A container for use with a hemodialysis apparatus having a vent structure is described. The vent structure is porous, and allows air in the container to vent when the container is being filled with fluid, but expands when the vent structure becomes wet, when filling is complete, thereby closing off the pores and inhibiting (e.g., preventing) fluid from flowing through the vent structure, and reentry of air into the container. The vent structure can also include a micro-porous membrane.
