Absorption of Ions and Water Flashcards
What is the surface area of the small intestine increased by?
- Macroscopic folds of Kerckring (plicae circulares)
- Villi + microvilli
What ions does absorption of water depend on primarily?
- Na+
- Cl-
Mechanism of water absorption
- Transport of Na+, Cl- and HCO3- into lateral intercellular spaces
- High NaCl near apical end of spaces means it is hypertonic
- Osmotic flow of water from lumen into spaces
4 different transport routes of entry for Na+
- Na+/glucose transport (or Na+/amino acid)
- Na+/H+ exchanger
- Parallel Na+/H+ and Cl-/HCO3- exchange
- Epithelial Na+ channels
Mechanism of parallel Na+/H+ and Cl-/HCO3- exchange
- Water + CO2 brought in from blood
- Via carbonic anhydrase, these are turned into H+ and HCO3-
- These are transported into lumen separately, bringing in Na+ and Cl- respectively
Which substances regulate parallel Na+/H+ and Cl-/HCO3- exchange, and what happens if their levels increase?
- cAMP
- cGMP
- Intracellular Ca2+
- Increase in any decreases NaCl absorption
How do mineralocorticoids increase Na+ absorption?
- Increase opening of apical channels
- Insertion of preformed channels from sub-apical epithelial vesicle pools into apical membrane
- Increase synthesis of apical channels + Na+/K+ pumps
What effect do absorptagogues have on intestinal electrolyte transport?
Promote absorption
What effect do secretagogues have on intestinal electrolyte transport?
Promote secretion-diarrhoea
What are examples of absorptagogues?
- Angiotensin
- Aldosterone
- Somatostatin
- Enkephalins
- Noradrenaline
How does angiotensin increase Na+ absorption?
- Dehydration + drop in effective circulating volume stimulates RAAS
- Angiotensin levels increase
- Enhances NaCl absorption by upregulating apical membrane Na+/H+ exchange
How does aldosterone increase Na+ absorption?
- Dehydration + drop in effective circulating volume stimulates RAAS
- Aldosterone levels increase
- Stimulates Na+ absorption through epithelial Na+ channels
4 categories of secretagogues
- Bacterial enterotoxins
- Hormones + neurotransmitters
- Products of cells of immune system
- Laxatives
How does the bacterial cholera toxin (enterotoxin) cause secretory diarrhoea?
- Enterotoxin induces intracellular cAMP concentrations
- This increase Cl- and K+ secretion + inhibits parallel Na+/H+ and Cl-/HCO3- exchange
Net uptake of Ca2+ in a day (using amounts absorbed and secreted)
- Absorbed = 500mg/day
- Secreted = 325mg/day
- Net = 175mg/day uptake
Active and passive movements of Ca2+ in the small intestine
- P = passive absorption of Ca2+ via paracellular pathway (not under control of VDR)
- A = active, trans-cellular uptake by epithelial cells of duodenum (under control of VDR)
What form must iron be in to be absorbed, and how does it get to this state?
- Fe2+
- Fe3+ reduced in stomach acid or by the ferrireductase enzyme duodenal cytochrome B (Dcytb)
Which protein absorbs Fe2+?
DMT1
3 fates of Fe2+ after being absorbed into the cell
- Used within cell
- Stored in protein ferritin
- Effluxed into blood via ferroportin (FPN), re-oxidised to Fe3+ and bound to transferrin
If iron is contained in haem, how is it absorbed and how is iron released?
- Haem-Fe absorbed via haem carrier protein 1 (HCP1)
- Haem oxygenase 1 (HO1) deconstructs inside cell to release Fe2+
How is iron transported into non-enterocytes?
- Fe-transferrin captured by receptors on cells requiring iron
- Transported in and iron released
Where are the major stores of iron?
- Liver
- Spleen
- Bone marrow
What do iron stores use to communicate with the small bowel to regulate iron absorption?
Hepcidin
How is hepcidin used to regulate iron absorption?
- Binds to ferroportin
- Prevents transport of Fe2+ out of enterocytes
