Acid-Base Balance

Question 1

What are the main buffer systems?

Answer 1

1. HCO₃^- (26mM) + H⁺ <–> CO₂ + H₂O
2. Hb <–> HbH⁺
3. plasma proteins (PPR) eg albumin (10mM): PPR^2- + H⁺ <–> PPRH^-
4. phosphates (2mM): HPO₄^2- + H⁺ <–> H₂PO₄^- + H⁺ <–> H₃PO₄

Question 2

Most of the CO2 produced metabolically is

Answer 2

converted to bicarb in RBCs (can freely diffuse membranes bc nonpolar)

Question 3

How do kidneys regulate pH?

Answer 3

• alter HCO3- by changes in production and excretion
• alter pH by changes in H+ excretion

Question 4

What are the causes of metabolic acidosis?

Answer 4

non-volatile acids (aka acids that are not CO2):

• suplphuric and phosphoric acids from protein and lipid metabolism (70mmol/day)
• lactic acid from anaerobic metabolism
• keto acids from fatty acid metabolism

Question 5

What is normal kidney [HCO3-]?

Answer 5

24mmol/L

Question 6

What is normal pCO2 at the lungs?

Answer 6

40mmHg (1.2Lmmol/L in blood)

Question 7

How much acid is produced per day?

Answer 7

70mmol

Question 8

Drop in bicarbonate concentration (n: 24-26mmol) suggests

Answer 8

metabolic acidosis; a non-volatile acid has built up and protonated bicarbonate

Question 9

Metabolic acidosis is indicated by

Answer 9

low plasma pH and decreased [bicarb]

Question 10

What is lost in vomiting and what is the pH result?

Answer 10

lose acid (HCL) resulting in metabolic alkalosis

Question 11

What is lost in diarrhoea and what is the pH result?

Answer 11

lose bicarbonate causing metabolic acidosis

Question 12

What is the anion gap?

Answer 12

• cations (Na, K) and anions (HCO3, Cl) in the plasma should match, theoretically
  
  • Na + K = 150mmol/L
  • HCO3 + Cl = 35-40mmol/L
  • ~12mmol/L are missing, this is the anion gap = unmeasured anions:
    
    • 80% albumin
    • phosphates
• normal: ~12mmol/L
• abnormal: <20mmol/L
  
  • ​eg massive increases in lactic acid/lactate

Question 13

An increased anion gap reflects

Answer 13

presence of a non-volatile acid (increase in unmeasured anions) because the anion gap is usually due to a fall in bicarbonate (fewer anions)

Question 14

Which causes of acidosis result in high anion gap?

Answer 14

• lactic acidosis (lactic acid)
• diabetic ketoacidosis (keto acids)
• renal failure (both chronic and acute)

Question 15

How is bicarbonate reabsorbed?

Answer 15

• filtered HCO3- combines with H+ (from Na/H+ exchanger in PCT cells) to produce H2O and CO2
• CO2 diffuses into PCT cell
  
  • some diffuses into plasma
  • rest combines with H2O and is converted to HCO3- and H+ by carbonic anhydrase
    
    • HCO3- is co-transported with Na+ on the basolateral membrane and reabsorbed into circulation
    • H+ is exchanged for Na+ on apical surface and goes into the lumen

Question 16

What is the maximum concentration of H+ that can be secreted?

Answer 16

pH 4.4

Question 17

How is H+ excreted when it is in excess (ie metabolic acidosis when all bicarb has been reabsorbed)?

Answer 17

• carriers:
  
  • phosphate, creatine, NH3 (a good base)
  • NH3 comes from glutamine metabolism in the PCT cells, soaks up H+ in lumen to excrete it