In the process of inspiration, the first event that occurs is d) diaphragm descends, thoracic volume begins to increase and rib cage rises.
This action is essential for initiating the breathing process. When the diaphragm contracts and moves downward, it increases the volume of the thoracic cavity. As the volume increases, the pressure inside the thoracic cavity (intrapulmonary pressure) decreases relative to the atmospheric pressure. This pressure difference causes air from outside the body to flow into the lungs.
In summary, the contraction of the diaphragm is crucial as it is the very first step that sets the whole breathing mechanism into motion, enabling the subsequent steps to follow.