A deaeration device includes a vent structure with a porous material capable of swelling when moistened such that the vent structure can inhibit liquid from escaping while allowing for the removal of gases from liquids. The deaeration device is integrated into a blood chamber of an extracorporeal circuit. The blood chamber may further include sensor components for actively performing measurements on the blood while the deaeration device passively removes air from the blood. In an embodiment, the blood chamber may be an optical blood chamber including an optical measurement sensor that performs measurements used in a determination of a hematocrit level in the blood.
