2 - Acid Base Physiology Flashcards
nl PCO2
40 mmHg
nl bicarb
24 mM
what amount of acid/base do the kidneys have to correct for every day just from body processes?
70 mmol of acid for a 70 kg person (about 1 mmol/kg)
henderson hasselbach eqn
[H+] = 24 x PCO2 / [bicarb]
3 main points / ways kidneys deal with acid base balance
proximal acidification (bicarb reabs, no acid excretion) titratable acids and ammonia (acid excreted) distal acidification (acid excreted)
clinical way to tell if someone has proximal RTA
give them bicarb for like a week and they will stay low (~16-18) because they just can’t reabsorb any more
mechanism of proximal RTA
dec capacity for bicarb reabsorption (defect in Na-H exchange, CA enzyme, etc) > can’t maintain high enough bicarb level > acidosis
Fanconi syndrome
proximal tubular damage causing bicarbonaturia, glycosuria, aminoaciduria and phosphaturia
causes of RTA
idiopathic, myeloma, familial, heavy metal poisoning, CA inhibitors
how does titratable acid formation work?
water is converted to OH and H
OH joins CO2 to form NEW bicarb (reabsorbed)
H is secreted and joins HPO4 to form H2PO4
since the HPO4 is filtered, this is GFR DEPENDENT
how does ammonia excretion work?
NH4 and OH are formed in kidney cells from metabolism
OH joins CO2 to form NEW bicarb (reabsorbed)
NH4 is broken apart, both NH3 and H are secreted, and then they reform NH4 in the tubule
since this is generated by the kidney cells, it can respond to acid loads as long as the GFR > 40
what is the primary site of distal acidification?
type A intercalated cell
how does the distal acidification work?
2 ways:
secretion of H ions from breaking down H20 through proton pumps > forms NH4 in tubule
H-K exchange ATPase that secretes 1 H for every 1 K reabsorbed
mechanism for distal RTA
defect in hydrogen secretion in distal tubules
what effect does hypokalemia have on ammonia conc in urine?
inc secretion of NH3 b/c H moves into cells to maintain charge (intracellular acidosis) so more ammonium is secreted to compensate
effects of distal RTA
tubule lumen is more negative K excretion enhanced > hypokalemia urine not as acidic osteoporosis as bone is used as a buffer hypercalciuria kidney stones
how does distal RTA cause kidney stones?
CaPO4 released from bones in response to acidemia
acidosis > retention of citrate in proximal tubule
high urine pH + inc CaPO4 > precipitation into stones
causes of distal RTA
idiopathic familial rheumatologic (RA, Sjogren's, SLE) drugs (ampho B, ifosfamide, lithium) renal transplant cirrhosis sickle cell
type 4 RTA mechanism
low aldosterone is underlying cause (or principal cell Na channel defect that mimics low aldosterone)
> excess Na, inc charge in tubule lumen > less H pumped out into lumen
also low aldosterone causes hyperkalemia > inhibits proximal NH4 production and bicarb generation falls
causes of type 4 RTA
diabetes
urinary obstruction
medications (bactrim, K sparing diuretics)
renal interstitial inflammation (allergic, SLE)
which type of RTA has lowest bicarb levels generally?
distal
which type of RTA will have hyperkalemia?
type 4
which type of RTA will have high pH urine?
distal
nephrolithiasis and nephrocalcinosis are assoc w/ which RTA?
distal
which type of RTA is most likely to present along with aminoaciduria and/or glycosuria?
proximal
urine “anion gap”
NH4 is indirectly measured in urine
if Cl > (Na+K) > inc NH4 in urine in response to acidosis
if “gap” is pos, NH4 is absent > kidney is not making ammonium > RTA
