Intestinal absorption Flashcards
Epithelia types in sections
Small intestine = leaky, paracellular occurs
Large = tight epithelium, transcellular so more highly regulated
Water reabsorption mechanism
Osmosis secondary to solute reabsorption
Na+ movement underlying
Through tight junctions and aquaporins
Types of transporter
Passive diffusion = ENaC
Na/H exchanger = NHE
Na+ coupled exchanger
Na/K ATPase
Aldosterone action
Increases Na+ absorption
Absorption in small intestine (start)
Coupled transport of amino acids and glucose with Na+
NHE on apical creates gradient for Na+ to move into cell, also creates gradient for Cl- to be reabsorbed paracellularly
Absorption in later small intestine
HCO3- into lumen and Cl- into cell exchanger
Cl- into cell
Cl- leaves on basolateral with K+ (KCCl)
NHE operates in parallel with an anion exchanger
CO2 hydration occurring in cell with carbonic hydrate
Colon and rectum
Cl- accumulated using the gradient of HCO3- from CA reaction
Apical Na+ influx through ENaC
Cl- efflux on basolateral
NHE on basolateral removes H+
- Na+ less important, paracellular less important
Potassium absorption/ secretion
Can be absorption or secretion
Passive absorption paracellular in small intestine,
Active absorption with H/K ATPase in colon, then K-Cl symporter on basolateral takes K+ into interstitium
Can have active secretion in colon, through action of Na-K ATPase and diffuse across apical
can have passive secretion through paracellular
Calcium absorption
Occurs in upper duodenum, against a transepithelial gradient
Calcium enters through specific channels (ECaC), binds to protein in cytosol (calbindin) and exits on basolateral via a ATPase
Iron absorption
Apical membrane divalent cation transporter, with copper and zinc as well
Becomes Fe3+ in cell
Can be stored or in the blood bound to ferritin
Types of uptake of nutrients
- Hydrolysis in lumen and then uptake (proteins)
- Hydrolysis on the apical membrane (disaccharides)
- Absorption into and hydrolysis within cell (di/tri peptides)
Carbohydrate breakdown
Salivary and pancreatic amylases
Brush border enzymes hydrolyse these products to give glucose, fructose, galactose
Carbohydrate absorption
D-Glucose and D-galactose absorbed on apical SGLT1
Monosaccharides exit basolateral on glut2 (hexoses)
Occurs by midi-jejunum
Protein breakdown
Pepsinogen -> pepsin in the stomach
Further digestion at brush border can occur
Pepsin, trypsin, chymotripsin
Enterokinase activates intestinal peptidases
Protein absorption
Amino acids are absorbed by apical carriers, Na+ dependent
Di/tri peptides can be directly absorbed by H+ dependent apical carrier proteins, then broken down inside cell
By the end of the jejunum
Exit cell on basolateral by Na+ independent carrier
Lipid breakdown
Muscular movements of the stomach emulsify fats, aided by lingual lipases
Most digestion in small intestine
Pancreatic lipase digest triglycerides -> monoglycerides and free FA
Products incorporated into bile salt micelles
Lipid absorption
Micelles diffuse to unstirred acidic layer adjacent to apical membrane, components dissociate at cell surface
Components are reassembled in the SER, form microscopic chylomicron particles
Chylomicrons exported to Golgi and pass into lymph
Lymph -> left subclavian vein via thoracic duct
Water soluble vitamin absorption
B1, B2, B12
Passive diffusion, specific Na+ coupled carrier proteins
Lipid soluble vitamin absorption
Steroids, vitamin A,D,E,K
Absorbed in bile micelles, mostly exit the epithelial cell unmodified in chylomicrons
where does protein breakdown begin
acidic stomach
enzymes in protein breakdown
pepsin = from stomach pancreatic peptidases (trypsin/chymotrypsin/elastase/carboxypeptidases) responsible for the bulk of digestion of proteins to oligopeptides and aa
where does further digestion of oligopeptides to amino acids occur
brush border peptidases
protein absorption
amino acids absorbed by apical carriers, mostly Na+ dependent, different categories of transporters
di and tri peptides directly absorbed by H+ dependent. apical career protein
role of intracellular peptidases
converts the absorbed di and tri peptides into amino acids in cells
absorption of amino acids finished by
end of jejunum
break down of carbohydrates
- salivary amylase initiates
- pancreatic amylase renders most starch in the form of disaccharides, trisaccharide and alpha limit dextrin
- brush border disaccharides release glucose, fructose and galactose
active absorption of carbohydrates
active absorption of D-glucose and D-galactose across apical membrane by Na+ dependent (SGLT-1)
fructose absorption
facilitated diffusion GLUT 5
exit of hexoses across basolateral
via GLUT2
oral rehydration solutions
given for diarrhoea to provide electrolytes
isosmotic salt/glucose solution (1:1)
contain sodium and potassium
what happens to unabsorbed carbohydrates
enter the colon as fuel for gut microflora, basically is fermented
bicarbonate secretion in duodenum
neutralise stomach acid
- create bicarbonate from hydration of CO2 using CA
- exchanger with Cl- on apical puts HCO3- into lumen
regulation of calcium absorption
ECaC and calbindin synthesis regulated by the vitamin D derived hormone calcitriol
cell polarity in GI
apical = lumen basolateral = interstitium/ blood
different methods of macromolecules absorption
hydrolysis in lumen followed by absorption (proteins -> aa)
hydrolysis on apical membrane (disaccharides)
absorption into and hydrolysis within cell (di/tri peptides)
- following luminal hydrolysis, substances may be taken top and then resynthesises in the cell, eg. triglycerides
disaccharides split into
lactose -> glucose and galactose (via lactase)
maltose -> glucose (maltase)
sucrose -> maltose (sucrase)
hydrolysed before absorption in small intestine
importance of brush border disaccharidases
hydrolyse products of main amylase enzymes and sucrose and lactose to give glucose, fructose, galactose
importance of brush border disaccharidases
hydrolyse products of main amylase enzymes and sucrose and lactose to give glucose, fructose, galactose
