Respirometry

Respirometry is the process of measuring an animal’s gas exchange with its environment. The devices used are called respirometers. For studies of metabolic rate, the most common type of respirometry is the measurement of an animal’s rate of O₂ consumption. In Box Extension 7.4 we offer illustrations and explanations of the two basic types of respirometry configurations that are used to measure O₂ consumption: (1) closed configurations, in which an animal is housed in a fully sealed chamber with a relative fixed volume of nonmoving air and (2) open configurations, in which air flows through the animal chamber during measurement.

Figure A A closed respirometer

In closed respirometry (Figure A), air does not flow through the animal chamber during measurement. The principal practical challenge with closed respirometry is to prevent temperature-induced changes in gas volume from confounding the results. This problem is solved in the closed respirometer shown in Figure A by connecting two identical chambers to either side of a manometer, a U-shaped water column that serves to measure gas-pressure differences. If the environmental temperature changes, the gas pressure inside both chambers rises equally, exerting equal and opposite increases of pressure on the two sides of the manometer; in this way, the position of the water in the manometer is unaffected by changes of environmental temperature.

In the closed respirometer, as the animal uses O₂, the CO₂ absorbent in the animal chamber removes exhaled CO₂ from the air in the chamber. Thus the animal’s O₂ consumption causes the total gas volume in the animal chamber to decrease relative to that in the other (animal-free) chamber; this decrease in the gas volume in the animal chamber causes the water in the manometer to shift, rising higher in the left arm than the right. At timed intervals, enough pure O₂ is injected from the syringe into the animal chamber to make the manometer return exactly to the initial, unshifted position shown in Figure A. The amount of O₂ injected at any one moment in this procedure must equal the amount of O₂ that the animal consumed during the interval of time preceding the injection. By measuring the O₂ injected and knowing the length of the preceding time interval, a researcher can calculate the animal’s O₂ use per unit of time.

Figure B An open respirometer

In open respirometry (Figure B), air flows through the animal chamber (or through a mask worn by the animal) during the measurement of O₂ consumption. The rate of airflow is measured carefully. Moreover, a precision O₂ meter, typically using an electrochemical or paramagnetic cell for O₂ detection, measures the O₂ concentration of the flowing air just before the air enters the animal chamber and just afterward. The researcher calculates the animal’s rate of O₂ consumption by taking into account the volume of air passing through the chamber per unit of time and the amount of O₂ extracted from each unit of volume. Although open respirometry requires the use of far more costly equipment than closed respirometry, it permits continuous, minute-by-minute (even instantaneous) monitoring of an animal’s rate of O₂ consumption.