + input from autonomic, sympathetic and parasympathetic nervous system (VAGUS)
Functions of oral cavity
- Salivary secretions
- Salivary glands: parotid glands (ear), sublingual (below tongue), submandibular (mandible) - main is submandibular
- Moistens and lubricates food
- Dissolves food
- Initiates digestion of polysaccharides (amylase) and lipids (lipase)
- Forms bolus (compact ball of food)
antibacterial actions (IgA and lysozyme)
What are the three pairs of salivary glands?
Parotid, sublingual and submandibular
Function of stomach
Stores ingested food, secretes (intrinsic factor, HCL, pepsinogen, mucus) to form gastric juice, mechanical breakdown of food = chyme
Key regions of stomach
cardiac system, cardia, fundus (pacemaker cells), body (oxyntic glands), antrum (pyloric glands), pyloric sphincter, rugae (deep folds to increase volume) = receptive relaxation
How is vit B12 broken down?
Intrinsic factor
Function of small intestine
Digestion and absorption of nutrients
Three sections of small intestines
Duodenum, jejunum and ileum (vit B12 ingested here)
Function of duodenum
- Receives secretions from liver, gall bladder and pancreas
- Sphincter of Oddi stops bile entering duodenum
- Precipitation of bile = gall bladder stones
Function of sphincter of oddi
Stops bile entering duodenum
Function of pancreas
Release of digestive enzymes and alkaline fluid into duodenum
- Enzymes and bicarbonate secretion
- Bile: bile salts (cholesterol), lecithin (phospholipid), alkaline secretions and metabolic wastes including bilirubin (from RBCs break down in liver)
- Bile emulsifies and forms micelles
- Majority of bile salts recycled back to liver via hepatic portal system
Function of hepatocytes
Synthesise bile into bile duct
What is bile?
bile salts (cholesterol), lecithin (phospholipid), alkaline secretions and metabolic wastes including bilirubin (from RBCs break down in liver)
Substrates of trypsin
Proteins (attack different peptide bonds) -> small peptides and amino acids
Substrates of lipase
Triglycerides -> monoglycerides and fatty acids
Substrates of amylase
Starch (amylose and amylopectin) -> maltose and a-limit dextrin
Areas of large intestine
Cecum (and appendix), ascending, transverse, descending and sigmoid colon and rectum
Functions of large intestine
reabsorb water and salt from chyme, stores and compacts faecal matter prior to defecation
Defecation reflex
mass movement of faeces - stretch of rectum - internal anal sphincter relaxes and contraction of rectum and sigmoid colon. Defecation occurs when external anal sphincter relaxed
Function of haustra
Moves material down
Gut microbiome functions
- Ferment dietary fibre: fatty acids for absorption and gas
- Synthesise vitamin B12, folate and K
- Protects gut from invasion by pathogens and educate your immune system
Helicobacter pylori
- Helicobacter pylori survives acidic environment by secreting ammonia
- Helicobacter pylori causes stomach ulcers
Nervous innervation of stomach movement
Vagus nerve and myenteric plexus (enteric N/S)
Out of stomach
steady flow of partially digested liquid chyme, largely sterile
Function of oxyntic glands
Oxyntic glands release acid by parietal cells
Function of pyloric glands
Pyloric glands contain regulatory G and D cells
Mucous secreting pits
At neck of pits are mucous secreting cells - parietal cells (HCl acid and intrinsic factor) - chief cell (pepsinogen) - G and D cells (gastrin and somatosensin)
Function of gastrin
stimulates gastric acid secretion and motility and is released into mucosal blood vessels
Intrinsic factor and vit B12
- Released from parietal cells
- Vit B12 is water-soluble vitamin and must be bound to intrinsic factor to be absorbed which facilitates absorption into terminal ileum
- B12 transported by transcobalamin
- Pernicious anaemia: loss of parietal cells due to auto-immune attack of stomach = no intrinsic factor
Gastric acid production
- Produced by parietal cells
- CO2 enters parietal cell
- CO2 combines with water to form carbonic acid (carbonic anhydrase)
- Carbonic anhydrase dissociates into bicarbonate and hydrogen ions
- H+ enters stomach anterior through proton pump
- H+ in means K+ out (electroneutral exchange of ions)
- Antiporter system: HCO3- out and Cl- in - passive
- After meal, fluid leaving stomach is alkaline
- Cl- that enters in proton pump enters stomach lumen and combines with H+ to form HCl
Gastric acid function
- Kills ingested bacteria
- Denatures proteins - particle breakdown
- Facilitates gastric lipase
- Converts pepsinogen to pepsin for protein digestion
- Proton pump inhibitors and H2 receptor antagonists used as treatment for acid
Gastric acid complications
- Stomach mjucosa protected from gastric acid by gastric-mucosal barrier
- A layer of bicarbonate-rich mucus
- Luminal membranes impermeable to H+ ions
- Tight junctions between mucosal cells
- Replacement of mucosal cells every 3 days
- Peptic ulcers arise by breaking down of stomach lining and acid reflex into oesophagus and excess acid entering duodenum
- Treat using antibiotics (helicobacter pylori), PPIs and H2 histamine receptor antagonists
Neuronal regulation - intrinsic nerve plexus
- Enteric nervous system
- Myenteric plexus: smooth muscle contraction and tone
- Submucosal plexus: controls GI secretions and blood flow
- Short reflexes: stimulus - nerve plexus - response
- ACh and NO increase motility
Neuronal regulation - external nerve plexus
- P/S vagus nerve and sympathetic influence on motility and secretions
- Long reflexes: stimulus - CNS - nerve plexus - response
Smooth muscle pacemaker
GI tract has basic electrical rhythm generated by pacemaker cells in muscularis extern
Gastric secretions and stomach emptying
- Cephalic: initiated by smell/taste, involved higher brain centres, Ghrelin hormone, initiates gastric secretions and motility
- Gastric: initiated in stomach, mediated by enteric and P/S nervous system and gastrin - increased gastric secretions and motility
- Intestinal: initiated in duodenum, mediated by inhibitory hormones, slows gastric emptying to optimise digestion and absorption within small intestine, enterogastric reflex
Cephalic phase
- Sight, smell, taste or thoughts of food
- CNS - parasympathetic (vagus)
- Submucosal plexus
- Mucous cells secrete mucus, chief cells secrete pepsinogen and parietal cells secrete HCl
- Ghrelin - released from flaccid stomach, acts on hypothalamus to stimulate appetite
- Ghrelin levels increase after weight loss, lower in people with obesity, higher in anorexia nervosa, higher in Prader-Willi syndrome
gastric phase
- Arrival of food in stomach = secretions and contractility
- Stretch receptors - submucosal and myenteric plecus
- Mucus cells - mucus, chief cells - pepsinogen, parietal cells - HCl, G cells - gastrin
- CNS (parasympathetic vagus)
- Histamine stimulates parietal cells to release HCl
- Gastrin released when pH falls - increases secretions and motility
Inhibiting stomach emptying
- Duodenal distension
- Irritation of duodenum mucosa
- Increase in fats and proteins
- Increase in acidity
- Increase in osmolarity
- Act on receptors in duodenum to trigger enterogastric reflex
Function of leptin in fat
Maintains fat stores
Intestinal phase
- Enterogastrones inhibit gastric acid secretions and gastric emptying
- Stretch and chemoreceptors - myenteric plexus (negative feedback)
- Stretch and chemoreceptors - CCK, secretin and somatosensin
- Negative feedback on chief cells (decreased pepsinogen) and parietal cells (decreased HCl)
- CCK: released by L cells in small intestine, stimulates pancreatic secretions and gall bladder contraction
- Secretin: released by S cells in response to low pH, stimulates pancreatic and bile secretions
- Somatostatin: released by D cells, inhibits everything
Function of leptin
Maintains fat store - released from adipose tissue and acts on hypothalamus to suppress appetite
How to inhibit stomach emptying
- Duodenal distension
- Irritation of duodenum mucosa
- Increase in fats and proteins
- Increase in acidity
- Increase in osmolarity
- Act on receptors in duodenum to trigger enterogastric reflex
Function of somatosensin
Released by D cells, inhibits everything
Function of secretin
Released by S cells, in response to low pH - stimulates pancreas and bile secretions
Function of cholecystokinin
Released by I cells in small intestine, stimulates pancreatic secretion and gall bladder contraction
