intestinal physiology Flashcards
how are proteins absorbed in the duodenum?
protein in stomach stimulates release of pepsinogen from chief cells. pepsinogen gets converted into pepsin by the presence of HCl, optimum pH is 2. pepsin has a positive feedback loop on itself to convert more into pepsin. pepsin is responsible for 20% of protein breakdown so is not essential but speeds it up. protein also mops up H+ in the stomach, raising the pH, which further stimulates the release of HCl from parietal cells. once in the duodenum, protein breakdown continues with proteases from the pancreas. trypsinogen converted into active form trypsin by enteropepsidases from intestinal mucosa. trypsin had positive feedback loop and also converts chymotrypsinogen into chymotrypsin. these break protons down into small peptides. small peptides either absorbed into enterocytes with H+ secondary active transport or they are broken down into amino acids by aminopepsidases on the luminal membrane or carboxypepsidases released by the pancreas. amino acids are absorbed with Na+. once in the cell they are broken down into amino acids by proteases and diffuse through the basolateral membrane into the blood.
how are carbohydrates absorbed in the intestine?
parotid gland secretes salivary a-amylase in response to parasympathetic stimulation. this starts to digest but can only break the 1-4 glycosidic bonds. SAA gets denatured by low pH in stomach. carbohydrates continue breakdown 1-4 bonds in duodenum by pancreatic amylase. brush border enzymes break down the 1-6 bonds to produce monosaccharides. fructose diffuses into enterocyte vis GLUT transporter. galactose and glucose enter via SGLT transporter by secondary active transport. All monosaccharides differ through a GLUT on basolateral membrane. most carbohydrates reabsorbed in first 20% of small intestine.
how much of water is reabsorbed?
98%
what is the daily water input ? how much water is excreted in the stool per day?
200ml
where is most of the water absorbed?
jejunum (5500ml), ileum (2000ml), colon (1300ml)
how is water reabsorbed?
3Na/2K pumps on basolateral membrane using ATP. facilitates secondary active transport of Na/glucose from luminal membrane. water follows by osmosis.
how are luminal contents moved alone in the small intestine and why?
segmental contraction, maximises abrobtion because a lot of contact with intestinal wall.
what happens in the colon? why is active transport needed?
Na+ actively reasorbed, water follows. exchanged for K+, and HCO3- is secreted and Cl- reabsorbed. creates very alkaline lumen. AT needed because contents are iso-osmotic so water isn’t going to follow an osmotic gradient.
describe the absorption of fats.
fats are initially broken down by salivary a-lipase secreted by the sublingual gland. this enzyme is denatured upon entry to the stomach. the presence of fat in the stomach stimulates a number of things. the chief cells secrete gastric lipases which continue to break down the fat into droplets. it also stimulates the resease of CCK and secretin from the I and S cells in the duodenal lining. CCK stimulates the contraction of the gallbladder and relaxation of the sphincter of oddi, so that bile is secreted into the duodenum through the ampulla of vater. CCK also stimulates pancreatic enzyme release. fats are broken down mechanically in the stomach into lipid droplets. These lipid droplets need to be emulsified by the bile salts from bile so that they do not stick back together. however, because the bile salts are amphipathic, the pancreatic lipases cannot attach on to the droplet to break it down, and they require collapse from the pancreatic secretions to bind the lipase to the droplet. the droplets can now be broken down into monoglycerides and free fatty acids. these can exist as free fatty acids or monoglycerides and fatty acids contained within a micelle. micelles are formed from cholesterol phospholipids etc and they exist in equilibrium with the free fatty acids. fatty acids are absorbed by diffusion into the enterocytes and this upsets the equilibrium therefore the micelles release more fatty acids. this concentration gradient is maintained by the fact that when in the enterocyte, triglycerides reform. this happens until all of the fatty acids have been absorbed. once in the cell, they are packaged into chylomicrons and transported out of the cell into lacteals which link to the lymph system. they are transported in the thoracic duct to the left subclavian vein.
how is iron absorbed?
iron mainly absorbed in duodenum. converted from Fe3+ to Fe2+ by cytochrome 2 ferric reduces enzyme on border of enterocyte. enters cell by DMT1. here it is either incorporated into ferritin for storage or transported into blood by ferroportin1, then oxidised to Fe3+ again to be transported in blood attached to transferrin.
how are fatty acids synthesised from glucose?
ingested glucose travels to the liver and is broken down into alpha glycerol phosphate and a fatty acid. the liver manufactures triglyerides from fatty acids and alpha glycerol phosphate. triglycerides are then transported in VLDLs to adipose tissue. these are broken down by lipoprotein lipases on the endothelium of the adipose capillaries and the fatty acids diffuse into the adipose tissue where they reform triglycerides. the rest of the VLDL is recycled back to the liver.
what are the 3 stores of triglycerides?
glycogen that can release glucose and convert into triglycerides. glusoce that has been converted and triglycerides straight from ingestion.
what are VLDLs, LDLs, and HDLs.
the lower the density, the more triglycerides and the less protein there is. LDLs carry cholesterol to tissues for manufacture of steroid hormones and creation of phospholipid bilayers. HDLs take cholesterol from the tissue back to the liver to be converted into bile acids by cytochrome P450 and then conjugated to bile salts and excreted in bile.
what is the enzyme required for the formation of cholesterol? how is cholesterol production inhibited?
HMG CoA reductase. cholesterol itself has a negative feedback loop and inhibits this enzyme.
how much of cholesterol is endogenous and what is the implication of this?
90%. your body is deciding how much cholesterol you have, so having a low cholesterol diet has little effect to reduce cholesterol.
