Alimentary System Flashcards
Give the basic structure of the gut wall.
Mucosa: epithelium, lamina propria (loose connective tissue).
Submucosa: connective tissue (contains submucosal nerve plexus)
Muscularis: smooth muscle (contains myenteric nerve plexus). Circular muscle deep to longitudinal muscle.
Serosa/ adventitia: connective tissue +/- epithelium, holds gut to body wall.
Give features of the epithelial transition at the gastro-oesophageal junction.
From non-keratinising stratified squamous to simple columnar.
What are rugae?
Gastric folds - so the stomach can expand to accommodate food and reduce in size when empty.
Give features of the secretions of different parts of the stomach.
Cardia and pyloric region = mucus only
Body and fundus = mucus, HCl and pepsinogen
Antrum = gastrin.
Give the 2 types of contraction in the stomach.
Peristalsis (20%). Propels chyme towards colon. More powerful and moves from LOS to pyloric sphincter.
Segmentation (80%). Weaker. Fluid chyme towards pyloric sphincter, solid chyme pushed back to body.
Detail a gastric chief cell.
Protein secreting epithelial cell, with abundant rER and Golgi apparatus. Masses of apical secretion granules containing pepsinogen. Also secretes gastric lipase.
What is the result of HCO3- being trapped in mucus gel at the epithelial border?
The pH the epithelial cells are exposed to is 6-7, compared to 1-2 in the lumen.
Explain how HCl is produced by a secreting parietal cell.
Tubulovesicles (containing H+/K+ pumps) fuse with canaliculi.
Na+ / K+ pump –> K+ diffuse into canaliculi
CO2 and H2O converted into H+ and HCO3- by carbonic anhydrase
H+ exchanged with K+ in canaliculi by H+/K+ ATPase
HCO3- exchanged with Cl- on basal membrane of cell by antiporter.
Cl- diffuses into canaliculi and combines with H+
Canaliculi continuous with gastric lumen.
What do omeprazole and ranitidine do?
Omeprazole = inhibits H+/K+ antiporter on canaliculi membrane. Ranitidine = interferes with histamine receptor (H2 antagonist).
What do G cells, a type of enteroendocrine cell, do?
Secrete gastrin. Found in high concentration in pyloric antrum.
Travels in blood and binds to enterochromaffin-like (ECL) cells, which release histamine.
Result is parietal cells secrete more HCl (gastric juice).
Give three hormones which have an inhibitory effect on gastric function and are secreted in the intestinal phase of gastric secretion.
Gastric inhibitory peptide (from K cells).
Cholecystokinin (I cells).
Secretin (S cells).
All secreted by enteroendocrine cells at the epithelial surface of the duodenum.
What are the caudate and quadrate lobes of the liver described as, and describe their anatomical relations.
Arising from the right lobe but functionally distinct.
The quadrate lobe is bounded on the left by the fissure of the ligamentum teres and on the right by the fossa of the gallbladder.
The caudate lobe is bounded on the left by the fissure of the ligamentum venosum and on the right by the groove for the IVC.
Describe a liver lobule.
A lobule is hexagonal morphologically, and has 6 portal triads.
Each portal triad consists of a bile duct, HPV and hepatic artery.
The HPV and hepatic artery combine to form a sinusoid. All 6 sinusoids drain into the central canal.
A bile canal drains bile from near the central canal TOWARDS the bile duct.
How is glucose released by glycogenolysis?
Broken down into glucose-1-phosphate, converted into glucose-6-phosphate by phosphoglucomutase. G-6-P can be fed into glycolysis.
Describe the Cori cycle.
Lactate from anaerobic glycolysis travels to the liver, where it is converted into glucose, requiring 6ATP. Glucose returns to the muscle cell, where lactate is reproduced and 2ATP is produced for the muscle.
How many ATPs are required to be invested in the glucose-alanine cycle?
6 ATP for the conversion of pyruvate to glucose (gluconeogenesis).
4 ATP for deamination of glutamate, producing urea.
Describe ketogenesis in the liver.
2 acetyl-CoAs derived from fatty acids can be combined to form Acetoacetyl CoA, which delivers acetyl-CoA to cells.
Give the functions of the lipoproteins produced by the liver.
VLDL - transports FAs to tissues
LDL - transports cholesterol to tissues
HDL - empty, picks up excess cholesterol from plasma.
What is glycocalyx?
A rich carbohydrate layer covering the epithelial border of the small intestine, protecting the cells from the digestive enzymes in the lumen. Also traps a layer of water and mucus called the “unstirred layer” which regulates the rate of absorption from the small intestine.
What happens to the abundance of goblet cells as you pass through the small intestine and colon?
The abundance increases, as mucus facilitates passage of material and the contents of the lumen become less and less wet as you progress since water is absorbed.
What are Paneth cells?
Cells found in the Crypts of Lieberkühn, containing large acidophilic granules containing lysozymes, glycoproteins and zinc. Engulf some bacteria and protozoa.
How can you distinguish between different regions of the small intestine?
DUODENUM: Brunner’s glands present. Submucosal coiled mucous glands which secreted alkaline fluid into the base of crypts. (Neutralises acidic chyme).
JEJUNUM: numerous, large folds in the submucosa called plicae circulares (valves of Kerckring). Longer vasa recta (straight arteries) and less prominent arterial arcades than ileum.
ILEUM: lots of Peyer’s patches (lymph nodules in submucosa). Well positioned to prevent bacteria from colon moving up into small intestine.
What is a migrating motor complex?
Cycles of smooth muscle contraction, beginning at the stomach and migrating through the small intestine towards the colon. This prevents migration of colonic bacteria into the ileum and may ‘clean’ the intestine of residual food.
Occurs more often in fasted state.
Name the transporters used to move glucose and galactose and fructose across the epithelial wall of the small intestine.
Glucose and galactose transported by SGLT-1: sodium-glucose linked transporter, by secondary active transport.
Fructose moved across by facilitated diffusion by GLUT 5.
GLUT 2 facilitates exit at basolateral membrane, for all 3 monosaccharides.
How is trypsinogen activated and what is its role?
Trypsinogen is cleaved to form trypsin by enterokinase - an enzyme located on the duodenal brush border. Trypsin activates other proteases.
What happens to di- and tripeptides which are absorbed across the brush border by facilitated diffusion and secondary active transport?
Absorbed across brush border by AA/H+ symporter (as opposed to AA/Na+ symporter used for AAs).
They are broken down by cytoplasmic peptidases before crossing the basolateral membrane.
What is the role of colipase?
Prevents bile salts from displacing lipase from the fat droplet
(lipase converts TAGs to monoglyceride and 2FAs).
Describe the difference between the secretions of the acinar cells and of the centroacinar and duct cells.
Acinar cells secrete low volume, viscous, enzyme-rich (zymogens) fluid.
Centroacinar and duct cells secrete high volume, watery, HCO3- rich fluid.
Describe how HCO3- is produced by a pancreatic duct cell.
CO2 and H2O converted into H+ and HCO3- by carbonic anhydrase.
H+ pumped into blood by H+/Na+ ATPase.
Cl- and HCO3- are exchanged on the apical membrane.
Cl- diffuses back out the cell through the CFTR.
Na+ and H2O diffuse paracellularly into the lumen.
Why is gastric blood alkaline and pancreatic blood acidic?
In the stomach, HCO3- secreted into blood.
In pancreas, H+ secreted into blood.
Explain how the pancreas protects itself from auto-digestion and how a blockage of the pancreatic duct can lead to acute pancreatitis.
Enzymes synthesised and stored in zymogen granules.
Pancreas also has a trypsin inhibitor to prevent activation.
If there is a blockage, protection may be overloaded, resulting in autodigestion.
What is orlistat?
An anti-obesity drug which inhibits pancreatic lipases. Leads to steatorrhoea.
Explain the control of HCO3- secretion.
S-cells in the duodenum, a type of enteroendocrine cell, secrete SECRETIN in response to high concentration of protons. Uses the second messenger cAMP. Upregulates chloride channels, leading to chloride efflux. This encourages the anion exchanger, leading to more HCO3- secretion.
Explain the role of cholecystokinin (CCK).
I-cells, a type of enteroendocrine cell, secrete CCK in response to fats and peptides in the duodenum. This stimulates both bile secretion and enzyme secretion from the pancreas (uses Ca2+/PLC model).
CCK also potentiates effects of secretin, leading to more HCO3- secretion (ONLY when stimulated by secretin).
What is the Z-line?
The epithelial border between the oesophagus and stomach, present in the lower oesophageal sphincter.
Non-keratinised stratified squamous epithelium –> simple columnar epithelium.
Light pink –> bright red.
What are the functions of a hepatic stellate cell?
Store vitamin A in cytoplasmic droplets.
Become activated (fibroblasts) in response to liver damage.
Proliferative, chemotactic and deposit collagen in extracellular matrix.
What are the 2 muscles of the upper oesophageal sphincter?
Constrictor pharyngeal muscle: circular muscle
Constrictor pharynges inferior: longitudinal muscle.
Briefly outline the stages of swallowing.
Stage 0 - oral phase. Chewing and saliva prepares bolus
Stage 1 - pharyngeal phase. Bolus guided to oesophagus by pharyngeal muscles.
Stage 2 - Upper oesophageal phase. Upper sphincter closes. Superior ring of circular muscle contracts. Lower ring dilates. Contraction of longitudinal muscle guides bolus.
Stage 3 - Lower oesophageal phase - LOS closes and peristaltic wave pushes bolus into stomach.
