Respiration stops in the last stage of expiration, in forced expiration b/c of:
**Core Concept**
In the last stage of forced expiration, airway pressure increases due to the contraction of accessory muscles and the diaphragm. This results in the closure of the glottis, leading to a temporary cessation of airflow and respiration.
**Why the Correct Answer is Right**
During forced expiration, the diaphragm and accessory muscles contract to increase airway pressure. As pressure builds up, the glottis (the opening between the vocal cords) closes, causing a brief pause in breathing. This mechanism is crucial in forced expiration, as it allows for the efficient expulsion of air from the lungs. The closure of the glottis is mediated by the contraction of the cricothyroid muscles, which pull the vocal cords together.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the mechanism of forced expiration. While the external intercostal muscles do assist in inspiration, they are not primarily responsible for forced expiration.
* **Option B:** The internal intercostal muscles help to compress the lungs and ribs during forced expiration, but they do not directly contribute to the closure of the glottis.
* **Option D:** The abdominal muscles do assist in forced expiration by increasing intra-abdominal pressure, but they do not directly cause the closure of the glottis.
**Clinical Pearl / High-Yield Fact**
In patients with respiratory failure, the ability to maintain a patent airway during forced expiration is crucial for effective ventilation. Clinicians should be aware of the importance of glottic closure during forced expiration and take steps to ensure adequate oxygenation and ventilation in critically ill patients.
**Correct Answer:** C.