The breathalyzer works by blowing into the air. The current of air generated flows into the device, where there is a chemical agent, which will react if exposed to ethyl alcohol.
The equipment needs more than one litre of air to make the measurement, the equivalent of at least five seconds of exhaled air. So, prepare your lungs.
In the device, the "amount of" reaction is measured by circuits and sensors, which presents it in milligrams of alcohol per litre of air. The chemical process eliminates all the alcohol received, and you're ready for the next blower.
Having alcohol in the lungs happens because our body absorbs part of everything we ingest (digestive process). And this absorption takes the contents into the bloodstream, including the alcohol in what we eat and drink.