Which ONE of the following statements about the post-inspiratory phase is CORRECT?

Question

Which ONE of the following statements about the post-inspiratory phase is CORRECT?

BlackTom AI · Accepted Answer

Post-inspiration (the post‑I phase) is characterized by active laryngeal braking: intrinsic laryngeal adductor muscles contract to narrow the upper airway and slow expiratory flow.

1. Relevant knowledge points and concepts
- Respiratory cycle phases: inspiration, post‑inspiration (early expiration), and late/forced expiration. Post‑I is an active, regulated phase that modulates expiratory flow.
- Neural control: post‑I motor activity arises from respiratory neurons in the ventral respiratory column and brainstem nuclei that drive motor neurons supplying laryngeal muscles (via branches of the vagus, including the recurrent laryngeal nerve).
- Laryngeal mechanics: adductor muscles (e.g., lateral cricoarytenoid, interarytenoid) close/narrow the glottis; abductor muscles (posterior cricoarytenoid) open it. Narrowing increases airway resistance and slows flow, producing an expiratory braking effect that aids gas exchange and prevents rapid lung emptying.

2. Step‑by‑step reasoning
- At the end of inspiration, passive elastic recoil would normally produce a rapid expiratory airflow.
- Post‑I neuronal activity transiently activates laryngeal adductors, narrowing the glottic aperture.
- Narrowing raises resistance to outflow; for a given driving pressure, flow decreases (Poiseuille-type behavior: increased resistance → decreased flow).
- The slowed expiration gives more time for gas mixing and stabilizes airway and alveolar pressures.

3. Intermediate/physiological steps (conceptual rather than numeric)
- Drive to expiratory flow = (alveolar/subglottic pressure) / airway resistance.
- Increasing adductor-mediated narrowing → increases resistance → reduces flow for the same pressure gradient.
- Result: slowed outward airflow during post‑I.

4. Why the correct answer is correct
- The statement "The adductor muscles of the upper airway contracts to decrease the diameter of the airways and so slow the rate of outward airflow" accurately describes the laryngeal braking function of post‑inspiration and matches known neurophysiology (post‑I motor drive to laryngeal adductors via vagal motor pathways).

5. Why the other options are incorrect
- Option 1 (Both phrenic and recurrent laryngeal active): the phrenic (diaphragm) activity ceases as inspiration ends; post‑I is mainly laryngeal motor activity rather than continued phrenic inspiratory drive.
- Option 2 (Constriction decreases sub‑glottal pressure, increasing outward airflow): constriction increases resistance and typically reduces flow; it does not increase expiratory flow. The statement confuses pressure/flow relationships.
- Option 3 (Abductor muscles contract to increase diameter and so decrease airflow during inspiration): abductor contraction increases diameter and would increase, not decrease, flow; plus post‑I concerns expiratory braking, not inspiratory abduction.

Conclusion: The correct choice is "The adductor muscles of the upper airway contracts to decrease the diameter of the airways and so slow the rate of outward airflow," which matches the provided answer.

Which ONE of the following statements about the post-inspiratory phase is CORRECT?Single choice

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