What is the primary function of the accessory muscles of respiration?

The primary function of the accessory muscles of respiration is to facilitate deeper breathing when needed. These muscles are typically not engaged during normal, quiet respiration but come into play during periods of increased respiratory demand, such as during exercise, illness, or respiratory distress.

When the body requires more oxygen or needs to expel carbon dioxide quickly, the accessory muscles—such as the sternocleidomastoid, scalene muscles, and pectoralis major—help expand the thoracic cavity to increase lung volume. This action allows for greater airflow into and out of the lungs, thereby supporting more vigorous breathing efforts when it's necessary. In such scenarios, these muscles play a crucial role in supplementing the work done by the diaphragm and intercostal muscles, which are the primary muscles for breathing.

While cardiac output, support for digestion, and posture are important aspects of overall physiological function, they are not the primary roles of the accessory muscles during respiration. These muscles are specifically adapted to enhance respiratory effort, particularly in situations where increased ventilation is required.