An organism’s metabolic rate is the amount of energy expended by that organism in a given time period - usually daily.
At rest, meaning in periods of inactivity, the metabolic rate is known as the basal metabolic rate (BMR). At rest, the BMR is low compared to when the body is undergoing activities like exercise.
During this resting state the body only needs to use energy to keep vital organs such as the heart, lungs and brain functioning properly.
The metabolic rate can be measured in different ways:
As a general rule, the greater the mass of an organism the higher that organism’s metabolic rate is.
Organisms with high metabolic rates require more efficient delivery of oxygen to cells.
However, BMR is higher per unit of body mass in small animals compared to larger ones. This is because the higher metabolic rate of small animals needs a greater delivery of oxygen to tissues around the body.
Also, the smaller animals have a greater surface area to volume ratio, so more heat is lost.
An organisms metabolic rate can be measured using several different pieces of apparatus including respirometers, oxygen probes, carbon dioxide probes and calorimeters.