Week 6 - Acid Base Balance Flashcards
What is alkalaemia/ acidaemia?
- pH greater than 7.45 -> alkalaemia
- pH less than 7.35 -> acidaemia
What is the effect of alkalaemia on calcium ions? What is the consequence of this?
- Lowers free calcium by causing the ions to come out of solution -> bound to albumin instead of H
- Leads to increased neuronal excitability causing paresthesia and tetany
What effect does acidaemia have on K? What is the major consequence of this?
-Causes hyperkalaemia as H+ are taken into the cell in exchange for K+ in order to lower H+ conc
Why is a lower pH disruptive?
-Denatures proteins causing enzyme disturbances and affecting muscle contractility, glycolysis and hepatic function
Below what pH is life threatening?
-7.0
How is plasma pH determined?
-Ratio of [HCO3]/CO2
What is the henderson-hesselbach equation?
- pH=pK+log([hco3]/(CO2 x0.23))
- pK is a constant = 6.1 at 37 degrees
What is the normal ratio of HCO3:CO2?
-20:1
What ultimately determines the pCO2?
-Respiration controlled by the chemoreceptors
What controls the concentration of HCO3?
-Kidneys
What is the effect of hypoventilation on partial pressures and acid base balance?
-Decreased pO2, increased pCO2 -> respiratory acidosis
What is the effect of hyperventilation on partial pressures and acid base balance?
-Increased pO2, decreased pCO2 -> respiratory alkalosis
What controls respiration?
- Detectoion of pO2 and pCO2 by peripheral and central chemoreceptors respectively
- Send information to respiratory centre in brain
How are respiratory changes in acid base balance corrected?
-Change in ventilation rate
How are respiratory changes in acid base balance compensated?
-Change in [HCO3]
What produces the main source of bicarb in the blood?
-RBCs
When is metabolic acidosis caused?
-When there is an increase in the production of acids produced by tissues which reacts with HCO3 and decreases pH
How are metabolic changes in acid base balance corrected?
-Increased HCO3 production or increased excretion
How do the kidneys increase [HCO3]?
- Recover all filtered HCO3
- Make new HCO3
How do the kidneys produce HCO3?
- Kidneys have high metabolic rate and produce lots of CO2
- CO2 reacts with water to form HCO3 and H+
- HCO3 enters plasma
- H+ enters filtrate
Describe the recovery of HCO3 in the kidneys
- 80% resorption in PCT
- NaKATPase sets up an Na gradient
- NHE brings Na into the cell in exchange for H
- H reacts with HCO3 to form CO2 and H2O
- CO2 diffuses across membrane
- CO2 reacts with H2O inside cell to produce HCO3 and H+
- HCO3 is transported out of the basolateral membrane into plasma on a transporter with Na
Which enzyme is present on apical tubular membrane that speeds up CO2 reactions?
-Carbonic Anhydrase
Besides from CO2, how else do the kidneys make HCO3? give an example. What substance is excreted in urine because of this?
- From Amino acids -> Glutamine to a-ketoglutarate
- Produces HCO3 and NH4+ (enters urine)
What is different between HCO3 resorption from CO2 in PCT and HCO3 production in DCT?
- In PCT Na gradient is sufficient to drive H+ out of cells into the tubule
- In DCT, Na is mostly resorbed so the gradient is not sufficient. Therefore H+ is actively secreted into the lumen by H+ATPase
How is H+ buffered in PCT and DCT?
- PCT-> by secreted NH3
- DCT-> by secreted NH3 and by filtered HPO4
In which cells of the DCT is HCO3 made?
-a-intercalated cells
How are metabolic changes in acid base balance compensated?
-Changes in ventilation rate to increase/decrease pCO2
How is metabolic acidosis compensated for?
-Increased breathing -> increased CO2 blown off -> decreased pCO2
How is metabolic alkalosis compensated for? Why is this limited?
- Decreased breathing -> decreased CO2 blown off -> increased pCO2
- Limited by decreased in oxygen when decreasing breathing
Commonly, when would metabolic alkalosis occur?
-After vomiting -> excreted all acid but HCO3 remains -> absorbed into blood stream -> alkalosis
What is the minimum pH of urine?
-4.5
How and why is acid excreted?
- In the urine reacted with NH3 to produce NH4 or reacted with phosphate
- If acid wasnt excreted it would react with HCO3 and acidosis would occur due to excessive HCO3
What are the cellular responses to acidosis in the kidney?
- Increased NHE activity -> more H+ secreted into lumen -> more HCO3 produced
- Increased ammonium production in PCT to buffer H= for excretion
- Increased H+ATPase in DCT
- Increased capacity to export HCO3 from tubular cells into ECF
What acids are often increased in metabolic acidosis?
- Lactic acid
- Ketoacids
What is the anion gap?
- The difference between [Na]+[K] and [Cl] and [HCO3]
- Usually 10-15 mmol/l
When does the anion gap increase?
-H+ from excess acid reacts with HCO3 and the HCO3 is replaced by the anion of that acid
What is the benefit of the anion gap? Give an example of when it is increased
- Shows wether there is acidosis due to a certain anion which has replaced HCO3
- Seen in ketoacidosis
From which organ does metabolic acidosis occur that does not effect the anion gap as HCO3 is replaced with Cl?
-Renal problems
How is metabolic alkalosis corrected?
-Increased excretion of HCO3
When is correction of metabolic alkalosis problematic?
-When there is also volume depletion because Na recovery is prioritised which increased NHE activity and thus increases HCO3 recovery
What is the relationship between metabolic acidosis and hyperkalaemia?
- metabolic acidosis -> increased H+ in ECF -> exchanged for K+ -> hyperkalaemia
- Hyperkalaemia -> increased K+ in ECF -> exchanged for H+ -> metabolic acidosis
What is the relatinship between metabolic alkalosis and hypokalaemia?
- Metabolic alkalosis -> H+ moved into ECF in exchange for K+ -> hypokalaemia
- Hypokalemia -> K+ moved into ECF in exchange for H+ -> metabolic alkalosis
What is the range in which plasma pH has to be maintained?
-7.35-7.45
Which acid is titratable in the urine?
-That which is bound to phosphate
