Which muscles are the two primary respiratory muscles?

• A. Internal intercostals and abdominal muscles
• B. Diaphragm and external intercostals
• C. Pectoralis major and serratus anterior
• D. Diaphragm and internal intercostals

Answer: (B)

During quiet, unforced breathing, air intake is driven mainly by the diaphragm and the external intercostal muscles. When the diaphragm contracts, it flattens and moves downward, increasing the vertical space in the thoracic cavity. Simultaneously, the external intercostals raise the ribs, expanding the chest from front to back and side to side. This combined widening lowers intrathoracic pressure, drawing air into the lungs.

The other muscles listed aren’t primary breathers. Internal intercostals assist mainly with forced expiration by pulling the ribs down, and abdominal muscles help push air out during heavy or forced expiration by increasing intra-abdominal pressure. Pectoralis major and serratus anterior can aid in deep or labored breathing, but they are not the main drivers of normal inspiration.

So, the two primary respiratory muscles are the diaphragm and the external intercostals.