How is the thoracic pressure decreased during inspiration?

The thoracic pressure is decreased during inspiration by the contraction of the diaphragm and the external intercostal muscles. The diaphragm is the main muscle of inspiration and it contracts to flatten and descend, increasing the volume of the thoracic cavity. The external intercostal muscles also contract to pull the ribs upward and outward, further increasing the volume of the thoracic cavity. As the volume of the thoracic cavity increases, the pressure inside decreases, which causes air to flow into the lungs.