Acid Base Balance Flashcards
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
What’s the difference between respiratory and metabolic acidosis?
Metabolic: primary problem affects [HCO3-]
Respiratory: primary problem affects CO2 excretion
Outline compensation
Ratio of HCO3-:CO2 that gives pH
Therefore abnormality affecting one parameter can be compensated for by the other
Tries to minimise changes in pH and restore to normal
In compensated disorders BOTH HCO3- and CO2 lie outside their normal ranges (in same direction ie both raised/lowered)
Outline respiratory acidosis (what it is, causes and compensation)
Low pH due to increased CO2 (retention)
Causes: any disorder affecting respiration/lungs
Compensation: kidney increases HCO3- production/reabsorption
Outline respiratory alkalosis
Raised pH due to decreased CO2
Causes: hyperventilation, anxiety, altitude
Compensation: kidneys decrease HCO3- production
Outline metabolic acidosis
Low pH due to decreased HCO3-
Causes: excessive HCO3- loss or addition of acid - failure of H+ secretion, severe diarrhoea losing HCO3-
Compensation: lungs increase ventilation to decrease CO2
Outline metabolic alkalosis
Raised pH due to increased HCO3-
Causes: addition of alkali or excessive H+ loss - dehydration, prolonged vomiting
Compensation: lungs decrease ventilation to increase CO2
