A nurse is caring for a patient with respiratory distress and observes the use of accessory muscles during inspiration. Based on the understanding of respiratory mechanics, what is the primary physiological mechanism that facilitates air movement into the lungs during inspiration?

A. Passive recoil is a phenomenon that occurs primarily during exhalation, not inspiration. The lungs and chest wall are elastic structures that, after being stretched during inhalation, naturally return to their resting size. This recoil pushes air out of the lungs, but it does not actively draw air in. Therefore, it does not contribute to the primary mechanism of inspiration. B. The diaphragm relaxes during exhalation, moving upward into the thoracic cavity. This increases intrathoracic pressure, which actually forces air out of the lungs, opposite of the process needed for inspiration. Relaxation of the diaphragm does not facilitate air entering the lungs. C. During normal inspiration, the diaphragm contracts, moving downward and flattening. At the same time, external intercostal muscles contract, lifting the ribs and expanding the thoracic cavity in all dimensions. This expansion increases the thoracic volume and decreases intrapulmonary (intrathoracic) pressure below atmospheric pressure. As a result, air flows into the lungs along the pressure gradient. This is the fundamental physiological mechanism of normal breathing. Accessory muscles, such as the sternocleidomastoid and scalene muscles, are recruited during increased respiratory effort, but the primary mechanism remains diaphragmatic contraction and thoracic expansion. D. Contraction of the abdominal muscles occurs during forced exhalation, such as coughing or active expiration. This action increases intrathoracic pressure by pushing the diaphragm upward and expels air from the lungs, rather than drawing air in. It is not part of normal inspiration and cannot facilitate airflow into the lungs.