During quiet breathing inspiration, which muscle contracts primarily?

• A. Scalenes
• B. Internal intercostals
• C. Diaphragm
• D. Sternocleidomastoids

Answer: (C)

The diaphragm is the primary muscle that contracts during quiet breathing, particularly during the phase of inspiration. Situated at the base of the thoracic cavity, the diaphragm is a dome-shaped muscular structure that separates the thoracic and abdominal cavities. When the diaphragm contracts, it moves downward and flattens, which increases the volume of the thoracic cavity. This increase in volume creates a negative pressure relative to atmospheric pressure, allowing air to flow into the lungs and facilitating the process of inhalation.

This action is crucial for maintaining effective gas exchange in the lungs, as it actively draws in air, allowing oxygen to enter and carbon dioxide to be expelled. In quiet breathing, which is typically unconscious and requires little effort, the diaphragm is the principal player, while other muscles involved in respiration, such as the external intercostals, may assist but do not primarily facilitate the initial phase of the breath.

The other muscles listed in the options play roles in more forceful types of breathing or are involved in expiration rather than quiet inhalation. Hence, the prominence of the diaphragm in quiet breathing underscores its importance in the mechanics of ventilation and gas exchange.