Acid-base disturbances

Question 1

Physiological buffering systems

Answer 1

• Buffering H+ in blood:
  1. Plasma proteins: H+ + R-NH2 ⇆ R-NH3+
  2. Haemoglobin: HbO2 ⇆ Hb.H+
• deoxyHb is much better at binding hydrogen ions (H+ acceptor)
  3. CO2/HCO3- system: H+ + HCO3- ⇆ H2CO3 ⇆ H2O + CO2
• Increases in partial pressure of CO2
• Buffer system but not that efficient as a buffer as it is not near its pKa when working pH 7.4
• Phosphate system has a pKa closer to physiological pH range, but bicarbonate system with lower pKa from physiological range is used as there is higher concentration of HCO3- than H+ and both components of system can be independently regulated (CO2 regulated by lungs, HCO3- regulated in kidneys)
• CO2/HCO3- system: pH = pKa +log[HCO3-]/0.23 X [PaCO2] (normal arterial blood will have a pH of 7.4)
• Ratio of pH = pKa + log [kidney]/[lungs] around 20, if ratio increases above 20 then it will increase pH → alkalosis, ratio below 20 will decrease pH → acidosis

Question 2

Disturbances and effects

Answer 2

• Causes can be respiratory (due to PCO2 change) or metabolic (pH changes due to HCO3- change)
• Disturbance in one system compensated by other system (not reset back to normal)
• Disorders include:
  1. Metabolic acidosis: reduction in HCO3- concentration lowers pH, non-respiratory disorder, PCO2 is unaffected
  2. Respiratory acidosis: lungs retain excess CO2, PCO2 elevated above normal, lowers pH
  3. Metabolic alkalosis: HCO3- concentration elevated PCO2 remains on the 40 mmHg isopleth
  4. Respiratory alkalosis: lungs release too much CO2 → lowers PCO, increases pH
• Compensatory mechanisms:
  1. Acidosis: in metabolic PCO2 falls, pH increases (lower right of davenport graph) but in respiratory pH increases and PCO2 remains increases due to inability of lungs to expel air
  2. Alkalosis: in metabolic pH lowers, PCO2 remains elevated (requires conservation of CO2), in respiratory pH lowers, PCO2 remains lower until disorder is treated
• Perfect compensation: blood profile end-states reflect both original disorders and compensatory mechanism
  1. If original disorder was respiratory acidosis/metabolic alkalosis, both HCO3- concentration and PCO2 are elevated above normal
  2. If original disorder was metabolic acidosis/respiratory alkalosis, both HCO3- concentration and PCO2 are reduced