Digestion & Absorption Flashcards
Where does protein digestion start?
In stomach (pepsin and HCl)
What cell releases pepsinogen?
Peptic/chief cell
What cell releases HCl?
Parietal/oxyntic cell
What does chyme from the stomach contain?
Solubilised, slightly digested carbohydrates and proteins
Slightly digested fats
Intrinsic factor
Describe protein digestion in the duodenum. (4)
- Enterokinase activates pancreatic trypsinogen to trypsin which activates other pancreatic proteases
- Pancreatic trypsin, elastase and chymotrypsin cleave interior peptide bonds
- Pancreatic carboxypeptidases remove amino acids from carboxyl ends
- Aminopeptidases remove amino acids from amino ends
Where is enterokinase found?
Brush border of duodenal enterocytes
Where are aminopeptidases found?
Brush border of duodenal enterocytes
What type of protease is chymotrypsin?
Endopeptidase
How are amino acids absorbed in the small intestine?
Na-linked luminal transporters and exit on BL membrane
How are di/tripeptides absorbed in the small intestine?
H-linked luminal transporters
Hydrolysed to amino acids
Exit on BL membrane
Why is the infant small intestine wall permeable to peptides?
Allows absorption of growth factors and antibodies from colostrum
What substance makes up dietary fibre?
Cellulose
How is starch digested?
Salivary amylase in mouth begins process
Pancreatic amylase cleaves straight chains to from short oligosaccharides, maltose and maltriose
Further digestion of these and other disaccharides by brush border enzymes
What transporters are involved in monosaccharide absorption? (4)
Driven by BL Na/K-ATPase
Apical SGLT - Na-linked glucose and galactose secondary active transport
Apical GLUT5 - fructose facilitated diffusion
Basolateral GLUT2 - facilitated diffusion of all three hexoses
What enzymes are involved in lipid digestion?
Mainly pancreatic lipase with some gastric lipase
What aids lipid digestion in the duodenum?
Colipase
Bile salts
What is beta-lipoprotein used for?
Package absorbed lipids into chylomicrons for export
Describe fat digestion in the duodenum. (4)
- Fat emulsification using bile salts
- Lipase actions converts TG to FA and MG
- Formation of micelles (FA, MG, bile salts…)
- Diffusion of micelles to epithelial brush border
Describe fat absorption in the duodenum. (4)
- Simple diffusion of free FA and MG into cell
- Intracellular resynthesis of TG in SER
- Incorporation of TG into chylomicrons together with beta-lipoprotein, cholesterol…
- Exocytosis of chylomicrons into nearby lacteals
Where is the majority of bile reabsorbed?
Terminal ileum
What is enterohepatic recirculation of bile salts?
Reabsorption of bile salts in terminal ileum
Transported back to liver via hepatic portal vein to be re-extracted by hepatocytes to be re-exported in bile juice
What is the overall percentage loss of bile salts?
5-10%
What hormone stimulates water reabsorption in the colon?
Aldosterone
What is produced by the resident microflora in the colon?
Vitamin K and folic acid
Where are the fat-soluble vitamins generally absorbed?
In small intestine with lipids
In what state are drugs best absorbed?
Non-ionised
Where are drugs absorbed?
Small intestine and colon
What can cause failure to digest macromolecules in chyme? (3)
Interruption of enterohepatic recirculation of bile salts (fats)
Failure to deliver pancreatic enzymes
Poor coordination of gastric emptying with delivery of pancreatic and biliary secretions
What can cause failure to absorb lactose?
Absence of brush border lactase (alactasaemia)
What can cause failure to absorb lipids?
Lack of beta-lipoproteins
Failed lipid digestion
What can cause insufficient water absorption?
Increased solute load
Increased salt and water secretion by bacterial infection
What does vit. B12 malabsorption in the ileum cause?
Fewer, immature, but macrocytic RBCs (pernicious anaemia)
What can cause vit. B12 malabsorption?
Lack of intrinsic factor due to gastric atrophy or an autoimmune condition
What does Fe2+ malabsorption cause?
Microcytic RBCs due to insufficient Hb
How can anaemia affect the oral appearance?
Pale gums
Glossitis and changes to tongue surface
What is osmotic diarrhoea? (4)
Increased solute load in lumen causes water retention
Increased volume of lumen contents causes increased peristalsis
Undigested fats have laxative effect
Undigested material fermented in colon by microflora = distension
What is secretory diarrhoea?
Response to bacterial infection
Increased secretory activity of intestinal crypt cells which affects Na movement
What diseases can cause intestinal mucosal damage (causing diarrhoea)?
Coeliac disease
Crohn’s disease
What is characteristic of the small intestine in Coeliac disease (gluten)?
Crypt hyperplasia
Decreased surface area (villous atrophy)
What is Crohn’s disease?
Inflammatory condition mainly affecting ileum and colon
