Homemade ventilator

There was a Horizon program about flu pandemics on last night. They seem to come along about every 40 years and vary between a very bad flu season and killing 2% of the population.
One of the points made by the program was that ventilators can save lives in flu patients but that there is no excess capacity of ventilators as would be needed for a pandemic.
So could you make up a ventilator? Our lungs tidal volume is about 500ml[1]. We breath 12 times a minute. Air has to be pushed into your lungs but exhalation is passive.

So could you design a homemade ventilator, they seem like just a timed pump and a release valve to me? Other factors such as nursing levels are important but fundamentally without something to breath for them while they fight the illness many flu victims will die.


[1]http://en.wikipedia.org/wiki/Lung_volumes