You Always leave the screens open at least a bit. Thats why we have such a large screen in the front and both of the windows with screens as well. These are your primary sources of ventilation. If you close everything up you will suffocate. vents are no different. They also have a screen. If you close a vent, you will have the same results . It's no different then the 3 screens provided. You must have air flow. Thats why they call them vents.
It is short for "Ventilation". The 3 screens are your "vents".
Always make sure you have a proper source of air flow, and never completely close all your screens.