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