Bicarbonate Flashcards
What is the primary role of bicarbonate?
buffer of ECF and blood
What is the equation for the equilibrium of bicarbonate and its catalyst?
CO2 + H20 -> H+ + HCO3-
- carbonic anhydrase (present in renal tubules)
What is the relation of pH to bicarbonate?
pH is proportional to HCO3- / pCO2
What are the 3 actions the kidney has with bicarbonate?
- bicarbonate filtration
- bicarbonate reabsorption
- bicarbonate regeneration by:
- titratable acid excretion (cannot be increased)
- ammonium excretion (can be increased)
What is the pH at the start of the proximal tubule and how does this change as you move distally?
7.4 (same as blood) and progressively gets more acidic
Where is bicarbonate reabsorbed in the nephron?
- 85-90% in PCT
- 10-15% in DCT and collecting tubule
What are the ways that bicarbonate can be transported from the tubular lumen to the blood?
- combines with H+ to be converted to CO2 and H2O which is capable of diffusing across membrane into the cell
- can change back into bicarbonate in the cell due to the presence of carbonic anhydrase
Proximal tubule reabsorption
- powered by Na/K-ATPase which generates the chemical gradient by pumping Na+ out
- steep inward sodium gradient so Na+ from the urine will diffuse into the cell coupled with H+ being pumped out
- bicarbonate and Na+ inside the cell will be transported out into the blood
Describe the features of the distal tubules
- no carbonic anhydrase so process is much slower
- intercalated cells are tighter so ions which are transported out of the cell and into the urine tend to stay there
Distal tubule reabsorption
- H+ is being transported out of the cell into urine by active transport (against concentration gradient)
- there it will bind to remaining bicarbonate to form CO2 and H20 which diffuses into the cell
- bicarbonate then transported into the blood by HCO3-Cl transport mechanism
In what instances can ammonium excretion increase?
- pathology
- loss of nephrons
Describe titratable acid excretion
In PCT:
- secondary active transport moves Na+ into cell from urine and H+ out
- instead of binding with bicarbonate, H+ binds with non-bicarbonate buffers especially phosphate
- after this the H+ ion is stuck in urine and cannot return to the cell
- CO2 and H20 in urine can diffuse into cell and combine to form new bicarbonate ion and H+
- bicarbonate ion is taken into bloodstream
Other than phosphate what are the other non-bicarb buffers found in urine?
- urate
- creatinine
- beta-hydroxybutyrate
Describe ammonium excretion
- glutamine is produced by liver and muscles and is taken up by proximal tubule cell
- metabolised to alpha-keto-glutarate
- can be changed to glucose or CO2 + H20
- ammonium produced as by-product and excreted into the urine
- new bicarbonate is made as H+ is not being transported out the cell (like it was before) and is now being used up for it
Describe what happens in respiratory acidaemia
- rise in PCO2
- parallel change inside renal tubule cells (as it can easily diffuse)
- intracellular acidaemia
- increases uptake and use of glutamine (and in turn ammonium excretion)
- increases bicarbonate regeneration
- low intracellular pH increases tubular proton secretion and ensure optimum reabsorption of bicarbonate
