Questions to the 72 hour visa

Hi
We are travelling to Vietnam, flying via Beijing on the way there and via Guangzhou on the way back. Can we stay in Beijing for 72 hours and on the way back there is not enough time to get the connecting flight to leave the airport, so we don't really have time to leave the airport. Can we do this or does the 72 hours have to include the transit back, so we spend only 2 days in Beijing. ?

Please help as really confused.

Thanks

Heidi