radiator and over flow question

[CG]

ok hope this doesnt make me sound stupid but i just havent figured this one out yet...if your radiator is under pressure how come it doesnt constantly blow out the overflow hose? as far as overflow bottles go ive seen every shape and size it seems...how does the water that goes into the overflow bottle get back into the system? im sure this is all 101 stuff, unfortunatly there are some things im barely at the 1 level...thanks