ventilation - a little confused

Ok, im confused about ventilation and really cannot seem to find an asnwer anywhere. I look at grow tents with exhausts and carbon filters, but what about intake? is an extractor fan all that is needed, ? what about the bottom left for intake?

REALLY struggling to get my head round this, much help is appreciated !
Ill be using 1 400w hps/hid
tents im considering....

http://cgi.ebay.co.uk/110623504265
http://cgi.ebay.co.uk/260587150213

thanks alot for your help

you only need holes for passive intake, the exhaust fan pulls air to the outside and fresh air replaces it through intake hole passively

Thank you for your response sir, very helpful as all this time ive been searching for fines to bring air into the tent (silly me.)

Thanks again !

What about smell control? Will they leak out of the passI've hole(s)?

Yes, using a intake would help. Use about 1/3 the size of a fan for your intake as you do your exhaust, or maybe 1/2. Liek if your exhaust fan is 440cfm, use 150-250 cfm for the intake fan. This helps make sure air is constantly being pulled through your tent and exits out your tent continuously.

YES YOU WILL HAVE SMELL. GET A CARBON FILTER AND SOME ONA GEL. Smell will smell regardless of whether you have some tiny ass intake vents open or not.

I believe you can get carbon crystals that neutralize smells, similar to the stuff contained inside carbon filters, you just put them near the vent inside or out of the grow room, heres one im thinking of getting. This is supplementary to a carbon filter to stop any leaks

http://www.3ch.co.uk/environment/odour-control/ona-block-odour-control/prod_967.html

Just don't put the gel or spray or whatever inside with your plants; keep it out. I learned the hard way...