Acid Base Balance Flashcards
(30 cards)
What’s the normal blood pH range?
7.36 - 7.44
What’s the main buffer system that aims to restore blood pH?
Bicarbonate buffer system:
H20 + Co2 <> H2CO3 <> H+ + HCO3-
Lungs regulate [CO2] and kidneys regulate [HCO3-]
How do the lungs and kidneys maintain pH homeostasis?
Kidneys control ECF pH by excreting more/less H+ in urine and amount of HCO3- reabsorbed in PCT
Lungs increase/decrease ventilation rate to alter amount of Co2 puffed off
By what mechanism does PCo2 and HCO3- affect blood pH, referring to Henderson-Hasselbalch equation?
Ratio of [Co2]:[HCO3-] determines pH, 20:1 maintains pH at 7.4
If concentration of either is changed then equation will become imbalanced, changing the pH
Describe respiratory acidosis - how might the kidneys compensate?
Raised pCO2 and reduced pH
Kidney compensation: reabsorbing more HCO3- and excreting more H+ in urine, so [HCO3-] will increase if compensated
Define acid
H+ proton donator
Define base
H+ proton acceptor
What’s the difference between a strong and weak acid?
Strong acids (HCL) completely dissociate in water, releasing lots of H+
Weak acids (H2CO3) incompletely dissociate in water, reaching equilibrium with its conjugate base, forming a buffer pair that responds to changes in H+ by reversibly binding H+
What’s a 1 unit pH change equivalent to, with regards to [H+] and why?
1 unit pH change = 10-fold [H+] change
Because negative log base 10 to H+ gives pH scale 1-14
How is homeostasis of most ions controlled, compared to H+?
Balance of intake, production and excretion (mainly by kidney)
Acid-base regulation controls H+ concentrations
What are 2 reasons why H+ concentration needs to be tightly regulated?
Small changes alter protein/enzyme activity
Alters binding of other ions (low H+ increases Ca2+ binding to Albumin)
What are the 2 sources of H+ in the body?
Volatile acids (easily vapourised and excreted by the lungs)
Non-volatile acids (eg organic acids excreted by the kidneys)
What are the 3 main mechanisms to minimise changes in pH?
Buffer systems - rapid chemical reactions that minimise sudden change in pH
Lungs - rapidly adjust excretion of CO2
Kidneys - slowly adjust excretion of H+ into urine and alter HCO3- reabsorption
What’s a buffer?
Any substance that can reversibly bind H+
Buffer + H+ <> HBuffer
What are the 3 buffer systems in the body?
Bicarbonate buffer system (extracellular)
Phosphate buffer system (intracellular and urine)
Protein buffer system (mainly intracellular)
Why is urine acidic?
pH 6
Kidneys must excrete H+ from non-volatile acid production to maintain the acid-base balance
Where is the majority of filtered HCO3- reabsorbed and how?
PCT
In tubular lumen HCO3- + H+ (that’s transported via Na+/H+ exchanger) to form H2CO3
H2CO3 dissociates and CO2 diffuses across luminal membrane
When inside tubular cell H2O + CO2 -> H2CO3 via carbonic anhydrase which then dissociates to H+ + HCO3-
HCO3- moves across basolateral membrane into kidney interstitial fluid with Na+
HCO3- can’t be directly transported from the lumen - what does it need?
Carbonic anhydrase and sectreted H+
Where in the kidney is H+ secreted and how?
Late distal and cortical collecting tubules
In type A intercalating cells: H+/K+ ATPase pump
What two things can increase the activity of the H+/K+ ATPase pump?
Aldosterone
Hypokalaemia
Increase H+ secretion and K+ reabsorption
What are the 2 main urinary buffers and what are they essential for?
Ammonia and phosphate
Comfort and allow sufficient H+ to be excreted in urine
How is H+ excreted by the urinary phosphate buffer?
H+ secreted into tubule lumen by Na+/H+ exchanger
H+ + NaHPO4- = NaH2PO4 (excreted in combination)
Outline the urinary ammonium buffer
NH4+ synthesised from glutamine in PCT
NH3 + H+ -> NH4+
Ammonia secreted in collecting duct where it picks up excess H+ and is excreted at ammonium NH4+
Why are renal responses to acid base imbalance slower than the lungs? (think about ammonia)
Decrease in pH stimulates renal glutamine metabolism = increased H+ secretion
Slower as requires protein synthesis
