True or False: As the thoracic cavity expands, the density of the gases filling the lungs decreases.

The statement is true because when the thoracic cavity expands, the volume of the lungs increases. According to Boyle's Law, which states that pressure is inversely related to volume in a closed system, as the volume increases, the pressure within the lungs decreases.

When the pressure drops below the atmospheric pressure, air flows into the lungs. As the lungs fill with air, the density of the gases decreases because density is defined as mass per unit volume. With the increase in lung volume, although the total mass of the air (gases) remains the same, the space it occupies increases, resulting in lower density.

This concept is fundamental to understanding respiratory physiology. When the diaphragm and intercostal muscles contract during inhalation, they create a larger thoracic cavity, allowing for the reduction in pressure and ultimately decreasing the density of the gases within the lungs.