Physiology Of Intestines Flashcards
How is water secreted by the small intestine?
Intestinal epithelium secretes Na+ Cl- and HCO3-
H20 follows by osmosis
Where is mucous secreted from in the small intestine?
Brunners glands in the duodenal mucosa
What hormones does the small intestine secrete?
What do they do?
Secretin & CCK - inhibit gastric motility and secretion and control pancreatic/biliary secretion
Motilin - stimulates migrating motor complexes via ENS and ANS
VIP (vasoactive intestinal peptide) - increases blood flow to GIP tract
GIP (gastric inhibitory peptide) - inhibits gastric secretion and stimulates insulin secretion
The endocrine pancreas secretes insulin and glucagon. What does the exocrine pancreas secrete?
Ducts cells secrete alkali - isotonic solution rich in HCO3-
Acinar cells secrete digestive enzymes: Trypsin Chymotrypsin Pancreatic amylase Lipases
How are pancreatic enzymes activated?
Trypsinogen activates inactive enzymes (zymogens) by membrane bound molecule entereokinase.
Trypsin then made which can activate other enzymes in the intestinal lumen
Enzyme and alkali secretion by the pancreas are controlled separately, how?
Enzyme: stimulated by CCK and ACH (PNS Vagus)
Alkali: secretin and potentially by CCK and ACh
What molecules are digested in the lumen of the duodenum and proximal jejunum?
What enzymes are involved?
Fats > monoglyceride and fatty acids by pancreatic lipase
Starch > disaccharides > monosaccharides by pancreatic amylase and then brush border enzymes
Proteins > peptide fragments > amino acids by trypsin and chymotrypsin then carboxypeptidase and aminopeptidase
How are fats digested and absorbed in the small intestine?
Fats are emulsified by bile salts and phospholipids
Digested by pancreatic lipase and products (monoglyceride + fatty acids) held in micelles
Micelles diffuse into unstirred layer and fatty acids and monoglycerides diffuse into cell membrane - once in the cell they’re reassembled to fats
Triglyceride droplets packaged into chylomicrons and exported across basolateral membrane to lymph
How are carbohydrates digested/absorbed?
Broken down and transported across intestinal membrane via either SLGT1 (glucose/galactose) or GLUT5 (fructose)
Then across the basolateral membrane via facilitated diffusion, GLUT2
What regulates Ca2+ absorption?
Vitamin D
What are the two types of iron and how are they absorbed?
Haem - absorbed by receptor mediated endocytosis to release Fe3+ which is then reduced to Fe2+
Free Iron - reduced from Fe3+ to Fe2+ by duodenal cytochrome b ferric reductase in the brush border membrane and transported by DMT1
In both cases Fe2+ then binds to ferroportin 1 in basolateral membrane to be exported
How is Iron absorption regulated?
By demand: intracellular iron-ferritin pool LOS as cell is slouched in faeces
If iron is required by body then becomes absorbed into the blood across ferroportin 1 (receptors are unregulated)
How is vitamin B12 absorbed?
Binds to R protein in the stomach
Released from R protein and binds to intrinsic factor in the duodenum/jejunum
Vitamin B12-IF complex is absorbed by receptor mediated endocytosis in the terminal ileum
What can malabsorption arise from?
Failure/deficiency in: Pancreatic enzyme secretion Brush border enzymes Absorption Bile secretion
Name some diseases of malabsorption and their site of pathology
Coeliac disease - duodenum/jejunum, fat absorption
Pancreatitis - pancreas, fat digestion
Crohn’s surgical resection - ileum, vitamin B12 and bile salt absorption
Lactase deficiency - small intestine, lactase hydrolysis
What are symptoms of malabsorption?
Weight loss Abdominal distension Diarrhoea Steatorrhoea Pernicious anaemia
Outline lactose intolerance
In healthy individuals lactose is digested by lactase producing normal stools
In lactose intolerance there’s a lack of lactase enzyme so lactose is broken down by gut bacteria leading to fermentation producing gases, organic acids, osmotically active molecules and leading to bloating/increased motility so alter stools
Outline what excessive vomiting or diarrhoea leads to with regards to acid-base balance in digestion
Vomiting: large loss of secreted acids = metabolic alkalosis
Diarrhoea: excessive loss of fluid and therefore NaHCO3 in faeces = metabolic acidosis
What are the functions of the large intestine?
Extract Na+ and H20 from luminal contents
Make and store faeces
Move faeces towards the rectum
What does the large intestine secrete and what is its role?
Alkaline mucous from goblet cells - protects epithelium from acid/abrasion/bacterial activity and provides adherent medium for holding faecal matter together
How is water absorbed in the colon?
Na+/K+ ATPase pumps Na+ out across basolateral membrane
H20 follows transcellulary as gap junctions too tight
What’s the role of the ileocaecal valve?
Separates terminal end of ileum from the caecum
One way valve/sphincter to prevent backflow of faecal contents from the colon and control the rate at which ideal chyme enter
What’s the role of movements of the colon?
What are the two types of movement?
Facilitate efficient absorption of water and salts and permit orderly evacuation of faeces
Haustral contractions and mass movements
Longitudinal muscle of the colon is aggregated into 3 bands cal taeniae coli - what is their role?
Combined contractions of taeniae coli and circular muscle layer cause colon to bulge into ovoid segments called haustrae
Haustral contractions mix contents to facilitate water absorption
