Final answer:
During inspiration, the spill valve is typically closed to allow the lungs to expand and air to be inhaled, which is necessary for maintaining the pressure difference for inhalation.
Step-by-step explanation:
During inspiration, or inhaling, the state of the spill valve is typically closed. This allows for the increase in lung volume that causes air to flow into the lungs. When we talk about gas dynamics, if a valve is open, gas molecules will spontaneously distribute evenly, as evidenced when a valve opens between a flask containing gas and a second flask containing a vacuum. However, during the respiratory process, the goal of inspiration is to draw air into the lungs, not to allow it to escape or distribute evenly. Therefore, to achieve this, the spill valve must be closed to maintain the necessary pressure difference that facilitates inhalation.