MLT GlobeChemistry: Acid-base & blood gases
63 Chemistry MCQs on Acid-base & blood gases with answers and explanations.
At a plasma pH of 7.40, the ratio of bicarbonate to carbonic acid is approximately:
From Henderson–Hasselbalch, 7.40 = 6.1 + log(HCO3−/H2CO3), so the ratio is about 20:1. Reversing it (1:20) would describe severe acidosis.
Which arterial blood pH value indicates acidaemia?
The adult reference interval is 7.35–7.45. A pH of 7.30 is below it (acidaemia); 7.38 and 7.42 are normal and 7.48 indicates alkalaemia.
In a healthy adult, arterial blood pH is normally held within:
Buffer systems, lungs and kidneys keep blood pH near 7.4. Values below 7.35 indicate acidemia and above 7.45 alkalemia.
HCO3− is 24 mmol/L and pCO2 is 40 mmHg. Which pH is calculated from Henderson–Hasselbalch?
Dissolved CO2 = 0.0307 × 40 ≈ 1.2 mmol/L; 24/1.2 = 20; log 20 = 1.30; pH = 6.1 + 1.3 = 7.40.
pH 7.49, pCO2 28 mmHg, HCO3− 21 mmol/L in an anxious patient. This is:
High pH with low pCO2 is respiratory alkalosis. HCO3− falls only about 2 mmol/L per 10 mmHg in the acute phase, fitting an acute disorder such as anxiety-related hyperventilation.
A blood gas sample was left at room temperature for 60 minutes before analysis. Which change is expected?
Leukocytes and other cells continue to consume O2 and produce CO2 and lactate, so pO2 and pH fall while pCO2 rises. Capping prevents air exchange but not metabolism.
The Clark electrode measures:
The Clark electrode is amperometric: oxygen diffuses through a membrane and is reduced at a platinum cathode, producing current proportional to pO2. The Severinghaus electrode measures pCO2.
On a blood gas analyser, bicarbonate is usually:
Blood gas analysers measure pH, pCO2 and pO2 and derive HCO3− using Henderson–Hasselbalch. Chemistry analysers measure total CO2 enzymatically or by ISE.
Which change shifts the oxyhemoglobin dissociation curve to the left (increased oxygen affinity)?
Alkalosis, hypothermia, low 2,3-BPG, carboxyhemoglobin and fetal hemoglobin shift the curve to the left. Acidosis, fever, high CO2 and high 2,3-BPG shift it to the right.
For lactate measurement, the blood sample should be:
Glycolysis in red and white cells continues after collection and raises lactate. Prolonged tourniquet and fist clenching also increase lactate. Fluoride tubes or prompt analysis on ice are advised.
A venous sample was mistakenly labelled as arterial. Which result pattern most suggests this?
Mixed venous blood normally has pO2 about 35–45 mmHg and pCO2 a few mmHg higher than arterial, with slightly lower pH. Unexpected hypoxemia in a well patient should prompt this question.
What is the usual reference range for arterial pO2 in a healthy adult breathing room air at sea level?
Arterial pO2 is normally about 80–100 mmHg (10.7–13.3 kPa) and falls slightly with age. 35–45 mmHg is the normal arterial pCO2 range.
Which is the most important buffer inside red blood cells?
Hemoglobin, especially deoxyhemoglobin, binds H+ generated when CO2 is converted to bicarbonate, making it the main intracellular buffer in RBCs. Bicarbonate is the main plasma buffer.
Which acid is called "volatile" because the body removes it mainly through the lungs?
Carbonic acid is in equilibrium with CO2, which is exhaled. Non-volatile (fixed) acids such as sulfuric, lactic and keto acids must be buffered and excreted by the kidneys or metabolized.
Before collecting an arterial sample from the radial artery, the modified Allen test is performed to check:
The Allen test confirms that the ulnar artery can supply the hand if the radial artery is damaged or blocked by the puncture. It does not assess blood pressure or coagulation.
The pH electrode on a blood gas analyser works by:
The pH electrode is potentiometric: a special glass membrane develops a potential proportional to H+ activity, measured against a reference electrode. Current from O2 reduction is the amperometric pO2 principle.
A patient found unconscious after an opioid overdose has slow, shallow breathing. Which blood gas pattern is expected?
Opioids depress the respiratory center, causing hypoventilation, CO2 retention and acute respiratory acidosis. Low pH with low HCO3− describes metabolic acidosis.
A patient with diabetic ketoacidosis breathes deeply and rapidly (Kussmaul breathing). This breathing pattern represents:
Low pH stimulates the respiratory center, increasing ventilation to blow off CO2 and limit the fall in pH. It is a compensatory response, not a primary respiratory disorder.
The usual reference range for arterial pCO2 in a healthy adult is:
Normal arterial pCO2 is 35–45 mmHg (4.7–6.0 kPa). 80–100 mmHg is the pO2 range and 22–26 is the bicarbonate range in mmol/L.
The usual reference range for arterial plasma bicarbonate in adults is about:
Arterial bicarbonate is normally about 22–26 mmol/L. 7.35–7.45 is the pH range, 35–45 is pCO2 in mmHg and 135–145 is sodium.