What is decreased within the lung as thoracic cavity expands during inhalation?

As the thoracic cavity expands during inhalation, the overall gas density decreases within the lungs. This decrease in gas density occurs because the volume of the thoracic cavity increases, leading to a reduction in pressure according to Boyle's Law, which states that as the volume of a gas increases, its pressure decreases if the temperature remains constant.

When you inhale, the diaphragm and intercostal muscles contract, increasing the volume of the thoracic cavity. This expansion creates a partial vacuum, allowing air to flow in from the atmosphere. As more space becomes available for the air, which consists of various gases, the density of these gases decreases because there is now more room for the same amount of gas particles.

Additionally, although the composition of gases such as oxygen and carbon dioxide remains constant in terms of their partial pressures, their density decreases due to increased volume. This is a crucial concept in understanding respiratory mechanics and how gases behave during breathing.