When you step on the accelerator of your car, the carburetor feeds more gasoline to the engine. The engine runs faster — that is, the chemical energy of the fuel is changed into heat energy and mechanical energy at an increased rate. But it is not enough to supply the engine with more gasoline; if that were all, your car would soon be on the scrap heap. A great many other factors are involved.
In order to burn the fuel, a greater supply of oxygen is needed; hence, the carburetor must suck in air to the engine in greater quantities than before. A good deal more heat is generated at high speeds. To keep the engine from becoming overheated, more cooling must be provided by the water pump, which circulates water through the engine jacket, and by the fan, which blows air over the radiator. The ignition must produce more sparks per minute to fire the fuel in the cylinders; more oil must flow in order to lubrÃcate the working parts. Many operations are required, therefore, in order to bring about an increased rate of energy transformation in your automobile.
So it is with the human body when it passes from rest to exercise. The muscles must transform chemical energy into mechanical energy at a faster rate than before, but a good many other things must happen, too.
To burn more fuel, the muscles need a greater supply of the oxygen that is carried to them by the circulation of the blood. Therefore, the flow of blood to the muscles must increase greatly. To charge the increased blood flow with suficient oxygen, the lungs must pump more air. Notice that your breathing becomes deeper and faster the minute you start exercising.
As the muscles become more active, more chemical energy is transformed into heat energy. This extra heat would quickly raise the body temperature to fever pitch if effective cooling were not provided.
The increased circulation of blood to the surface of the body — that is, the skin — helps to carry the heat away faster than normally. The sweat glands all over the body surface burst into activity and drench the body with perspiration: as the sweat evaporates, the body loses heat to its surroundings. These cooling devices are quite effective, but they do not suffice if a person engages in particularly strenuous activitv. The body temperature of the most perfectly trained athlete is bound to rise if he runs a mile race, or plays several fast sets of tennis or boxes ten rounds.
