Absorption + secretion Flashcards
Semester 1 year 1
What is secretion in the GI tract controlled by?
Hormonal, paracrine + neurocrine agents
Where is fluid absorbed in the GI tract?
-most absorbed in small intestine
-rest in large intestine or lost in faeces
What does the salt and water balance help do?
-regulate ECF volume
-maintain blood pressure
What are 3 main functions of the stomach?
-secretions
-motor - regulate food intake/transit, mix to decrease food particle size
-humoral regulation - gastrin + somatostatin (regulate secretions)
What are the proximal secretions in the stomach?
-HCl - decrease stomach pH
-pepsinogens - precursor to pepsins (hydrolysed to become active pepsins)
-intrinsic factor
-mucins/bicarbonate ions
Why do pepsins begin as pepsinogens?
Ensure the enzymes don’t attack the wall as they’re in inactive form
What are the distal secretions in the stomach?
-gastrin
-somatostatin
-pepsinogens
What are basal secretions from the stomach secretory cells rich in?
-Na+
-when stimulated, become rich in H+ (from HCl)
What cells make up oxyntic glands in the body and what do they secrete?
-epithelial cells - HCO3^-
-mucous neck cells - mucus
-parietal cells - HCl + intrinsic factor
-enterochromaffin-like cells - histamine
-chief cells - pepsinogen
-enterochromaffin cells - serotonin, VIP, substance P
-D cells - somatostatin (hormone)
What cells make up antrum pyloric glands and what do they secrete?
-same as the body oxyntic glands, except no parietal cells
-also have G cells - gastrin hormone
What is the structure of resting parietal cells?
-cytoplasmic pool of tubulovesicular membrane on apical side
-contain acid secreting H, K-ATPase
What is the structure of stimulated parietal cells?
-stimulation induces cytoskeletal changes
-fusion of tubulovesicular + canalicular membranes
-increase SA
-appearance of microvilli
-insertion of H, K-ATPase pump + K+ Cl- channels
Describe the process of gastric acid secretion
-H2CO3 dissociates
-H+ secreted across apical membrane via H+-K+ ATPase - Cl- follows via channel
-HCO3^- absorbed into blood via Cl-, HCO3^- exchanger
-net secretion of HCl + absorption of HCO3^-
-K+ recycled into lumen via K+ channels
Are the structures and functions of the small and large intestine similar or not?
-similar
-both have crypts of Lieberkühn
What do the small and large intestine absorb?
-absorb fluids + electrolytes via villus cells/surface epithelia respectively
-Na+, K+, Cl-, HCO3^-
What additional thing does the small intestine absorb that the large doesn’t?
-hydrolysed food after digestion
-via luminal + border brush enzymes
What do crypt epithelial cells do?
-secrete fluid + electrolytes
-protects against bacteria + toxins
What do ‘polar’ cells have?
Apical + basolateral membranes separated by tight junctions
What is transcellular movement?
Usually active across at least 1 membrane
What is paracellular movement?
Generally passive + determined by tight junctions
What is fluid movement coupled to?
-coupled to solute movement
-sometimes the reverse - solvent drag
Why does the small intestine have a large surface area?
-folds of Kerckring (plica)
-villi (+ crypts)
-microvilli
What are the duodenum + jejunum in the small intestine?
Primary site of Na+, Cl-, K+, HCO3^- absorption
In the large intestine, what is absorbed and secreted?
-net absorption - H2O, Na+, Cl-
-secretes K+, HCO3^-
Describe cellular transport (absorption) in the jejunum
-all Na+ transcellular absorption occurs via Na+-K+ ATPase
-low IC Na+ acts as driving force for Na+ entry via Na+ channels or Na-H ion exchanger + Na+-glucose transporter (secondary active transport)
-luminal HCO3^- increases Na+ reabsorption by stimulating the apical Na-H ion exchanger
-net absorption of NaHCO3
Describe cellular transport (absorption) in the ileum
-same transport mechanism as jejunum + Cl-, HCO3^- exchanger + Cl- transporter
-H+ and HCO3^- generated, H+ secreted into lumen via Na-H ion exchanger
-HCO3^- secreted into lumen by Cl-,HCO3^- exchanger
-net absorption of NaCl
Describe how pancreatic secretion occurs using ductal cells
-acinar cells secrete enzymes (CDK receptors)
-centroacinar cells secrete isotonic aqueous portion
-HCO3^- secreted into pancreatic juice by Cl-HCO3 exchanger
-H+ transported into blood via Na-H exchanger
-net secretion of HCO3^- + absorption of H+
Describe how absorption occurs in the large intestine (colon)
-synthesis of Na+ channels induced by aldosterone
-increased Na+ channels = increased Na+ entry across apical membrane = increased Na+ across basolateral membrane via Na-K ATPase
-causes increase in K+ in cell = increased K+ secretion across apical membrane
Describe how carbohydrates are absorbed in the small intestine
-glucose + galactose - co-transport against electrochemical gradient (high intracellular conc.)
-use sodium gradient to drive movement - low Na intracellular conc. due to ATPase on basolateral
-fructose - facilitated diffusion
Describe how proteins are absorbed in the small intestine
-amino acids move in with Na down electrochemical grad. (secondary active transport)
-dipeptides + tripeptides move in with H+ using co-transport
-peptidase converts them to amino acids which then move into blood via facilitated diffusion
Describe the first part of lipid absorption in the small intestine (prior to moving into the cell)
-pancreatic + other lipases, aided by bile salts complete lipid hydrolysis in duodenum + jejunum
-products of lipid digestion - cholesterol, lysophospholipids, monoglycerides + free fatty acids
-lipid digestion products solubilised in mixed micelles
-micelle exterior lined with amphipathic bile salts
Describe the second part of lipid absorption in the small intestine (moving into the cell)
-mucus gel layer lining epithelial surface contain fatty acids that become protonated + cross luminal surface of enterocyte
-once inside cell, products are re-esterified with free fatty acids in SER
-packaged into chylomicrons
-cross basolateral membrane + absorbed into lacteals - glycerol enters blood supply
-bile salts are recycled - enterohepatic circulation
How can the fatty acids in the mucus gel layer cross the luminal surface of enterocyte?
-diffusion
-incorporated into enterocyte membrane
-carrier mediated method
