The vent is an opening that allows air, gas, or liquid to pass or into a confined space. In this case, I am mainly referring to the vent that is the outlet and inlet for circulating air to prevent asphyxiation, air poisoning, and maintaining clean air for the humans living in the building. The vent is a flat steel grille, often powder-coated has its grid that is often vertical or horizontal or even intertwined. It could be found indoors on the wall, ceiling, in the closet, or outdoors on the facade of the building, etc. The vent is constantly ignored. People remember its existence until the system operates- The faint white noise or the sound of roaring steel vibrating comes from them.  

It is interesting to note that the vent of interior space is welcomed and treated kindly because it represents the circulation system- clean air, plus, is the cold AC in the summer and the heater in the winter that makes people feel comfortable in a confined space. In contrast, the exterior vent is not welcome because it is often surrounded by unpleasant smells- the smell of dirty air exhaled by others. 

You can’t tell by looking at it if the air is sucking in or out until you touch it with your hand or use a thin light object from an external source like paper to know its direction. Unfortunately, the vent is a gate of the airflow, is an enter or exit. Only if it is attached to the system, it has the effect of ventilation; otherwise, the vent is just a systematic mechanism of airflow- It reveals a vest mechanism is running or perhaps was running in the space.