Structure + Function: Upper GIT, Liver, Small Intestine and Large Intestine Flashcards
Which structures in the Oesophagus regulate movement into and out of it?
Upper and Lower Oesophageal Sphincters
Describe the structure of the Stomach wall
▷Serosa: (intraperitoneal!)
▷Muscularis Externae: 3 Layers: Longitudinal (outer), Circular (middle), Oblique (inner)
▷Submucosa and Mucosa: folded (= rugae) when empty → stretch as stomach fills
▷Lumenal surface (Mucosa): surface mucus cells → gastric pits → gastric glands → mucus neck, parietal and chief cells
Describe the structure of the Oesphageal wall
▷Mucosa: Non-keratinised stratified squamous epithelium
▷Submucosa: submucosal glands -> secrete mucus into ducts
▷Muscularis Externa: upper 1/3 = skeletal muscle, lower 2/3 = smooth muscle
▷Adventitia: (retroperitoneal!)
What are the functions of the Stomach?
▷Temporary store of ingested material
▷Dissolves food particles and initiates digestive process
▷Controls delivery of contents to small intestine
▷Sterilises ingested material
▷Produces Intrinsic Factor (Vitamin B12 absorption)
How is chewing controlled?
▷Voluntary: somatic nerves -> skeletal muscles of mouth/jaw
▷Reflex: Food enters mouth -> pressure of food against gums, hard palate tongue -> increased pressure triggers mechanoreceptors -> inhibition of jaw muscles (jaw drops) -> reduced pressure -> contraction (jaw shuts) -> increased pressure -> continues…
What is in Saliva?
What are the functions of each?
▷Water -> solvent
▷Mucins -> major protein component -> mucin + water = mucus -> lubricates
▷ a-Amylase -> catalyses breakdown of polysaccharide to disaccharide
▷Electrolytes -> tonicity/pH
▷Lysozymes -> bacteriocidal -> cleaves polysaccharide component of bacterial cell wall
What is the reflex control mechanism of Swallowing?
▷Oesophageal phase: peristaltic wave sweeps along oesophagus -> propels bolus to stomach (10s)
▷LOS relaxes as bolus nears stomach -> allows entry of bolus into stomach
▷Receptive relaxation of Stomach -> via Vagal reflexes -> thin, elastic Gastric Fundus and Body relax -> Stomach volume = 50ml -> 1500ml (with no change in pressure)
What are the Lobes of the Liver?
4 Lobes:
▷Right (Biggest Lobe)
▷Left
▷Caudate
▷Quadrate
What is the Porta of the Liver?
▷Carries blood vessels (hepatic portal vein, hepatic artery), lymphatic vessels, ducts (right/left hepatic ducts, common hepatic duct) and nerves (hepatic nerve plexus) to the inferior surface of Liver
Which ducts lead to the Major Duodenal Papilla?
▷Common Hepatic duct (Liver)
▷Joined by Cystic Duct (Gallbladder)
▷Combines to form Common Bile duct
▷Common Bile duct + Pancreatic duct → Major Duodenal Papilla (Sphincter of Oddi)
What is located at each corner of each Hepatic Lobule?
What is located in the centre of each Hepatic Lobule?
▷Portal Triad: hepatic portal vein, hepatic artery, hepatic duct
▷Central vein (drains hepatic sinusoids -> hepatic veins -> IVC)
What is the function of the Liver hepatocytes?
▷Bile synthesis (-> canaliculi -> hepatic ducts)
▷Nutrient storage (glycogen, fat, vits (B12, A, D, E, K), Cu, Fe)
▷Nutrient interconversion (ie. AAs into glucose for gluconeogenesis)
▷Detoxification
What is the main component of Bile Pigments and what is this component made from?
▷Main component of Bile Pigments = Bilirubin
▷Bilirubin is made from breakdown products of Hb from old/damaged RBCs (majority broken down in the Liver)
Why are faeces brown and why is urine yellow?
▷Liver breaks down Hb in old/damaged RBCs to Bilirubin -> Bilirubin = yellow
▷Bilirubin which passes through the GIT is modified by bacterial enzymes which makes it brown -> brown faeces!
▷Bilirubin which is reabsorbed across the intestinal wall gets excreted in the urine -> yellow urine!
Where are Bile acids made and what are they made from?
How much is made per day?
▷They are synthesised in the Liver from cholesterol
▷0.5g/day -> replaces the bile salts which are lost in the faeces! (5%)
How increases Bile salt solubility?
How are Bile salts recycled?
▷Conjugation with glycine or taurine
▷Secreted Bile salts are recycled via the Enterohepatic circulation (reabsorbed at the Terminal Ileum)
What triggers Bile secretion?
Fat in the Duodenum -> release of CCK -> relaxes Sphincter of Oddi, contracts GB
What is the function of the Gallbladder?
Concentration of Bile (5-20x)
absorbs Na+ and H2O
What are the combined effects of Secretin and CCK?
▷Secretin -> Acid Neutralisation
▷CCK -> Digestion of Fats/AAs
How does Secretin neutralise Gastric Acid?
Triggered by Acid in the Duodenum
▷Inhibits Gastric Acid secretion ▷Inhibits Gastric emptying ▷Increases Duodenal HCO3- secretion ▷Increases Pancreatic HCO3- secretion ▷Increases Bile duct HCO3- secretion
Gastric Acid Neutralisation switches off Secretin release
How does CCK enable Fat/AA digestion?
Triggered by Fats/AAs in the Duodenum
▷Inhibits Gastric emptying
▷Increases Pancreatic enzyme secretion
▷Gallbladder contraction
▷Relaxes Sphincter of Oddi
What is the function of Villus and Cypt cells?
▷Villus cells = absorption of nutrients + H2O
▷Crypt cells (epithelial cells lining the Crypts of Lieberkuhn) = secretion of Cl- (NB. CFTR channels) and H2O
What is the function of the Duodenum, Jejunum and Ileum?
▷Duodenum = Gastric acid neutralisation, Digestion, Iron absorption
▷Jejunum = Nutrient absorption (95%)
▷Ileum = NaCl and H2O absorption (chyme dehydration -> faeces!)
What is the function of Microvilli, Villi and Circular Folds (Plicae) in the Small Intestine?
To enhance absorptive surface area
What are the effects of the Parasympathetic NS on intestinal motility?
Which Nerve is involved?
▷Increases intestinal motility -> increased contractions
▷Vagus Nerve
What are the effects of the Sympathetic NS on intestinal motility?
▷Decreases intestinal motility -> decreased contractions
What are the effects of the Autonomic NS on BER?
▷No effect of ANS on BER
What is the Law of the Intestine?
What is it mediated by?
▷If intestinal smooth muscle is distended (eg. by bolus of chyme): muscle on oral side of bolus contracts and muscle on anal side of bolus relaxes -> moves bolus into area of relaxation towards colon
▷Mediated solely by Neurones in Myenteric Plexus (ENS) -> ie. would occur even in an isolated small intestine!
What is the difference between Segmentation and Peristalsis?
▷Segmentation = most common during meal -> rhythmic and contraction relaxation of intestinal segments -> provides thorough mixing of digestive enzymes with chyme, and brings chyme into contact with absorbing surface
▷Peristalsis = begins when segmentation stops (post-prandial) -> MMC
What is the function of the BER?
Sets the frequency of the segmentation contractions
What is responsible for generating the BER?
Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer (NB. gastric motility!)
What hormone initiates the MMC and what is the MMC’s function?
▷Motilin
▷The MMC is the pattern of peristaltic activity travelling down the small intestine (starting in the gastric antrum) -> it aims to move undigested material into the large intestine AND limit bacterial colonisation of the small intestine
(bc then the bacteria would just accumulate over time feed on whichever food u ingest in the future!)
What causes cessation of the MMC?
▷Arrival of food in the stomach -> bc it initiates Segmentation contractions so Peristalsis stops! (ergo: MMC stops)
What is the Gastroileal Reflex?
▷Stimulated by the presence of food in the stomach and gastric peristalsis
▷Peristaltic contractions reaches the Ileum
▷Ileocaecal valve opens
▷Chyme enters colon
▷Distension of colon
▷Reflex contraction of Ileocaecal valve (preventing reflux of contents back into the small intestine
What are Teniae Coli?
What are Haustra?
▷Three bands of longitudinal smooth muscle on the colon surface -> (longitudinal smooth muscle layer is incomplete) -> runs the entire length of the colon
▷Small, segmented pouches of bowel separated by haustral folds (folds of Mucosa) -> gives the colon its “segmented” appearances
What type of epithelium is present in the Colon?
Simple Columnar Epithelium with Large, straight crypts (flat), and a large number of goblet cells (lubrication)
What is the function of the Colon?
Active absorption of Na+ -> osmotic absorption of H2O
NB. no important nutrient absorption role in humans
Why is there bacterial colonisation in the Colon?
What do intestinal bacteria ferment, and what are the products of bacterial fermentation?
▷Because chyme spends the longest time in the Colon than any other area of the GIT
▷Bacteria ferment cellulose from plant matter (bc they contain cellulase and humans don’t!)
▷Bacteria produce short-chain FAs, Vitamin K, and Gas (flatus)
What is the difference between the Internal Anal Sphincter and the External Anal Sphincter?
▷Internal Anal Sphincter = smooth muscle (thicker muscularis mucosa than rest of GIT)
▷External Anal Sphincter = skeletal muscle
What type of junctional epithelium is present in the Anal canal?
Ano-Rectal junction
▷Simple Columnar epithelium -> Stratified Squamous epithelium
What is the Defecation Reflex?
▷Contraction of Rectum -> distension of rectal wall by faecal material -> stimulates mechanoreceptors in rectal wall -> stimulates urge to defecate
▷Relaxation of Internal and contraction of External Anal sphincters
▷Increased peristaltic activity in Colon (MMC) -> increased pressure on External Anal sphincter -> relaxes under voluntary control, causes expulsion of faeces -> Voluntary delay of defaecation - descending pathways
What is Constipation and what causes it?
▷Person has difficulty emptying the colon -> fewer than 3 bowel movements/week
▷Symptoms are due to distension of the rectum
What is Diarrhoea? What is the main cause of Diarrhoea?
▷Too frequent passage of faeces which are too liquid
▷Enterotoxinogenic Bacteria: ie. Vibrio Cholerae, Escherichia Coli
What is the mechanism of action of Vibrio Cholerae for causing Cholera?
▷Vibrio Cholerae produces enterotoxins which maximally turn on intestinal Cl- secretion from Crypt cells -> increases H2O secretion
▷Acts by elevating intracellular secondary messengers (ie. cAMP)
▷H2O secretion swamps the absorptive capacity of the Villus cells -> profuse watery diarrhoea (25L/day)
What is the treatment for secretory Diarrhoea and why?
▷Enterotoxins do not damage Villus cells -> therefore they are still able to absorb H2O from the lumen
▷Give sodium/glucose solution (ORS) -> absorbed by Villus cells and drives H2O absorption via the paracellular pathway
