Basic and Clinical Sciences Examination (BCSE) Practice Exam 2025 – Comprehensive All-in-One Guide for Exam Success

The diaphragm and external intercostal muscles play a crucial role in the process of inspiration, which involves drawing air into the lungs.

The diaphragm is the principal muscle of respiration, and when it contracts, it flattens and moves downward, increasing the volume of the thoracic cavity. This expansion creates negative pressure within the lungs relative to the atmosphere, allowing air to flow in. The external intercostal muscles are located between the ribs and assist in the process by elevating the rib cage when they contract, further increasing the volume of the thoracic cavity and aiding in lung expansion.

In contrast, the internal intercostal muscles primarily facilitate expiration, particularly during forceful breathing, rather than contributing to inspiration. Other muscles listed in the choices, such as abdominal muscles and serratus anterior, are more involved in forced expiration or stabilization rather than the primary act of inhalation. Similarly, the levator costae and scalenus medius assist in respiration, but their role is secondary compared to that of the diaphragm and external intercostals during normal inspiration.