What does the contraction of the phrenic nerves stimulate, resulting in increased volume and decreased pressure?

The contraction of the phrenic nerves stimulates the contraction of the diaphragm, which is the primary muscle involved in breathing. When the diaphragm contracts, it moves downward into the abdominal cavity, increasing the volume of the thoracic cavity. This increase in volume leads to a decrease in pressure within the thoracic cavity, allowing air to be drawn into the lungs as the pressure inside becomes lower than the atmospheric pressure outside.

This mechanism is essential for inhalation. The diaphragm's movement is crucial for effective ventilation, as it significantly influences the overall dynamics of breathing. During normal, quiet respiration, the diaphragm is the predominant muscle used, making it vital for sustaining the respiratory process.