Metabolism

Metabolism can be defined as the sum total of processes occurring in a living organism. Heat is produced during all of these processes, therefore the metabolic rate is the rate of heat production. All biological processes ultimately use oxidation, so the rate of oxygen consumption can theoretically be used to estimate metabolic rate.

Transduction is the conversion of energy from one form to another. There are 3 major stages of transduction in the biosphere: photosynthesis, cell respiration (energy given up, production of ATP), and cell work. Energy involved in cell work can be mechanical, synthetic, chemical, asmotic or electrical. Contraction is the change of chemical energy to mechanical energy.

Metabolism as a rate of heat production can be measured. All cellular events result in the production of heat. A calorie is the amount of heat required to raise 1g of water 1 degree Celsius. Direct calorimetry is the direct measurement of heat, and is very difficult to determine. Indirect calorimetry involves the measurement of oxygen consumption, or the measurement of carbon and nitrogen content of excreted materials to determine metabolic rate.

During exercise, indirect calorimetry devices must measure external work. Oxygen is not used immediately for energy (since immediate energy sources and glycogen provide the initial sources of energy and are nonoxidative sources). Also, lactic acid is produced during vigorous exercise and the body buffers this buildup with bicarbonate-carbonic acid system. Therefore R does not = RQ. During and after exercise, urine production is inhibited and nitrogen is also lost in sweat, so the release of nitrogen is difficult to measure. Knowing all these factors, one can account for them and still get an estimate of metabolic rate.

EPOC is Excessive Postexercise Oxygen Consumption. This refers to the persistence of metabolic response even after exercise stops.

Efficiency is the fraction of energy liberated as external work, expressed as a percentage.

Steady-state exercise occurs when oxygen consumption (VO2) is relatively constant. This is directly proportional to the submaximal work load.

When testing on a treadmill, the grade is added to create external work (lifting the body up the incline).