Lect 10 Flashcards
how is respiration regulated by plasma CO2
- CO2 diffuses across BBB, forms with water, and the dissociated H+ stimulates the chemosensitive areas of the medulla
- elevated PCO2 stimulates respiration to blow off more CO2
what is the basic role of the kidney in control of bicarbonate
stabilize [HCO3-] at 22-26 mEq/L
how does the kidney control bicarbonate (3 mechanisms)
- complete “recovery” of filtered bilcarbonate when plasma [HCO3-] is < 26 mEq/L
- synthesize new HCO3- above and beyond that entering in the glomerular filtrate
- excrete HCO3- when present in excess
describe the process of how HCO3- is recovered by the kidney
- H2O + CO2 -> H2CO3 inside tubular cells (carbonic anhydrase required).
- H2CO3 dissociates into H+ and HCO3-
- H+ is secreted (drives process)
- HCO3- enteres blood
- secreted H+ reacts with HCO3- in urine; HCO3- does not cross apical membrane to be reabsorbed
where in the nephron, is the most amount of bicarbonate reabsorbed
- proximal tubule: 85%
- **99.9% of all filtered HCO3- is generally reabsorbed
what happens to H2CO3 once it is formed in the tubule fluid from filtered bicarb and secreted H+
- H2CO3 is acted on by carbonic anhydrase to form H2O and CO2.
- CO2 can either be excreted or be reabsorbed into the tubule cell to help form more bicarbonate in the cell
via what mechanisms is H+ secreted in the proximal tubule
- sodium-proton antiport
- actively secreted (requires ATP)
via what mechanisms is HCO3- transported from tubule cell into ECF in the proximal tubule
- HCO3-, Na+ symport (driven by bicarb conc)
- HCO3-,Cl- antiport
via what mechanisms is H+ transported from tubule cell into tubule fluid in collecting duct (type A intercalated cell)
- proton ATPase
- potassium proton antiport (requires ATP)
one HCO3- is released into the peritubular capillaries for every X HCO3- neutralized in the tubule
for every 1 HCO3-
- 1:1
what causes net H+ extrusion to stop
- when HCO3- is gone from the filtrate
- luminal pH falls to 4.5 (pH gradient from 7.4 to 4.5 is 1000 fold)
what is titratable acidity
- primarily filtered phosphate
- pK for phosphate = 6.8 is excellent for buffering
- H+ picked up by phosphate allows synthesis of additional HCO3-
explain why the proxmal tubule metabolizes glutamine from blood
- glutamine metabolized to yeild NH3 and a-ketoglutarate
- NH3 enters tubular fluid and is protonated -> NH4 (diffusing trapping)-> urine
- a-ketoglutarate metabolized to HCO3- -> blood
- Each glutamine -> 2 HCO3- and 2 NH4+
synthesis of NH4 from glutamine is regulated by
intracelluar pH
- acidosis stimulates glutamine catabolism -> allows HCO3- to be returned to the blood to neutralize the H+
what effect does [K+] have on NH4+ synthesis
- hypokalemia stimulates NH4+ synthesis and hyperkalemia inhibits NH4+ synthesis
- related to H+/K+ exchange across cell membrane
