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- 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
- Parietal cell: HCl
- Mucous Neck cell: thin mucus
- Chief cells: pepsinogen
Pyloric glands and secretions
- mucus secreting cells
- G cells: gastrin
Types of gastric mucosa
- glandular, nonglandular
- 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)
- Three parts: 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.
- supply energy
- 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.
- 3 kinds: 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.
- 2 forms: 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.
- two types: 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.
- 2 kinds: 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.
- Carboxypeptidase A
- Carboxypeptidase B
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
- membranous peptidases
- intracellular peptidases
- 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)
- micelle formation
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