The relationship among flow rate, chamber volume and calculated rate of metabolism in vertebrate respirometry

A relationship exists among the calculated rate of metabolism of an animal enclosed in a chamber, chamber volume and air flow rate. A "critical" flow rate, defined as the minimal flow rate that produces a complete mixture of chamber gases, characterizes each chamber/animal combination. At flow rates below the critical flow rate, calculated rates of metabolism decrease with flow rate and approach zero as flow rate approaches zero. These calculated rates are unreliable estimates of rate of metabolism. At flow rates greater than the critical flow rate, calculated rates of metabolism are independent of flow rate because a reciprocal relationship exists between flow rate and the differential in oxygen content found between the air entering and exiting the chamber; they represent an accurate estimate of the animal's rate of metabolism under the conditions to which the animal is exposed. The critical flow rate increases with chamber volume, the animal's rate of metabolism and with any factor that increases rate of metabolism, including body mass, activity and in endotherms ambient temperatures below thermoneutrality, although chamber volume is its single most important determinant. Some evasions of the critical flow rate are discussed.