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:
1. Sucrose - ( cane sugar )2. Lactose - ( milk )3. 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
hydrolyzedSTOMACH: 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 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 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
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