Which muscle is primarily responsible for breathing by changing thoracic volume?

• A. Lungs
• B. Diaphragm
• C. Brain
• D. Heart

Answer: (B)

The diaphragm is the main muscle that changes the thoracic volume to drive breathing. When it contracts, it moves downward and flattens, increasing the vertical size of the chest cavity. This expansion lowers the pressure inside the thorax, allowing air to flow into the lungs. While other muscles, like the intercostals, can assist during deeper or labored breathing, the diaphragm does the primary job of creating the volume change that initiates inspiration. The lungs themselves aren’t muscles, so they don’t actively change thoracic volume; they simply fill as the chest expands. The brain regulates breathing rhythm, and the heart pumps blood, but neither is the primary driver of the thoracic volume change used for inhalation.