What changes first in the process of respiration that leads to a change in the second factor?

The correct option is based on the fundamental principles of gas behavior as described by Boyle's Law, which states that the pressure of a gas is inversely proportional to its volume when temperature is held constant. When the process of respiration begins, there is an initial change in the volume of the thoracic cavity due to the contraction of the diaphragm and the intercostal muscles. This increase in volume results in a decrease in pressure within the lungs.

As the pressure in the lungs decreases, air from outside the body flows in to equalize the pressure difference, leading to inhalation. This change in volume directly impacts the pressure inside the lungs and is crucial for the mechanics of breathing. As the lungs expand and the volume increases, the corresponding decrease in pressure allows for air to rush in.

Understanding this principle is vital for comprehending respiratory physiology as it illustrates the relationship between volume and pressure, which is foundational to how gases behave in the respiratory system.