How Does Exercise Affect the Body?

Many people today are interested in exercise as a way of improving their health and physical abilities. But there is also concern that too much exercise, or exercise that is not appropriate for certain individuals, may actually do more harm than good. Exercise has many short-term (acute) and long-term effects that the body must be capable of handling for the exercise to be beneficial. Some of the major acute effects of exercising are shown in Figure 1. When we exercise, our heart rate, systolic blood pressure, and cardiac output (the amount of blood pumped per heart beat) all increase. Blood flow to the heart, the muscles, and the skin increase. The body's metabolism becomes more active, producing CO₂ and H⁺ in the muscles. We breathe faster and deeper to supply the oxygen required by this increased metabolism. Eventually, with strenuous exercise, our body's metabolism exceeds the oxygen supply and begins to use alternate biochemical processes that do not require oxygen. These processes generate lactic acid, which enters the blood stream. As we develop a long-term habit of exercise, our cardiac output and lung capacity increase, even when we are at rest, so that we can exercise longer and harder than before. Over time, the amount of muscle in the body increases, and fat is burned as its energy is needed to help fuel the body's increased metabolism.

Figure 1 This figure highlights some of the major acute (short-term) effects on the body during exercise.

Chemical Changes in the Blood During Exercise

In previous tutorials ("Hemoglobin and the Heme Group: Metal Complexes in the Blood for Oxygen Transport", "Iron Use and Storage in the Body: Ferritin and Molecular Representations", "Maintaining the Body's Chemistry: Dialysis in the Kidneys") you learned about the daily maintenance required in the blood for normal everyday activities such as eating, sleeping, and studying. Now, we turn our attention to the chemical and physiological concepts that explain how the body copes with the stress of exercise. As we shall see, many of the same processes that work to maintain the blood's chemistry under normal conditions are involved in blood-chemistry maintenance during exercise, as well.

During exercise, the muscles use up oxygen as they convert chemical energy in glucose to mechanical energy. This O₂ comes from hemoglobin in the blood. CO₂ and H⁺ are produced during the breakdown of glucose, and are removed from the muscle via the blood. The production and removal of CO₂ and H⁺, together with the use and transport of O₂, cause chemical changes in the blood. These chemical changes, unless offset by other physiological functions, cause the pH of the blood to drop. If the pH of the body gets too low (below 7.4), a condition known as acidosis results. This can be very serious, because many of the chemical reactions that occur in the body, especially those involving proteins, are pH-dependent. Ideally, the pH of the blood should be maintained at 7.4. If the pH drops below 6.8 or rises above 7.8, death may occur. Fortunately, we have buffers in the blood to protect against large changes in pH.