Anatomy and Physiology II Practice Test 2026 - Free Anatomy Practice Questions and Study Guide

The diaphragm is considered the major muscle of respiration due to its central role in the breathing process. When the diaphragm contracts, it moves downward, increasing the volume of the thoracic cavity and causing a decrease in internal pressure. This pressure change allows air to flow into the lungs, facilitating inhalation. During exhalation, the diaphragm relaxes and moves upward, reducing the thoracic cavity's volume and pushing air out of the lungs.

While other muscles, such as the anterior scalene and sternocleidomastoid, assist in respiration, especially during deep or labored breathing, they are not the primary muscles responsible for the process. The rectus abdominis plays a role in forceful expiration by helping to compress the abdominal cavity, but its function is secondary to that of the diaphragm. Therefore, the diaphragm's fundamental and consistent involvement in the breathing cycle identifies it clearly as the major muscle of respiration.