GI physiology Flashcards
Name the three salivary glands and state their locations.
Submandibular - under mandible
Sublingual - under tongue
Parotid - on side of face
What is the composition of saliva and what do each of the components function as?
Water - acts as solvent, to avoid water flooding into mouth, softens and moistens food
Mucins - combine with water to form mucus, lubricating function
alpha-amylase - breaks down polysaccharides into maltose and glucose
Electrolytes - affect tonicity and pH, e.g. bicarbonate keeps pH high to avoid damage to the teeth
Lysozyme - bactericidal (cleaves polysaccharide component of bacterial cell walls)
What is the nervous innervation of the salivary glands?
ANS - parasympathetic and sympathetic both stimulator.
Parasympathetic - watery, high volume saliva, stimulated by glossopharyngeal and facial nerves.
Sympathetic - small volume, viscous. High amylase content stimulated by beta2-adrenoceptors. alpha1-adrenoceptors.
Reflex control - presence of food in mouth stimulates chemoreceptors/pressure receptors to trigger saliva production.
What is the histology of salivary glands?
Alveolar feeding into ducts. 3 types of alveoli: - mucus - secrete mucus - serious - secretes enzymes, e.g. amylase - mixed - serous and mucus
What are the lower and upper oesophageal sphincters? What are their functions?
Thickened rings of smooth muscle (contract/relax to control open/closed state).
UOS above level of pharynx, prevents food regard
LOS before cardia of stomach (peristalsis of oesophagus in coordination with LOS relaxation). LOS should only be open when food/liquid passing through.
Where are the three constrictions of the oesophagus?
- cervical - 15cm from incisors
- thoracic - crossed by arch of aorta and left main bronchus (22.5cm and 27.5cm from IT respectively)
- diaphragmatic - passing oesophageal hiatus (40cm from IT)
At what vertebral level does the oesophagus start and end?
Lower level of Cricoid cartilage (C6) to T11-12 (where it enters the stomach)
How is the oesophagus peritonised?
Retroperitoneal.
What is the function of the oesophagus?
Carry found via contraction of the circular muscle (peristalsis) to the stomach.
Describe the process of swallowing to food reaching the stomach.
Bolus pushed to back of mouth by tongue.
Presence of bolus initiates reflex contractions of pharyngeal muscles (coordinated by swallowing centre (medulla). UOS relaxes and epiglottis covers larynx opening. UOS contracts (once food through).
Propulsion of bolus to stomach (10s) by peristalsis.
LOS relaxes and bolus enters stomach.
Vagal reflexes cause relaxation of thin, elastic smooth muscle of gastric fundus and body.
Why doesn’t the stomach rip with the pressure of food entering it?
Stomach arranged in rugae (pleats), which unfold and allow for increased space for food, without an increase in pressure of the stomach. When food leaves the stomach again, the stomach can fold back into rugae.
Why do we chew?
Prolong taste, defence against respiratory failure.
What is chewing controlled by?
Voluntary by somatic nerves (skeletal muscles of jaw/mouth). Chewing can also be involuntary.
Reflex - contraction of jaw muscles creates pressure of food against gums/hard palate - sensed by mechanoreceptors that inhibit jaw muscles causing reduced pressure and another contraction.
What is the nervous supply to the oesophagus?
Vagus and sympathetic trunk.
How does the oesophagus form embryologically?
From cranial part of primitive gut tube as a laryngo-tracheal diverticulum from the ventral wall. A tracheo-oesophageal septum divides the forgot into the trachea and oesophagus.
What are the four tunics of the oesophagus?
Mucosa - non-keratinised stratified squamous epithelium.
Submucosa - ducts from glands responsible for lubrication.
Musculares externa - upper 1/3rd skeletal, lower 2/3rd smooth muscle.
Adventitia - connective tissue.
What are the 5 parts of the stomach?
Cardia Fundus Body Pyloric antrum Pylorus
What is the fundus mostly filled with? What is its function?
Gas
Storage (stretchy and thin)
What causes hiccups?
If stomach too full can push on diaphragm and irritate it and cause hiccups.
What is the function of the body of the stomach?
Mucus, HCl and intrinsic factor production.
What is the function of the antrum of the stomach?
Mixing and grinding. Gastric production (enters circulation).
What is the function of the pylorus?
Controls discharge of chyme into the duodenum.
What is the function of the rugae?
Pleats that allows for expansion of stomach without an increase in pressure.
What pH is the stomach lumen at?
2.
What are the functions of the stomach?
Temporary storage of food (prevents mass digestion and osmotic pull)
Initiation of digestion
Control of delivery to small intestine
Sterilization (HCl)
Intrinsic factor production (produced by parietal cells)
Where is the intrinsic factor B12 complex absorbed?
Ileum.
Peristaltic waves move from where to where?
Body to antrum.
Which part of the stomach has the most powerful contractions?
Antrum (thick muscle), body only has thin muscle (just where secretions move out), contraction of the pyloric sphincter creates rotation of blood.
Which muscle is responsible for the squeezing type motion that breaks down food?
Oblique muscle layer.
What cells are responsible for creating the peristaltic rhythm of the stomach?
Pacemaker cells (in longitudinal muscle where UOS is).
What is slo wave contraction and BER in the stomach?
Slow waves caused by spontaneous depolarisation to create basic electrical rhythm (BER) - fundamental property of longitudinal muscle fibres.
Slow wave depol is sub threshold so requires further depolarisation to induce APs leading to contraction.
What determines the stretch of contraction of the stomach?
Number of APs fired.
What things control gastric motility?
Gastrin - increases contraction
Food in stomach –> distension – vagus/ENS reflexes –> ACh –> increased contraction
Fat/AAs/hypertonicity in duodenum –> inhibition of motility.
What cells release gastrin?
G cells in the antrum of the stomach.
What junctions are parietal cells connected by?
Tight junctions.
Describe the cellular process by which parietal cells pump HCl into the stomach lumen.
CO2 diffuses into cell, combines with water –> carbonic acid (enzyme carbonic anhydrase catalyses this reaction). Carbonic acid splits into hydrogen ion and bicarbonate.
Hydrogen pumped out (actively by KH pump in exchange for K).
At basolateral membrane, bicarbonate transported out in exchange for Cl (=transient alkalinity of blood).
Cl exits apical membrane –> osmotic potential –> water follows via paracellular pathway.
Leads to pH 2 of stomach lumen.
On the basolateral surface of the parietal cell what receptors for what substances can be found?
Gastrin, histamine, prostaglandins and acetylcholine which all influence the KH pump.
What happens when gastrin binds to its receptor on the parietal cell?
It increases intracellular Ca, which activates its protein kinase A which activates KH pump –> decreased pH.
What happens when histamine binds to its receptor on the parietal cell?
Histamine binds unique stomach receptor (H2) - it is G coupled receptor which activates adenyl cyclase which converts ATP to cAMP. cAMP activates protein kinase A –> stimulates KH pump –> decreased pH.
What happens when ACh binds to its receptor on the parietal cell?
Acts on M3 muscarinic receptor –> increase in intracellular Ca –> activates its protein kinase C –> stimulates KH pump –> decreases pH.
What happens when prostaglandin binds to its receptor on the parietal cell?
Receptor coupled to inhibitory G protein which inhibits adenyl cyclase –> prevents ATP being made into cAMP and reduces KH activity –> increased pH.
What sort of chemicals are histamine and gastrin?
Gastric is a hormone and histamine is a paracrine substance.
What is involved in the cephalic phase?
Sight, smell, taste of food triggers vagus to produce ACh and triggers G cells to produce gastrin. Gastrin/ACh act on ECL (enterochromaffin like) cells to produce histamine.
What is involved in the gastric phase?
Food arrives in stomach causing distension of stomach –> vagal/enteric reflexes –> release of ACh, peptides in lumen stimulate G cell cells –> gastrin. ACh/gastrin act on ECL cells –> histamine –> increased parietal cell activity.
What are the mechanisms inhibiting gastric secretion?
Cephalic phase - stopped eating so vagal activity reduced.
Gastric phase - decreased pH means gastrin is reduced (neg feedback).
Intestinal phase - acid in duodenum triggers release of secretin and enterogastric reflex which both decrease in gastrin secretion, fat in duodenum causes GIP.
What is GIP?
Gastric inhibitory polypeptide - decreases gastrin secretion and parietal HCl secretion.
Why must you increase the pH of the chyme entering the duodenum?
Pancreatic enzymes denatured at pH 2 and only function above pH 5.
What are enterogastrones?
Hormones released from gland cells in the duodenal mucosa (secretin, CCK, GIP) in response to acid, hypertonic solutions, fatty acids or monoglycerides in duodenum. They prevent further build up in duodenum.
What are the two strategies enterogastrones use to prevent further build up of acid in the duodenum?
Reducing gastric emptying (inhibiting motility of wall/contract pyloric sphincter) and inhibit gastric acid secretion.
What cells secret pepsinogen?
Chief cells.
What does zymogen storage prevent?
Autodigestion.
What is the function of pepsinogen?
Pepsin liberated at low pH –> its back on pepsinogen and further cleaves itself (positive feedback).
What inactivates pepsins?
Neutral pH.
What kinds of cells are present in gastric glands and what do each secrete?
Mucous neck cells - mucus (also produced by surface epithelium)
Chief cells - pepsinogen
Parietal cells - HCl, intrinsic factor
What is the function of the mucus in the stomach?
Cytoprotective - prevents mucosal surface from mechanical injury, neutral pH (HCO3) protects against acid corrosion and pepsin digestion. Creates gel layer that prevents bicarbonate diffusing away.
Describe the embryological development of the stomach.
Wk 4 - gut tube dilates to produce stomach, differential growth leads to lesser and greater curvature.
Stomach undergoes 90 degree rotation around longitudinal axis and then anterior-posterior rotation.
What does the fact the stomach undergoes an anterior-posterior rotation mean in terms of its nervous supply?
R side of primitive stomach supplied by R vagus and L by L, so when stomach rotates, R vagus supplies posterior surface and L supplies anterior surface.
What are the four tunics of the stomach wall?
Serosa - connective tissue
Muscularis externa - longitudinal muscle, circular muscle, oblique muscle
Submucosa and mucosa (rugae)
Muscularis mucosa contraction helps expel contents of gastric glands.
What are the two major and minor lobes of the liver?
Major - right and left lobe
Minor - caudate and quadrate lobes
What does the falciform ligament of the liver separate?
Right and left lobe and attaches liver to anterior wall.
What does the coronary ligament attach the liver to?
Underside of the diaphragm on bare area.
What is the porta hepatis?
Fissure of the liver where the hepatic artery, portal veins and common hepatic duct, lymphatic and nerves enter/leave the liver.
What is the bare area of the liver?
Non-peritoneal area of R lobe where coronary ligament attaches to the liver.
How do secretions from the liver enter the duodenum?
Enters right and left hepatic ducts, which combine to form common hepatic duct, which combined with cystic duct of gallbladder to form the common bile duct which joints the pancreatic duct to enter the duodenal lumen via the major duodenal papilla.
What ribs is the liver lying deep to?
7-11.
What covers the liver?
Visceral peritoneum and connective tissue capsule.
What is the alimentary role of the liver?
Production of bile.
What are the components of bile?
Bile acids, lecithin (emulsification agent), cholesterol, bile pigments, toxic metals (modified by liver), bicarbonate.
What colour are hepatocytes?
Brown.
What are the functions of the hepatocytes?
Bile salt production, storage of excess nutrients, interconversion of nutrients and storage of important vitamins, Cu, Fe and removal of toxic substances (modification).
Where do bile pigments come from?
Breakdown of Hb from old/damage RBCs.
Bilirubin main pigment.
What does bilirubin change the colour of?
Makes bile yellow
Bacterial modification of bilirubin in the gut causes the brown colour of faeces and reabsorption from gut into blood stream and then excretion in urine makes urine yellow.
What are bile acids synthesised from?
Cholesterol.
What is the issue with bile acids?
They are very insoluble so need conjugation with glycine or taurine before secretion.
How are secreted bile salts recycled?
Via enterohepatic circulation (taken back up at end of ileum and transported back to liver by portal vein).
What controls the delivery of bile into the duodenum?
Sphincter of Oddi.
Describe the embryological development of the liver.
Liver primordium in 3rd wk (overgrowth of endoderm at distal foregut).
Liver bud (hepatic diverticulum) grows into septum transversum (mesodermal plate bw pericardial cavity and yolk sac).
Connection between liver bud and foregut narrows to form bile duct. Ventral outgrowth from bile duct forms gallbladder and cystic duct.
Diverticulum develops into core of mesochyme between heart and forgot.
Septum transversum gives rise to vessels of liver.
What is the blood supply to the liver?
Hepatic arteries (coeliac trunk branches) and hepatic portal vein.
The coeliac artery gives rise to common hepatic artery which gives rise to R and L hepatic arteries (supply R and L lobe of liver) and proper hepatic artery - supplies liver, gallbladder, part of stomach.
Blood from the hepatic arteries and portal veins travels through what in the liver?
Sinusoids (discontinuous capillaries).
What is the parasympathetic and sympathetic supply to the liver?
PS - vagus
S - coeliac plexus fibres
What is the lymphatic supply to the liver?
Lacteals found in centre of villi (absorb fat) and drain into mesenteric lymph nodes, then superior mesenteric or ileocolic lymph nodes before reaching cisterna chyli.
What creates the distinct septa of the liver.
Connective tissue capsule of portal hepatic extends inside liver and splits it into distinct septa (support). Vessels, ducts and nerves follow the septa and divide the liver into hexagonal lobules (at each corner there is a portal triad).
What is a portal triad?
Portal vein, hepatic duct and hepatic artery.
What is found at the centre of each lobule of the liver?
Central vein (these anatomise to form hepatic vein).
Where do the hepatic veins drain into?
IVC.
What structures radiate out from central veins?
Hepatic cords.
What lies in between each cord of hepatocytes?
Bile canaliculi (cleft like lumen).
What do bile canaliculi come to form?
Hepatic ducts.
What are the different anatomical parts of the gallbladder?
Fundus, neck and body.
Where is the gallbladder found?
Inferior surface of liver.
What is the function of the gallbladder?
Storage and concentration of bile (15-20x, more concentrated if not had fatty meal in a while) - absorbs Na and water follows.
How much bile can the gallbladder store?
Up to 50ml.
What is the function of the sphincter of Oddi?
Controls release of bile and pancreatic juice into duodenum. When contracted forces bile back up into gallbladder.
What causes the release of CCK?
Fat/amino acids in duodenum.
What is the function of CCK?
Relaxation of sphincter of odd and contraction of gallbladder muscularis to discharge bile into duodenum.
Also causes decrease in gastric emptying and increase in pancreatic enzyme secretion to increase digestion.
What is the function of secretin?
Decreases gastric acid secretion, gastric emptying and increases duodenal bile and pancreatic HCO3 secretion –> neutralisation.
What is secretin release in response to?
Acid in the duodenum.
How does the gallbladder dehydrate bile?
NaK ATPase on baso-lateral membrane of gallbladder cells creates constant inward driving force for sodium to enter through sodium channels, which pulls water with it –> dehydration.
What does the gallbladder form from embryologically?
Ventral outgrowth from primordial bile duct.
What is the blood supply to the gallbladder?
Cystic a. (from hepatic artery properly) and drainage via cystic vein (drains into portal vein).
Cystic arteries lie in which structure?
Triangle of Calot - formed by inferior surface of liver, common hepatic duct and cystic duct.
What is the parasympathetic and sympathetic supply to the gallbladder?
S (and sensory) - coeliac plexus
PS - vagus (leads to increased contraction of gallbladder)
Where does lymph from gallbladder drain into?
Cystic lymph nodes –> coeliac lymph node.
What are the 3 tunics of the gallbladder wall?
Mucosa (rugae)
Muscularis (contraction)
Serosa (connective tissue)
What are the different parts of the pancreas?
Head, tail, body.
What vessel forms behind the neck of the pancreas?
Portal vein.
What is the function of the pancreas?
Secretion of bicarbonate by duct cells.
Secretion of digestive enzymes by zymogen acing cells.
What is the optimum pH for zymogens?
7.
Why are enzymes release in zymogen form?
To prevent auto digestion.