Final answer:
The statement is true; when the diaphragm flattens, it increases the volume of the thoracic cavity, decreasing the pressure within, which allows air to flow into the lungs due to the principles of Boyle's law.
Step-by-step explanation:
When the diaphragm flattens during inhalation, it contracts and moves downward, increasing the volume of the thoracic cavity. According to Boyle's law, which states that the pressure of a gas is inversely proportional to its volume, this increase in volume results in a decrease in pressure within the thoracic cavity (thoracic pressure). As a result of the decrease in thoracic pressure, air rushes into the lungs until the pressure inside the lungs is equal to atmospheric pressure.
Additionally, the external and internal intercostal muscles contract, contributing further to the increased volume of the thoracic cavity. The increase in thoracic cavity volume and the subsequent decrease in thoracic pressure fuel the inhalation process. Conversely, during exhalation, the diaphragm relaxes and moves upwards, and the intercostal muscles relax, which decreases the volume of the thoracic cavity and increases the air pressure inside, pushing air out of the lungs into the atmosphere.