What triggers the Hering-Bruer deflation reflex?

• A. Overinflation of the lungs
• B. Deflation of the lungs
• C. Excess carbon dioxide in the blood
• D. Increased heart rate

Answer: (B)

The Hering-Breuer deflation reflex is primarily triggered by the deflation of the lungs. This reflex is a protective mechanism that helps to prevent the lungs from collapsing and to regulate breathing. When the lungs are deflated, sensory receptors in the lungs detect this change in lung volume, leading to an inhibition of the inspiratory center in the brainstem. This results in a cessation of inhalation and can promote the initiation of expiration, thereby helping to maintain adequate lung function and preventing further deflation.

The reflex plays a critical role during deep breathing and can be particularly relevant in situations like exercise or in patients with conditions that affect lung compliance. The other choices do not accurately represent the trigger for this reflex: overinflation of the lungs would instead activate stretch receptors to halt further inflation, while excess carbon dioxide or an increased heart rate does not directly initiate the Hering-Breuer deflation reflex. Understanding this reflex is important for recognizing the physiological responses to changes in lung volume and maintaining respiratory homeostasis.