Question 1
Multiple choiceHard1 markDuring a controlled observation, blood CO₂ is raised briefly and then allowed to fall. The measurements are:
| Stage | Blood CO₂ (relative units) | Blood pH | Ventilation rate (dm³ min⁻¹) |
|---|---|---|---|
| Baseline | 1.0 | 7.40 | 6 |
| After CO₂ rises | 1.3 | 7.32 | 12 |
| Recovery | 1.1 | 7.38 | 7 |
Which statements are consistent with the data and feedback control of ventilation?
I. The initial CO₂ increase can lower blood pH by increasing blood acidity.
II. The fall in pH causes brainstem chemoreceptors to signal the diaphragm and intercostal muscles to reduce ventilation.
III. Increased ventilation can remove CO₂ from the blood, raising pH and reducing chemoreceptor stimulation so ventilation falls.
Show answer and explanation
Answer: B. Increased blood CO₂ can lower pH. Brainstem chemoreceptors respond to the pH change by signalling the diaphragm and intercostal muscles to increase ventilation; greater CO₂ removal then helps pH recover and reduces the stimulus.
A: stops at the initial pH change and omits the feedback described in III.
C: reverses the response in II and omits the supported CO₂–pH relationship in I.
D: includes II, which incorrectly says ventilation decreases when pH falls.
Remember: increased ventilation removes CO₂, helping blood pH rise.