Exercise Physiology A Tale of Two Pumps

The Metabolic Demands

As every “couch potato” knows, exercise requires work, generally perceived as moving the body some distance. To accomplish this work, the body must generate energy in the form of adenosine triphosphate (ATP). And because no good deed goes unpunished, the metabolic processes that provide the energy to sustain exercise result in byproducts that, if we are to avoid harm, call upon compensatory responses from the respiratory system.

Aerobic Metabolism ↑

During the vast majority of our lives, we rely on aerobic metabolism to provide energy for our tissues. The body uses a combination of carbohydrates and fat to generate ATP, the primary energy currency. Protein may also be used to generate energy, but the body prefers to conserve amino acids for the growth and repair of organs and the enzymatic processes needed to support metabolism.

The amount of carbon dioxide produced as a byproduct of aerobic metabolism varies with the material being used as a fuel source. One is able to compare the different fuel sources in this regard by examining the respiratory quotient (RQ), the ratio of carbon dioxide produced per unit of oxygen consumed in the production of energy.

Oxygen consumption is a measure of metabolic activity in the body. You can assess it by measuring the difference between the amount of oxygen that enters the body in a given time interval, usually measured as milliliters per minute, and the amount that is exhaled, also measured as milliliters per minute.

Carbon dioxide production can be expressed in a similar fashion as the difference between the amount of CO₂ exhaled and the amount inhaled.

Because there is virtually no carbon dioxide in the gas we inhale from the atmosphere, carbon dioxide production is equal to the amount of carbon dioxide we exhale. Substituting 0 for the amount of carbon dioxide inhaled, the equation can be written as follows:

The RQ for carbohydrate is 1.0; for fats, it is 0.7; and for protein, it is 0.8. For someone who eats the average American diet, resting metabolism relies more on the consumption of fat than carbohydrates, and the RQ at rest is approximately 0.8.

During exercise (particularly high-intensity, short-duration exercise), however, the body shifts to greater utilization of carbohydrates, and the RQ changes accordingly with greater production of carbon dioxide.

Normal oxygen consumption at rest is approximately 250 mL/min, and normal carbon dioxide production at rest on a typical mixed diet is roughly 200 mL/min (hence, an RQ of 0.8). During exercise, oxygen consumption may increase to as high as 3000 mL/min (or even higher in athletes), and carbon dioxide production increases even more dramatically as carbohydrate becomes the primary source of energy. The respiratory system, in order to maintain homeostasis, must accommodate these metabolic needs by increasing ventilation.

Anaerobic Metabolism ↑

As the intensity of exercise increases, the body is unable to derive all of its energy needs from aerobic metabolism, and there is a shift to anaerobic processes. Generally, this occurs at a level of exercise that corresponds to 50% to 60% of the individual’s maximal oxygen consumption, that is, the oxygen consumption associated with the person’s exercise at the point that she says she can go no further (Note: in highly trained competitive athletes, anaerobic threshold [AT] may approximate 80% to 90% of the maximal oxygen consumption.) Tables of predicted values of maximal oxygen consumption based on the individual’s size and age can be used for a reference when assessing whether the AT is normal or abnormal. The addition of anaerobic metabolism to supplement energy needs leads to increased demands on the respiratory system.

Aerobic metabolism leads to the production of a byproduct, carbon dioxide, that must be eliminated via the respiratory system. Anaerobic metabolism results in the production of lactic acid. As discussed in Chapter 7, to minimize the effects on the pH of the blood when acid is produced, the body buffers the acid with bicarbonate, a process that ultimately produces carbonic acid, which dissociates to water and carbon dioxide. The anaerobic metabolism of a molecule of glucose, for example, can be summarized as follows:

Thus, anaerobic metabolism leads to the production of additional carbon dioxide via buffering of lactic acid. As with aerobic processes, this process adds an additional stress to the ventilatory pump as it strives to increase ventilation to maintain homeostasis. Anaerobic metabolism, although necessary to support intense exercise, is relatively inefficient compared with aerobic processes. Only two ATP molecules are produced per molecule of glucose metabolized via anaerobic mechanisms. In contrast, the body reaps 36 molecules of ATP for each molecule of glucose metabolized aerobically.

The point during exercise at which the body begins to increasingly rely on anaerobic metabolism to the degree that metabolic acid accumulates is termed the anaerobic threshold (AT). The AT is expressed in terms of the level of oxygen consumption at which lactic acid production can be detected. The point at which AT occurs depends primarily on two factors: the ability of the body to deliver oxygen to the tissues (determined by the function of both the cardiovascular and respiratory systems as well as the hemoglobin content of the blood) and the ability of the tissue to extract and use oxygen to support aerobic processes (determined largely by the density of mitochondria, the quantity of enzymes necessary to support aerobic metabolism in exercising muscles, and the density of capillaries delivering blood and oxygen to the tissue in the muscles). Both of these factors can be altered by your level of physical conditioning or fitness. As discussed in more detail later in this chapter, oxygen delivery depends on the contractile function of the cardiac pump that, similar to the level of aerobic enzymes in the muscles, responds positively to exercise training and is adversely affected by a sedentary lifestyle.

The AT can be assessed either by sampling arterial blood repeatedly during exercise to determine the level of oxygen consumption at which lactic acid begins to accumulate or by examining the slope of the graph of ventilation as a function of oxygen consumption during exercise (more about this shortly).