An animal’s overall rate of energy use is its metabolic rate. Metabolic rate can be measured by the animal’s rate of oxygen consumption, which reflects the rate of aerobic production of ATP.

Metabolic rate is affected by many factors, one of which is the activity level of an organism. When an animal shifts from rest to activity, its metabolic rate and oxygen consumption rise to meet its increased demand for ATP. The onset of activity requires immediate energy, which in animals is provided by specialized energy stores in their tissues. In vertebrate muscle cells, for example, phosphocreatine is a ready source of high-energy phosphates that is hydrolyzed to synthesize ATP directly from ADP at the onset of activity. This reaction is followed by the relatively rapid production of ATP by anaerobic glycolysis. Although glycolysis produces relatively few ATP molecules per molecule of glucose broken down, the reactions are extremely fast, providing a rapid short-term supply of energy for animals. Animals rely on anaerobic glycolysis for short bursts of intensive activity.

With longer activity, the ATP needs of the animal are met by aerobic respiration in mitochondria. Therefore, the rate of oxygen consumption initially increases, then levels off (Fig. 40.2). At this point, the animal’s need for energy is being met entirely by aerobic respiration. Once mitochondrial ATP production increases, the steady state production of ATP depends on adequate transport of oxygen to the tissue by the cardiovascular system (Chapter 39) as well as by diffusion of oxygen into mitochondria.

Sprinters rely heavily on anaerobic ATP production for intense short-term bouts of activity, but longer races are run at progressively slower speeds for increased endurance as the distance runner relies more heavily on aerobic metabolism to produce ATP. The world record for the 100-m sprint averages just above 10 m/s, but for the 1500-m race it is about 6.6 m/s.

When activity ends, the animal’s oxygen consumption rate declines but does not immediately return to resting levels, as shown in Fig. 40.2. The elevated consumption of oxygen following activity is the animal’s recovery metabolism. It represents the continued metabolic energy required to reestablish the resting metabolic state of the cells. During recovery metabolism, cells re-synthesize depleted ATP stores and metabolize the end products of fermentation, particularly lactic acid. The difference between an animal’s immediate energy need at the onset of activity and energy supplied by aerobic metabolism is referred to as the animal’s “oxygen debt.” This debt is “paid back” following exercise by the animal’s recovery metabolism. Recovery metabolism is associated with the elevated breathing and heart rate that you (and other animals) experience when resting after moderate to intense exercise.