Air circulation?

[Traceyash]

I know this may seem a silly question but im still learning and wanted to know. I have bought myself a plastic greenhouse from my favourite garden centre,tis 3ft 11" wide by 5ft 3" tall,4 shelves and a plastic cover with 2 front zips that allows the front to be rolled up. Know the one?
Well,i know how it holds the heat from opening it up this past couple of days and being hit by the heat,but how does air circulate in them? When it is used,do i need to leave a gap at the bottom zips for air circulation or is it breathable?
Thank you in advance.