The mix manifold is located behind the fwd cargo bay not 'in' the bay....so no air from the bay is used. In fact the cargo bay is effectively sealed from the cabin in flight. The air in the bay where the mix manifold is located is part of the under floor area. Air enters the pax cabin at the top of the cabin, circulates inside the cabin and exits at foot level in the side walls, to the underfloor area. It can then flow towards the outflow valve and over board and some can flow towards the recirculation system and the mix manifold where it is recirculated.