When does a small decrease in O2 concentration begin to stimulate hyperpnea?

• A. At 80 mmHg
• B. At 70 mmHg
• C. At levels dropping below 60 mmHg
• D. It does not stimulate hyperpnea

Answer: (C)

Stimulation of hyperpnea, or increased respiratory rate and depth, in response to decreased oxygen concentration is primarily related to the body's need to maintain adequate oxygen levels for cellular metabolism. Typically, the body utilizes both central and peripheral chemoreceptors to monitor levels of oxygen (O2), carbon dioxide (CO2), and pH in the blood.

Under normal physiological conditions, significant stimulation of the respiratory drive begins when arterial oxygen tension drops below approximately 60 mmHg. At this level, the peripheral chemoreceptors, primarily located in the carotid and aortic bodies, become increasingly sensitive to changes in oxygen saturation. Below this threshold, the body senses hypoxemia more profoundly, resulting in hyperpnea as an adaptive response to enhance oxygen delivery to tissues.

At higher levels, such as 70 mmHg or 80 mmHg, oxygen saturation is generally sufficient for the body's needs, and it does not trigger a strong respiratory response. Consequently, the correct understanding of the threshold for stimulating hyperpnea aligns with the cut-off point of approximately 60 mmHg, where compensatory mechanisms begin to actively respond to ensure adequate oxygenation despite the declining levels.