Gastrointestinal Physiology Dr. Meg-angela Christi Amores

Digestion by Hydrolysis CARBOHYDRATES Almost ALL in the diet are: large polysaccharides or disaccharides Combinations of monosaccharides H of 1 mono removed, OH of other mono removed = joined H2O is formed In DIGESTION: reversal of the process DIGESTION of CARBS: conversion to monosaccharides

Digestion by Hydrolysis FATS entire fat portion of diet are triglycerides (neutral fats) 3 fatty acids + glycerol = triglyceride removing 3 H20 m in DIGESTION: reversal: fat-digesting enzymes return three molecules of water to the triglyceride molecule and thereby split the fatty acid molecules away from the glycerol

Digestion by Hydrolysis PROTEINS Formed from amino acids linked in peptide bonds H from one AA removed, OH from another AA removed In DIGESTION: reversal or process

Digestion Basic chemistry: in the case of all three major types of food, the same basic process of hydrolysis is involved Only difference lies in the types of enzymes required to promote the hydrolysis reactions for each type of food

CARBOHYDRATE Digestion FOOD in the diet: 3 major sources of carbs in diet: Sucrose - ( cane sugar ) Lactose - ( milk ) Starch - (all non-animal food, potatoes, grains) Other carbs: amylose, glycogen, alcohol, lactic acid, pyruvic acid, pectins, dextrins, cellulose

CARBOHYDRATE Digestion MOUTH: Saliva – ptyalin (a amylase) STARCH --- maltose (di) and polymers of glucose (mo) Short stay in the mouth, <5% of starch is hydrolyzed STOMACH: gastric juice - acidic Amylase deactivates in stomach, but can digest about 30-40% while in the fundus

CARBOHYDRATE Digestion SMALL INTESTINE : DUODENUM Pancreatic amylase – more powerful 15 to 30 mins of arrival of chyme in duodenum – completely digested STARCH ------ maltose and glucose polymers JEJUNUM and ILEUM four enzymes (lactase, sucrase, maltase, and α-dextrinase) Lactase – GALACTOSE --- lactose and glucose Sucrase – SUCROSE ---- fructose and glucose Maltase – MALTOSE --- glucose and glucose

PROTEIN digestion In the diet: chemically long chains of amino acids bound together by peptide linkages

PROTEIN digestion STOMACH PROTEIN ----- Proteoses, polypeptides Pepsin – most active at pH of 2 to 3 (Needs HCl to be activated) Only initiates digestion (10 to 20% of total protein) Special ability to digest COLLAGEN PROTEIN ----- Proteoses, polypeptides

PROTEIN digestion UPPER SMALL INTESTINE Where most protein digestion occur Pancreatic enzymes: TRYPSIN, CHYMOTRYPSIN Proteoses, polypeptides ---- smaller peptides Pancreatic enzyme: PEPTIDASE (enterocytes) smaller peptides ----- amino acids

FAT Digestion In the diet: triglycerides, phospholipids, cholesterol, and cholesterol esters MOUTH, STOMACH small amount of triglycerides is digested in the stomach by lingual lipase that is secreted by lingual glands in the mouth and swallowed with the saliva (<10%, unimpt) INTESTINE Where essentially all fat digestion occurs

FAT Digestion Emulsification by BILE first step in fat digestion break the fat globules into very small sizes begins by agitation in the stomach to mix the fat most of the emulsification occurs in the duodenum under the influence of bile Contain BILE SALTS and LECITHIN to make the fat globules readily fragmentable by agitation with the water

FAT Digestion SMALL INTESTINE Pancreatic LIPASE triglycerides of the diet are split by pancreatic lipase into free fatty acids and 2-monoglycerides

ABSORPTION STOMACH SMALL INTESTINE poor absorptive area, no villi, with tight junctions Can absorb alcohol, aspirin SMALL INTESTINE Absorbs more than 7 L of fluid per day many folds called valvulae conniventes (or folds of Kerckring) with villi intestinal epithelial cell on each villus is characterized by a brush border with microvilli

ABSORPTION – small intestine Daily: Several hundred grams of carbohydrates 100 or more grams of fat, 50 to 100 grams of amino acids, 50 to 100 grams of ions, and 7 to 8 liters of water ABSORPTIVE CAPACITY: several kilograms of carbohydrates per day 500 grams of fat per day 500 to 700 grams of proteins per day 20 or more liters of water per day

ABSORPTION Water – diffusion by laws of osmosis Sodium – active transport

ABSORPTION Chloride – diffusion , thru electronegativity created by sodium Bicarbonates – combine with Hydrogen to form H2CO3 (carbonic acid), which dissociates to form H2O and CO2 . CO2 readily diffuses, released to lungs

ABSORPTION Carbohydrates - absorbed in the form of monosaccharides most abundant of the absorbed monosaccharides is glucose (80%) galactose and fructose ( 20%) by secondary active transport (Glucose co trasported with SODIUM)

ABSORPTION

ABSORPTION Protein are absorbed through the luminal membranes of the intestinal epithelial cells in the form of dipeptides, tripeptides and amino acids Energy supplied by active transport of sodium co-transport (or secondary active transport) of the amino acids and peptides

ABSORPTION Fat end products first become dissolved in the central lipid portions of bile micelles monoglycerides and free fatty acids are carried to the surfaces of the microvilli of the intestinal cell brush border and then penetrate into the recesses among the moving, agitating microvilli diffusion

ABSORPTION – large intestine 1500 mL of chyme enter Large Intestine Most of the water and electrolytes are absorbed leaving less than 100 milliliters of fluid to be excreted Most occur the proximal one half of the colon, giving this portion the name absorbing colon ABSORPTIVE CAPACITY maximum of 5 to 8 liters of fluid and electrolytes each day

DIARRHEA results from rapid movement of fecal matter through the large intestine Enteritis: inflammation usually caused either by a virus or by bacteria mucosa becomes extensively irritated, and its rate of secretion becomes greatly enhanced Cholera – secretion of 10 to 12 liters per day, lead to death. Max abs capacity of LI: 6 – 8L/d