The primary muscle used in breathing is the diaphragm. This dome-shaped muscle is located below the lungs and separates the thoracic cavity from the abdominal cavity. During inhalation, the diaphragm contracts and moves downward, creating negative pressure in the thoracic cavity, which allows air to be drawn into the lungs. Conversely, when the diaphragm relaxes during exhalation, it moves upward, pushing air out of the lungs. The diaphragm plays a critical role in respiration, working alongside accessory muscles during heavy breathing or exertion. Its efficient functioning is essential for maintaining adequate oxygen supply to the body.