text: For an open-system, design constraints include washout characteristics of the animal chamber and sensitivity of the gas analyzers.  However, the basic principle remains the same: What goes in must come out. The primary distinction between an open and closed system is that the open system flows air through the chamber (i.e., air is pushed or pulled by pump) at a rate that constantly replenishes the O depleted by the animal while removing the CO and water vapor produced by the animal. The volumetric flow rate must be high enough to ensure that the animal never consumes all of the oxygen present in the chamber while at the same time, the rate must be low enough so that the animal consumes enough O for detection. For a 20 g mouse, flow rates of about 200 ml/min through 500 ml containers would provide a good balance. At this flow rate, about 40 ml of O is brought to the chamber and the entire volume of air in the chamber is exchanged within 5 minutes. For other smaller animals, chamber volumes can be much smaller and flow rates would be adjusted down as well. Note that for warm-blooded or endothermic animals (birds and mammals), chamber sizes and or flow rates would be selected to accommodate their higher metabolic rates.