Questions: Which of the following is false? Changes in arterial pH resulting from CO2 retention or metabolic factors act through the peripheral chemoreceptors. When arterial PO2 falls below 60 mm Hg, it becomes the major stimulus for respiration. Oxygen levels are the primary stimulant of respiration. Under normal conditions, blood PO2 affects breathing only indirectly.

The answer is the third one: Oxygen levels are the primary stimulant of respiration.

Explanation:

1. Changes in arterial pH resulting from $\mathrm{CO}_{2}$ retention or metabolic factors act through the peripheral chemoreceptors.

   • This statement is true. Peripheral chemoreceptors respond to changes in blood pH, which can be affected by $\mathrm{CO}_{2}$ levels and metabolic factors.

2. When arterial $\mathrm{PO}_{2}$ falls below 60 mm Hg, it becomes the major stimulus for respiration.

   • This statement is true. When $\mathrm{PO}_{2}$ drops below 60 mm Hg, it significantly stimulates peripheral chemoreceptors to increase respiration.

3. Oxygen levels are the primary stimulant of respiration.

   • This statement is false. Under normal conditions, $\mathrm{CO}_{2}$ levels and the resulting pH changes are the primary stimulants for respiration, not oxygen levels.

4. Under normal conditions, blood $\mathrm{PO}_{2}$ affects breathing only indirectly.

   • This statement is true. Normally, $\mathrm{PO}_{2}$ affects breathing indirectly through its influence on $\mathrm{CO}_{2}$ and pH levels.