Digestion & Absorption (mccabe) Flashcards
which portion of the GI tract is responsible for absorbing majority of macronutrients?
proximal small intestine!!! (cuz the pancreatic enzymes enter here)
What are some key jobs of the duodenum/ early jejunum?
Secrete: CCK, secretin, disaccharidases (ie lactase)
Absorb: glucose (glucose transporters located here)
What are some key jobs of the ileum?
Re-absorb bile for re-cycling! (therefore Crohn’s have decrease bile absorption)
and
Absorb B12 attached to intrinsic factor
What are triglycerides?
a glycerol molecule w/ 3 attached fatty acids w/ varying number of carbons on them… if FA has > 12 carbons then it is defined as “long-chain”
How does long-chain fat digestion work?
in stomach: chief cells secrete gastric lipase which only removes one fatty acid chain resulting in diglycerides
in intestine:
*Acid in duodenum stimulates secretin (from S cells) from endocrine cell which 1) stimulates secretion of bicarb 2) moves pH in duodenum toward normal
Fatty acids in duodenum stimulate CCK (from I cells) which stimulates pancreatic enzymes, gallbladder contraction, causes relaxation of sphinter of oddi thus increase bile into the lumen!!!
How is pancreatic lipase different from gastric lipase?
It clips off 2 FA’s from the triglyceride!… free fatty acids and monoacylglycerides are readily absorbed
VS
gastric lipase which just clips off a single fatty acid (which is absorbed well) but the diacylglycerols are almost not absorbed at all
Explain bile synth and recycling
made in liver… excess is stored in gall bladder… used to help digest fatty acids… 95% is reabsorbed back into the ILEUM which sends it back to the liver
Explain fat absorption and the role of lipase, bile, and co-lipase
with fatty acid release lipase will bind the fat first, but then release of CCK will cause bile secretion… Bile has a much high affinity for fat than lipase, so it will bind fat and DISPLACE lipase… so co-lipase (not an ezyme just an anchor) stabilizes lipase on the fat so that lipase can bind to the bile salt-coated fat droplet –> “micelle” production which gets taken to the epithelial barrieri for FA’s can be absorbed… non of this would happen without bile!
How does bile help with absorption of fats?
Fats are lipids… which means they will be repelled by water… so when lipids approach intestinal epithelium they encounter an unstirred water layer which acts as a permeability layer… BUT when bile salts are bound around the fat (micelles)… it markedly increases the solubility of the lipids allowing them access to the epithelium
Explain chylomicron synthesis
absorbed fatty acids enter intestinal epithelial cells where they go to the golgi where they are packaged into vesicles that fuse with BM and get released into exrtracellular space (because too large to enter blood vesssels)… as a result they get absorbed into the lymphatics and THEN enter circulation… along the way they get modified with new apoproteins. As the chylomicron (fats w. apoproteins) get smaller and smaller it becomes a chylomicron remnant which is then absorbed by the liver and turned into LDL and VLDL
Name 6 ways medium chain fatty acids are different than long chain
1) more soluble
2) micelles are NOT needed
3) paracellular absorption is important
4) absorbed directly into portal blood (no need for chylomicron)
5) Taste way shittier than long-chain
6) # carbons = 6-12
What kind of diet should a patient with intestinal lymphatic problems be on/
one that avoids long-chain fats, and includes more medium chain fatty acids…
Why would a diet high in medium chain fatty acids be good for a patient with cystic fibrosis?
in CF you have reduced pancreatic bicarb secretion and reduced pancreatic enzyme secretion!!! Medium-chain fatty acids do not need pancreatic enzymes for absorption… vs long-chain which would stimulates CCK which would stimulate pancreatic enzymes to cleave off 2 fatty acids to allow for proper absorption…
How does pancreatitis cause steatorrhea?
If pt eats long-chain fatty acid foods then it will require pancreatic enzymes to be properly absorbed… but in the case of pancreatitis this will not happen… ONLY gastric lipase will be present creating very few FA’s and diacylglycerols which are must less readily absorbed so the excess “malabsorbed” fat goes out in the shit.
How would a bile problem/obstruction cause steatorrhea/malabsorption?
Recall that bile salts engulf the fat droplets to allow them to pass through the water barrier that sits over the epithelium… without it absorption will be significantly reduced!
