Below roughly what arterial oxygen pressure do the peripheral chemoreceptors sharply increase respiratory activity?

1. Peripheral chemoreceptors are responsible for sensing large changes in blood oxygen. 2. They only respond once blood oxygen becomes quite low, at about 60 mm Hg or less. 3. At that point they stimulate an increase in respiratory activity. 4. So the third option is correct. 5. About 100 mm Hg is the normal arterial value, so a receptor firing there would be signalling constantly. 6. About 40 mm Hg is the value in venous blood and in resting tissue, well past the threshold rather than at it. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 22 "The Respiratory System", section 22.3 The Process of Breathing_