Which muscles are primarily involved in inspiration?

• A. Internal intercostals and diaphragm
• B. Diaphragm and external intercostals
• C. Scalene muscles only
• D. Abdominal muscles

Answer: (B)

The primary muscles involved in inspiration are the diaphragm and the external intercostals. The diaphragm is the main muscle responsible for inhalation. When the diaphragm contracts, it moves downward and increases the volume of the thoracic cavity, which decreases the pressure in the lungs, allowing air to flow in. The external intercostal muscles assist by elevating the ribs, further expanding the thoracic cavity and facilitating airflow into the lungs.

The scalene muscles do contribute to inspiration, especially during deep breathing or when additional respiratory effort is required, but they are not the primary muscles involved. The internal intercostals are primarily involved in forced expiration rather than inspiration. The abdominal muscles play a significant role in expiration, particularly during forced exhalation, rather than during the inhalation process. Thus, the diaphragm and external intercostals clearly stand out as the two main muscles responsible for the act of breathing in.