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Oral Surgery · single question

A decrease in blood CO2 levels (hypocapnia) leads to which respiratory change?

  1. Increased rate and depth of breathing (hyperventilation)
  2. No change in breathing
  3. Apnea (complete cessation)
  4. Decreased rate and depth of breathing (hypoventilation) — low CO2 removes the drive to breathe
Show answer and explanation

Correct answer: D. Decreased rate and depth of breathing (hypoventilation) — low CO2 removes the drive to breathe

  • A. Increased breathing (hyperventilation) would occur when CO2 is HIGH (hypercapnia) — the opposite.
  • B. There is a direct respiratory change with CO2 levels — the body continuously adjusts breathing based on CO2.
  • C. Apnea is an extreme form of respiratory suppression — typically caused by opioids, not simply low CO2 physiologically.
  • D. Correct. CO2 (carbon dioxide) is the PRIMARY DRIVE for breathing. When CO2 is LOW (hypocapnia — from hyperventilation or low metabolic activity), hydrogen ion concentration in the brain's chemoreceptors decreases → respiratory drive DECREASES → breathing becomes SLOWER and SHALLOWER (hypoventilation). This is a homeostatic mechanism to allow CO2 to build back up to normal levels.

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