GI part II
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GI part II
GI part II domestic animal physiology for test 3
breaking down of food into subunits that are suitable for absorption
Mechanical, chemical, hydrolysis (catalyzed by enzymes)
Two classes of digestive enzymes
act within lumen (lumenal digestion)
act at the membrane surface of epithelium (membranous digestion)
enzymes act in lumen of gut.
Originate from major GI glands (salivary, gastric, and pancreas)
begin process of chemical digestion
break down into short-chain polymers (incomplete)
Membranous phase of digestion
Enzymes bound to cells in intestinal mucosa complete what luminous started with hydrolysis.
Followed closely by absorption
Intrinsic Endocrine system
cells that produce true hormones, secreted with no duct into blood (not lumen). Regulatory, not digestive
intrinsic paracrine system
molecules that are secreted by one cell and exert effect locally, travelling by diffusion from interstitial fluid to nearby target cells.
Regulatory, not digestive. secreted into interstitial (not into lumen)
secretion of proximal GI tract
saliva. Initated by anticipation of eating.
Sympathetic NS stimulates saliva before a fight. Has nothing to do with digestion
Functions of saliva in ruminants (3)
maintains fluid consistency of rumen
slightly alkaline to neutralize acids formed by fermentation
May help prevent frothing
Functions of saliva in non-ruminants
Lubricates food to facilitate swallowing
Contains antibodies and lysozymes
Some amylase in saliva of swine and rats, much more in humans. None in c/d. Works in mouth-fundus, inactivated by acid once mixed.
Secretion of cardiac glands
Fundic glands and their secretions
Mucous Neck cell
: thin mucus
Pyloric glands and secretions
mucus secreting cells
Types of gastric mucosa
Margo plicatus separates in horses
Nonglandular gastric mucosa
region in animals with simple stomach, chamber in animals with complex stomach.
small amount of fermentative digestion occurs. Mixing in proximal stomach, food protected from gastric gland secretions.
Glandular stomach (3)
: cardiac mucosa (very top), fundic mucosa(fundus and body), pyloric mucosa (lower end)
contain glands of similar structure but different secretions.
Openings to ducts into which gastric glands empty their secretions. Walls are formed mainly of surface mucosal cells.
clustered in the neck or proximal area of gastric gland in fundic region.
Secrete HCL, intrinsic factor (not cats) for absorption of B12
Max stimulation = isotonic HCl, pH below 1
H and Cl are secreted separately from hydration equation, combine in lumen.
(H+ in, K+ out, Cl in HCO3 out)
Mucous neck cells
In middle of gastric gland in fundic region.
secrete thin mucus. Progenitor cells for gastric mucosa--only cells that can divide, then migrate up or down.
in base of gastric gland in fundic region.
Secrete pepsinogen. HCl converts into pepsin (proteolytic enzyme), breaks down proteins into peptides.
ONLY enzyme found in stomach of adult animals. Initiates protein digestion.
Secreted as pepsinogen by chief cells, HCl converts into pepsin.
Once activated can activate other pepsinogen molecules.
proenzymes. Digestive enzymes are synthesized, stored and released as proenzymes then activated in lumen of gut so they don't eat their secreter cells.
secrete Gastrin in pyloric region. Endocrine gland cells.
HCl secretion in parietal cell is stimulated by
Histamine (secreted by mast cells and enterochromaffin-like cells in parietal mucosa, stim to secrete ACh and gastrin)
anticipation of eating
presence of food in stomach (parasymathetic vagus>ACH>Parietal (HCl), G cells> gastrin>parietal>HCl (x2)
increase in gastric pH (basic food coming in)
Cephalic phase of gastric secretion
responce of the stomach to anticipatory stimuli, like thinking of food.
Gastric phase of gastric secretion
food entering stomach and causing secretions
HCl secretion of parietal cells is reduced by
pH returning to 1/below. At 2, suppressed, at 1 abolished.
intestinal environment--acid exits stomach, lowers duodenal pH, neuro and secretin suppress secretion
Simulation of pepsinogen secretion
Same regulatory control as HCl (not much research).
enzyme secreted by abomasal mucosa of young ruminant
Coagulates milk proteins, facilitating digestion.
Duodenal mucosa: Cholecystokinin, Secretin
Cholecystokinin (secretion in small intestine). Stimuli and actions
Secreted from duodenal mucosa
Stimulated by chyme with high amino acids, high fatty acids, low pH
Inhibits gastric emptying, increases secretion of pancreatic enzymes and HCO3-, stimulates gall bladder contractions to increase bile.
Secretin (secretion in small intestine). Stimuli and actions
Secreted in duodenal mucosa
Stimulated by chyme with low pH, high fatty acids
Lowers HCl production, increases pancreatic HCO3- secretion, increases biliary HCO3- secretion
Secreted into duodenum.
Exocrine digestive enzymes (zymogens)
: Trypsinogen, chymotrypsinogen, procarboxypeptidase, proelastase
Stimulation of pancreatic secretions
Occurs in phases. Surface receptors stimulated by acetylcholine, cholecystokinin, secretin.
Max stimulation when all three are binding
Neuronal and endocrine
Cephalic(sight/smell), Gastric(distension of stomach) and intestinal(food enters duodenum) phases, each with neuronal and endocrine
Reaction of pancreas to secretin
Causes pancreatic digestive enzyme secretion, bicarb (to neutralize acid of stomach in duodenum)
Activation of trysinogen in small intestine
Pancrease secretes Trypsinogen.
CCK stimulates duodenal mucosal cells to produce ENTEROpeptidase
enteropeptidase activates trypsinogen to trypsin
trypsin activates Chymotrysinogen to chymotripsin, procarboxypeptidase to carboxypepdiase and proelastase to elastase
Trypsin then serves as autocatalytic agent, activates more trypsinogen and other digestive enzymes.
help absorption of lipids (fatty acids, monoglycerides, cholesterol, fat-soluble vitamins) from small intestine.
Emulsify dietary fats
synthesized by hepatocytes from cholesterol, stored in gallbladder (cyclic) or secreted into intestine (continuous, horse and rat)
Most important stimulus for gallbladder contraction and release of bile into small intestine
Bile acid absorption
absorbed in ileum
travel via hepatic portal vein to liver, absorbed from portal blood. Stimulate further bile synthesis until fats all absorbed
VERY LITTLE in systemic circulation. Used to test liver function.
Cholecystokinin (CCK) inhibition/stop
when fats are digested and absorbed, sphincter at entrance of bile duct to intestine, bile back to gallbladder
no more bile in intestine, no negative feedback, no more stimulation.
Bile secretion inhibition
when fats are digested and absorbed, no more CCK, sphincter at entrance of bile duct to intestine, bile back to gallbladder, no more bile in intestine, no negative feedback, no more stimulation.
carbohydrates, protein, fats
essential for life but do not supply energy
minerals, vitamins, water
nutrients containing C, H, O. Repeating simple sugar molecules.
come primarily from plants.
: fibers, sugars, starches
part of plant carbohydrates.
Structural part of plants, energy for herbivores, can't be digested by mammals, broken down by microbes.
energy transport molecules in plants.
: simple (single molecular unit), complex (two or more repeating saccharide subunits.
most broken down sugars. Glucose, galactose, fructose.
2 monomer sugars. Lactose (galactose and glucose) and sucrose (glucose and fructose)
muliple repeating simple sugar units.
disaccharides (2, most important), Trisaccharides (3), oligosaccharides (3-10)
energy-yielding nutrient in diets of omnivorous animals (pigs, rats and primates). Repeating subunits of GLUCOSE ONLY.
: branched (amylopectin) and unbranched (amylose)
complex, high molecular weight large molecules that contain a high percentage of amino acids
contain C, H, O and N
hydrolysis yields amino acids.
Dipeptides, Oligopeptides (2-10), Polypeptides (10-100), proteins (101+)
Essential amino acids
those that cannot be synthesized at all or enough to allow normal growth. Must be injested.
Nonessential amino acids
those that can be produced by the body in sufficient amounts to allow normal growth.
fats and related substances. Includes triglycerides, phospholipids and cholesterol
Minerals (2 kinds)
accessory food--inorganic foodstuffs that are essential for normal growth and reproduction.
: macrominerals (lots required) and trace minerals (small amounts required)
accessory foods--chemically unrelated organic compounds essential for life. Supplied by diet or synthesized by the body. (ruminant microbes supply water-soluble B)
function as metabolic catalysts or regulators.
hydrolysis of major nutrients by splitting of a chemical bond by the insertion of a water molecule.
Luminal or Membranous.
Luminal carbohydrate digestion
applies ONLY TO STARCHES.
uses alpha amylase (from pancreas and from saliva in omnivores)
makes short-shain polysaccharides (maltose, etc. short glucose chains)
Membranous phase digestion
carbohydrates and proteins.
Enzymes are embedded in intestinal membrane in brush border, so food has to come to it.
Membranous carbohydrate digestion
Specific enzymes for each polysaccharide. (maltase, isomaltase, sucrase, lactase)
Breaks disacch. down into 2 molecules.
Protein luminal digestion
variety of luminal-phase enzymes turn large molecular proteins into small peptide chains, some free amino acids.
includes endopeptidases and exopeptidases.
Begins in stomach (pepsin), finishes in SI (trypsin, chymotrypsin, elastase and carboxypepsidase A and B)
luminal protein digestion, breaks proteins at internal points, resulting in short chains (no singles).
Pepsin, chymosin (rennin), trypsin, chymotrypsin, elastase
luminal protein digestion, breaks individual amino acids off the ends of proteins. Secreted by pancreas.
Membranous protein digestion
Peptide-digesting enzymes or peptidases on enterocyte membrane surface. Hydrolyze products of luminal phase
make free amino acids, dipeptides and tripeptides
digest dipeptides and tripeptides allowed into the cell after membranous phase of protein digestion. Leave free amino acids.
Final product of protein digestion, 3 ways to get it
Always finish with free amino acids going into the blood.
Gotten by exopeptidases in luminal
movement of products of digestion across the intestinal mucosa and into the vascular system for distribution.
Flow from high to low concentration down gradient by diffusion
Charged ions and most organic nutrient molecules can't penetrate so need facilitated of some kind
secondary and tertiary active transport
use sodium ion electrochemical gradient to move nutrients into the cell.
Antiports or exchangers can be used, also E.C gradient. Example Na+/H+ exchanger
passive diffusion directly through tight junctions where enterocytes touch.
Absorption of glucose
diffuses until equilibrium, then piggybacks with Na+ from lumen of intestine into mucosal cell, diffuses into capillary
Absorption of proteins
diffusion or sodium co-transport systems. similar to carbohydrates
Water and electrolyte absorption
Transport molecules (3 ways for sodium, 3 ways for chloride, passive diff. for potassium, bicarb absorbed by neutralizing HCl and ion-exchange in ileum and colon)
all water absorption is passive.
Absorption into capillaries
diffusion. concentrated in interstitial (lateral), diffuse down concentration gradient.
Not water-soluble, so hard to digest/absorb.
primary is triglyceride, coming from plant and animal sources.
Also cholesterol, cholesteryl ester, waxes, phospholipids
Fat-soluble vitamins are ADEK
A, D, E, K
Lipid assimilation (4)
Emulsification of lipids
Process of reducing lipid droplets to a size that forms stable suspensions in water or water-based solutions. Higher surface area.
Begins in stomach, completed when bile acids and phospholipids surround to make smaller drops with higher surface area.
Hydrolysis of lipids
bile-coated or emulsified droplets are broken down by pancreatic enzymes including lipase and co-lipase
secreted in active form from the pancreas, helped by co-lipase
cleaves fatty acids into 2 fatty acids and one monoglyceride
Lipid-digesting pancreatic enzymes
lipase, co-lipase, cholesterolesterase, phospholipase
products of hydrolytic lipid digestion (fatty acids, monoglycerides, etc.) combine with bile acids and phospholipids to form micelles
Allow lipids to diffuse close to absorptive surface of the cell.
through enterocyte membrane by protein carriers and simple diffusion
All of micelle EXCEPT bile salt are transported through membrane
triglyerides re-formed then packaged into chylomicrons for transport (phospholipid and cholesterol bubble with triglyceride and cholesterol ester inside)
absorbed by lacteal, travel in lymphatic
what lipids turn into inside the cell.
spherical structures with triglycerides and cholesterol esters inside and phospholipids and cholesterol on the outside.
can't be absorbed in blood, travel through lymphatics