Function of the Intestine

Function of the Intestine • • Digestion and absorption o Proximally fats, proteins, carbs, iron are absorbed o Distally bile salts and vitamin B12 Molecules which are non-polar can simply pass through membrane but polar will need either active or passive transport Electrolyte and Water absorption • Input (9L) and Output 8.7 L (mostly intestinal absorption, 7L) • At the lumen, sodium and glucose travel through the SGLT1 pump • Fructose goes through the GLUT5 receptor • Na/K+ pump moves potassium back into the cell o Na+ will enter the cell from the gut lumen, down both a concentration and electrical gradient, thus causing water to follow passively Chloride Secretion • Net Cl- will be secreted by crypt cells which will travel across electrogenic channels into the lumen o This will be accompanied by sodium which will draw water o Since the release of chloride through electrogenic channels is regulated by cAMP, this can be stimulated such as during cholera, thus causing secretory diarrhoea Bicarbonate Secretion • Most important in the Pancreas but also happens in the small intestine→ bicarbonate will be secreted into the lumen Digestion and Absorption of Carbohydrates • Digestion of carbs and proteins takes place in two places o Enzymes secreted into intestinal lumen begin the digestive process ▪ Polysaccharides are composed of glucose units and are broken down to maltose (2 glucose units) by pancreatic amylase, with salivary amylase playing a small part o Membrane bound hydrolases located on the microvilli (brush border) of epithelial cells mediate next stage ▪ The ingested disaccharides (mostly sucrose and fructose), along with maltose, are digested to the component single sugars by their specific disaccharides in the glycocalyx lining on the outside of luminal membrane of the brush border of enterocytes. o • Glucose or galactose are then transported actively (i.e. use of energy to move molecule up a concentration gradient) across the enterocyte, with a linkage to sodium absorption. Fructose is absorbed passively (i.e. across its own concentration gradient). Different enzymes will break down particular sugars, for example lactose is broken down by lactase o Lactose intolerance is due a deficiency of lactase in the glycocalyx → therefore not converted into glucose and galactose and instead travels down intestine as lactose → fermented by bacteria → gas and increased motility Digestion and absorption of proteins • Pancreatic acinar cells are the source of all the enzymes in pancreatic juice o Contain zymogen granules which contain potent enzymes which can breakdown proteins o These enzymes are not activated immediately, otherwise they would digest proteins in the pancreatic acinar cells ▪ Instead trypsinogen (and thus all the other enzymes) are activated by enterokinase on enterocyte brush borders, but also secreted into the intestinal lumen ▪ Pancreatitis: Acinar cell injury (through alcohol or gallstones) can activate tripsinogen within the acinar cells→ cell activation→ mediators → abcess/necrosis • Major peptide transporter is PEPT1 o Must be broken down into either single peptide, dipeptide or tripeptide before it can be absorbed by the cell • Cytosolic proteolysis further breaks down peptides Coeliac Disease • Enterocytes normally have tight junctions which is important for preventing microbes entering the bloodstream from the lumen • In people who have coeliac disease (and other auto-immune conditions) there is a leakiness between enterocytes o This allows gluten prolamins from food to pass through, where there will be partial degradation (where normally they will be not be degraded) ▪ This is done by the immune system which treats gluten as a foreign antigen→ breakdown of gluten into gliadin and glutenin • The T cells activated by this process do damage to ordinary intestinal cells