W12 nutrient absorption and malnutrition Flashcards
What are secretory vesicles that leave the golgi called ?
Condensing granules
Describe enzyme synthesis and secretion in acinar cells
Synthesized and folded in RER on basal side —> golgi where they are glycosylated —> condensing granules leave the golgi —> mature zymogen granules (contain both zymogens and active enzymes) —> accumulate at apical end until stimulatory signal for exocytosis
What fats are produced by acinar cells ?
Lipases
Colipase
Cholesterol ester hydrolase
Phospholipase A2
What proteins are produced but acinar cells ?
Trypsin
Chymotrypsin
Elastase
Carboxypeptidase
What inactive enzyme fats are produced by acinar cells ?
Procolipase
Prophospholipase A2
What inactive protein enzymes are produced by acinar cells ?
Trypsinogen
Chymotrypsinogen
Proelastase
Pro-carboxypeptidase A and B
Where in the duodenum is enterokinase present ?
Brush border of the duodenal mucosal epithelial cells
What is the function of enterokinase in the duodenum ?
Acts on trypsinogen —> trypsin activation peptide (TAP) —> trypsin
What is the function of trypsin in the duodenum ?
Act on pancreatic pro-enzymes —> enzymes
Why are active enzymes important in the duodenum?
Cleave large peptides —> small peptides —> amino acids —> intestinal cells take up AAs
Describe pancreatic exocrine secretions during the cephalic phase
Thoughts, taste, smell of food —> pancreatic secretory stimuli
Vagal cholinergic nerves —> enteric intrinsic nerves —>
1. Release ACh to stimulate enzyme secretion by pancreatic acini
2. Release ACh and vasoactive intestinal peptide (VIP) to potentiate alkaline fluid secretion by ductal cells
Describe exocrine secretions of pancreas during the gastric phase ?
Stretch induced vasovagal reflex —> cholinergic nerves induce enteric intrinsic nerves to further:
- release ACh —> enzyme secretion by acini
- release ACh and VIP to activate alkaline fluid production by ductal cells
Peptides produced by digestion by pepsin —> enteroendocrine G cells in gastric epithelium —> Gastrin —> weak CCK receptor agonist to induce pancreatic secretions
Describe exocrine secretions of the pancreas during the intestinal phase
Neuroendocrine cells in intestine —> secrete hormones —> coordinate actions of food digestion
Chyme in SI —> secretin and CCK —> secretion of digestive enzymes from acini and alkaline fluid from ducts
CCK reinforces vago-vagal reflex arc —> maintain max pancreatic secretion
Vagovagal enteropancreatic reflex responds to presence of proteins and lipids —> acinar cells by inducing ACh release from nerves —> Ach binds to M3 receptor on acinar cells
_______ and ________ stimulate more secretion than _______ and _______ induce more secretion than ________
Monoglycerides Free fatty acids Proteins Proteins Carbohydrates
Describe ductal cell structure
Simple cuboidal or columnar
Form hollow tubes
Basally located nuclei
High # of mitochondria
How does bicarbonate secretion into the lumen occur ?
Via a Cl and HCO3 exchange mechanism
what detects low pH in the intestine ?
S-cells
What does detection of low pH of the chyme in the intestine stimulate ?
Secretion of secretin
What stimulates bicarbonate secretion in the small intestine ?
Ach (minor)
Secretin
Describe secretin —> bicarbonate secretion mechanism
Secretin —> bind secretin receptor on ductal cells and activates cAMP production —> activation of PKA —> CFTR (chloride ion channel) —> Cl- ions pumped into lumen —> unequal ion gradient —> H2O and Na+ pulled through intercellular junction —> contransport of Na+ and HCO3 across cell membrane brings bicarbonate into cytoplasm from submucosa
HCO3 is secreted into ductal lumen bu Cl- anti porter which recycles Cl-
What is critical for bicarbonate secretion ?
CFTR function
What are the consequences of reduced exocrine function ?
Exocrine insufficiency Acute pancreatitis Chronic pancreatitis Diabetes Pancreatic ductal adenocarcinoma
What are the 4 basic GI processes ?
Motility, secretion, digestion, absorption
What is digestion ?
Biochemical breakdown of nutrients into their molecular components
What are carbohydrates composed of ?
Monosaccharides
Disaccharides
Polysaccharides
What enzymes are involved in the breakdown of carbohydrates ?
Amylase, sucrase, lactase, maltase
What enzymes are involved in the breakdown of proteins ?
Pepsin, trypsin, chymotrypsin, carboxypeptidase, aminopeptidases
What enzyme is involved in the breakdown of fats ?
Lipase
Where does absorption of digested nutrients, water and electrolytes primarily occur ?
Across the membrane of epithelial cellls in the small intestine
What happens to carbohydrate and protein breakdown products when they travel through the epithelial cells ?
Enter the blood
What happens to fat breakdown products when they travel through the epithelial cells ?
They enter the lymphatic system
What is the purpose of the alkaline fluid secreted by the pancreas ?
Neutralize acidic gastric chyme
What do duct cells in the pancreas secrete ?
Secrete aqueous NaHCO3 solution
What do acinar cells in the pancreas secrete ?
Digestive enzymes
What 3 enzymes secreted by the pancreas hydrolyse peptide bonds?
Trypsinogen
Chymotrypsinogen
Pro carboxypeptidase
Why does the pancreas secrete trypsin inhibitor ?
To inhibit any activated trypsin in the pancreas
What activates trypsinogen (—> trypsin) ?
Enterokinase
What is the only enzyme that can digest fats ?
Lipase
What causes steatorrhea ?
Lack of lipase —> ingested fats remain too large to absorb
~60-70% of ingested fats are excreted in the feces
How much pancreatic aqueous alkaline solution is secreted each day ?
1-2 litres
How is bile delivered to the duodenum ?
The sphincter of Oddi
Where are the majority of bile salts reabsorbed ?
Terminal ileum
Composition of bile salts
Deprotonated molecules, negatively charged
Derived from cholesterol but with negatively charged hydrophobic tail
What is lecithin
A component of cell membranes
What regulates the release of bile ?
CCK triggers the contraction of the gall bladder and the relaxation of the sphincter of Oddi
Vagal efferents augment this response
What regulates the production and secretion of bile ?
Neuronal- vagal input increases bile production
Hormonal- secretin stimulates the production and secretion of the NaHCO3 component of bile
What causes the inhibition of bile ?
Fats have left duodenum:
- CCK levels drop
- the sphincter of Oddi closes
—> bile cannot enter the duodenum
when do hepatocytes stop manufacturing bile ?
When the circulating bile salt concentration in the enterohepatic circulation declines
How are fats digested in the small intestine and what components are absorbed ?
Completely hydrolysed by pancreatic lipase
Monoglycerides and free fatty acids are absorbed
How are proteins digested and absorbed in the small intestine ?
Reduced to small peptide chains and some unitary amino acids by pancreatic proteolytic enzymes
Peptide fragments require further hydrolysis by aminopeptidases in the epithelial brush border prior to their absorption
How are carbohydrates digested and absorbed by the small intestine
Broken into disaccharides and some monosaccharides
Further hydrolysed by disaccharidases in the epithelial brush border.
What secretes amylase ?
Salivary glands in mouth
Exocrine pancreas
What is the site of action for pepsin ?
Stomach antrum
Where do trypsin, chymotrypsin and carboxypeptidase act?
Small intestine lumen
Where do aminopeptidases act ?
Small intestine brush border
What secretes aminopeptidases ?
Small intestine epithelial cells
What increases the surface area of the small intestine for absorption ?
Circular folds
Villi
Brush border
- microvilli
What holds together epithelial and mucous cells?
Tight junctions
What does the brush border contain ?
Enzymes that hydrolyse peptides/disaccharides
Enterokinase
Transmembrane carrier proteins
What is contained in the crypts of Lieberkuhn ?
Mucous cells: primary locus for success entericus secretion
Stem cells that constantly produce new epithelial cells
Why is bile neccassary for the digestion of fats ?
Lipase can only act on the surface of fat aggregates
Bile salts act as a detergent and emulsify large fat aggregates into smaller pieces
How do fat droplets repel other droplets ?
The negatively charged hydrophilic tails of of bile salts repel each other
What is the function of colipase ?
Anchor lipase to the bile salts at the surface of a fat droplet
Describe the structure and function of micelles
4-7 nm in diameter
Composed of bile salts, lecithin and cholesterol (core), hydrophilic tail shell
Shuttle digested fat molecules to the epithelial surface
What hydrolyses disaccharides into their constituent monosaccharides ?
Disaccharidases in the brush border
How are glucose and galactose absorbed
Secondary active transport
- co-transported into the cell with Na+ ions
- passively diffuses into the interstitial fluid through a membrane channel
- enter a capillary