A chamber is described for use in an extracorporeal fluid system. The chamber has a bottom entry port and a bottom exit port. A microporous filter at the top of the chamber allows air in the fluid to vent from the chamber. In use, the chamber is filled with saline. Blood is then introduced into the chamber. A layer of saline is above a layer of blood in the chamber. The saline stagnates as the blood flows through the chamber. The saline keeps the blood from contacting the filter and depositing protein on the filter.