Acid/Base balance Flashcards
What’s the normal pH of the body? What concentration of H+ does this correlate to?
pH = -log[40*10^-9 eq/L] = 7.4
Which processes in the body produce acid?
Your body produces lots of acid: oxidation of amino acids, fats, carbs
What’s the major buffer we care about and why?
HCO3: it’s generated by the body
Its pKa is close to the body’s pH = 6.1
All the buffers are reflective of one another: the state of one is comparable to the state of another
Henderson Hasselbach Equation
pH = pKa + log [HCO3]/alpha*pCO2
pKa=6.1
alpha=0.03
What’s the kidney’s role in acid base balance?
Makes bicarb to replace what we lose in the lungs and in the kidney
Each day we lose 100 mEq of bicarb assuming 100 kilos body weight
HCO3 + H+ <–> H2CO3 <–> CO2 + H2O
How does the kidney reabsorb HCO3?
Via H+ secretion, in the proximal collecting tubule and collecting duct
(1) H+ is secreted, combines with bicarb in the urine to form H2O and CO2
(2) the CO2 goes back into the cell
Where is most of the bicarb reabsorbed?
In the proximal tubule, because it comes first! by the time it gets to the CD most of the bicarb is gone
What’s the difference between bicarb reabsorption/H+ secretion in the PCT and CD?
(1) different channels: in PCT, it’s a Na/H exchanger that absorbs Na, pumps out protons; H+ combines with bicarb forming CO2 and H2O, and the CO2 goes back into the cell, becomes bicarb, which is pumped back into the blood with bicarb/Cl- exchanger
In the CD, it’s a H+ transloc. ATPase = pump (allows you to create up to 1000 fold gradient of H+); the H+ combines with NH3 to be excreted
(2) By the time the urine gets to the CD, not much bicarb will be left; each H+ will remain in the urine and not be balanced by a return of a bicarb! This is so great because this is how the body gets a net gain of bicarb
What limits the capacity of the proximal tubule to reabsorb HCO3?
If the concentration of HCO3 increases too much
At a certain point, the reabsorptive capacity is saturated and HCO3 has a net excretion
What regulates HCO3 reabsorption into the proximal tubule?
Volume depletion –> increased reabsorption
Osmotic/hydrostatic forces
Ang II directly stimulates Na/H exchanger: Na in, H+ out, so CO2 is reabsorbed
What regulates H+ transport?
pCO2 = most important
aldosterone
K+
Acid-base status
NH3
pH gradient
Membrane potential (determined by Na absorption; when urine is neg, more H+ flows in)
How does pCO2 regulate H+ transport?
High CO2 stimulates exocytosis of H+ ATP vesicles in proximal and collecting tubules
How much acid do we actually excrete? How is this possible based on the pH/ volume of urine excrete/day? seems paradoxical
60 mEq/day
if the urine pH= 4.5-5, at a minimum we excrete 1L/day, so it’s only 10 microEq H+
Acid excretion is by other means than just “naked” protons: NH3 (generated from gluconeogenesis in kidney), HPO4
Whats the relationship between acid ingestion and NH3 synthesis?
Directly related
