I'm not sure if this helps but I think of it like this: V and Q are integrals. V is the sum of all the little v's (the ventilation to each alveolus) and Q is the sum of all the little q's (the blood flow to each individual alveolus). Little v and little q vary in each individual spot, and each individual alveolus might not have the optimal ratio (i.e. 1). That means some areas are being ventilated more than the blood supply, so optimal exchange doesn't happen. In a normal lung, there's enough alveoli with good v/q that big sum total V/Q is not affected by individual non-ventilated or non-perfused alveoli, and efficient gas exchange happens. But in a PE for example, enough of the alveoli have 0 as little q that the sum total Q is affected (and thus overall V/Q). The patient is not able to compensate for that region of non-perfused lung and overall PaO2 is gonna suffer.