MLT GlobeChemistry: Acid-base & blood gases – page 2
63 Chemistry MCQs on Acid-base & blood gases with answers and explanations.
Which is the main buffer system of plasma and extracellular fluid?
The bicarbonate–carbonic acid system is the major extracellular buffer because CO2 is controlled by the lungs and bicarbonate by the kidneys. Hemoglobin is the main buffer in red cells.
Which anticoagulant is used for arterial blood gas samples?
Blood gas syringes contain heparin, usually dry (lyophilised) and balanced for electrolytes. EDTA, citrate and fluoride alter pH or electrolyte results.
Respiratory acidosis is caused by:
When ventilation is inadequate, CO2 accumulates, raising carbonic acid and lowering pH. Hyperventilation causes respiratory alkalosis.
Metabolic alkalosis is primarily characterised by:
The primary change in metabolic alkalosis is a rise in bicarbonate. Changes in pCO2 as the first event define respiratory disorders.
Which organ compensates for respiratory acid-base disorders by changing the amount of bicarbonate retained and acid excreted?
In respiratory disorders, the kidneys compensate by adjusting bicarbonate reabsorption and acid excretion. The lungs compensate for metabolic disorders.
Mixing which pair of solutions produces a buffer?
Partial neutralisation leaves weak base NH4OH together with its salt NH4Cl, a buffer pair. Strong acid–strong base mixtures and excess strong base cannot buffer.
Arterial blood has a pH of 7.40. Its hydrogen ion concentration is approximately:
[H+] = 10^-7.4 mol/L ≈ 4.0 × 10^-8 mol/L, i.e. 40 nmol/L. Each 0.3 pH unit change roughly doubles or halves this value.
pH 7.25, pCO2 60 mmHg, HCO3 26 mmol/L. This is:
Low pH with high pCO2 means respiratory acidosis. Bicarbonate is still near normal, so renal compensation has not yet happened: an acute process such as hypoventilation.
pH 7.30, pCO2 30 mmHg, HCO3 14 mmol/L. This is:
Low pH with low bicarbonate is metabolic acidosis. The low pCO2 shows the lungs are blowing off CO2 to compensate, but pH is not yet normal.
A large air bubble left in an arterial blood gas syringe will most likely cause:
Gases equilibrate with room air, which has high O2 (about 150 mmHg) and almost no CO2. Losing CO2 also raises the pH. Bubbles must be expelled and the syringe capped at once.
The Henderson–Hasselbalch equation for the bicarbonate buffer system is:
pH = pKa (6.1) + log (bicarbonate / dissolved CO2), where dissolved CO2 = 0.0307 × pCO2 (mmHg) in mmol/L. Using 0.23 × pCO2 applies when pCO2 is in kPa.
pH 7.52, pCO2 48 mmHg, HCO3− 38 mmol/L. The most likely disorder is:
High pH with high HCO3− indicates primary metabolic alkalosis; the raised pCO2 reflects hypoventilation as compensation, and pH is still abnormal, so compensation is partial.
A patient with stable COPD has pH 7.36, pCO2 60 mmHg, HCO3− 33 mmol/L. This is best described as:
Raised pCO2 with pH at the low end of normal and markedly raised HCO3− indicates chronic respiratory acidosis compensated by renal bicarbonate retention. pH on the acid side points to the respiratory primary process.
Salicylate poisoning in an adult classically produces which early mixed disorder?
Salicylates stimulate the respiratory centre (respiratory alkalosis) and disturb metabolism, causing organic acid accumulation and a high anion gap metabolic acidosis.
Excess liquid heparin in a blood gas syringe most likely causes:
Liquid heparin has low pCO2 and dilutes the sample, lowering pCO2, bicarbonate and ionised calcium. Dry balanced heparin syringes are recommended.
In the Severinghaus pCO2 electrode, CO2 is detected because it:
CO2 crosses a gas-permeable membrane into a bicarbonate electrolyte, forming carbonic acid and changing pH, which a glass electrode measures. The Clark electrode uses reduction at platinum for pO2.
Co-oximetry determines hemoglobin fractions by:
Co-oximeters measure light absorbance at many wavelengths to quantify oxy-, deoxy-, carboxy- and methemoglobin. Saturation calculated from pO2 does not detect dyshemoglobins.
A house fire victim has pO2 95 mmHg, calculated SO2 98%, but co-oximetry shows oxyhemoglobin 70%. The most likely explanation is:
Carbon monoxide binds hemoglobin without lowering dissolved pO2, so calculated saturation is falsely normal. Co-oximetry measures carboxyhemoglobin directly and shows reduced oxyhemoglobin.
A patient given dapsone has chocolate-brown blood, normal pO2 and reduced oxygen saturation on co-oximetry. Which fraction is increased?
Oxidant drugs such as dapsone and local anaesthetics convert heme iron to Fe3+, forming methemoglobin, which cannot carry oxygen and gives brownish blood. Co-oximetry measures it at about 630 nm.
P50 is defined as:
Normal P50 is about 26–27 mmHg. A lower P50 means higher oxygen affinity (left shift); a higher P50 means lower affinity (right shift).