For example, some of the energy in the foods you eat is eventually converted into the kinetic energy of your moving arm. However, food fuels cannot be used to energize body activities directly. Instead, some of the food energy is captured temporarily in the bonds of a chemical called adenosine triphosphate (ATP) (ah-den’o-sen tri’Wfat). Later, ATP’s bonds are broken and the stored energy is released as needed to do cellular work. Chemical energy in the form of ATP is the most useful form of energy in living systems because it is used to run all functional processes.

■ Electrical energy results from the movement of charged particles. In your home, electrical energy is found in the flow of electrons along the household wiring. In your body, electrical currents are generated when charged particles called ions move along or across cell membranes. The nervous system uses electrical currents, called nerve impulses, to transmit messages from one part of the body to another. Electrical currents traveling across the heart stimulate it to contract (beat) and pump blood. (This is why a strong electrical shock, which interferes with such currents, can cause death.)

■ Mechanical energy is energy directly involved in moving matter. When you ride a bicycle, your legs provide the mechanical energy that moves the pedals. » Radiant energy, or electromagnetic energy (e-lek”tro-mag-net’ ik), is energy that travels in waves. These waves, which vary in length, are col-
lectively called the electromagnetic spectrum and include visible light, infrared waves, radio waves, ultraviolet waves, and X rays. Light energy, which stimulates the retinas of our eyes, is important in vision. Ultraviolet waves cause sunburn, but they also stimulate our body to make vitamin D.

Another important source of homeostatic imbalance occurs when the usual negative feedback mechanisms are overwhelmed and destructive positive feedback mechanisms take over. Some instances of heart failure reflect this phenomenon.

Examples of homeostatic imbalance are provided throughout this book to enhance your understanding of normal physiological mechanisms. These homeostatic imbalance sections are preceded by the symbol jjfEfj to alert you to the fact that an abnormal condition is being described.

The positive feedback mechanism in which oxytocin, a hypothalamic hormone, intensifies labor contractions during the birth of a baby is described in Chapter 28 (see Figure 28.16). Oxytocin causes the contractions to become both more frequent and more powerful. The increased contractions cause more oxytocin to be released, which causes more contractions and so on until the baby is finally born, an event that ends the stimulus for oxytocin release and shuts off the positive feedback mechanism.

Blood clotting is a normal response to a break in the lining of a blood vessel and is an excellent example of an important body function controlled by positive feedback. Basically, once vessel damage has occurred, blood elements called platelets immediately begin to cling to the injured site and release chemicals that attract more platelets. This rapidly growing pileup of platelets initiates the sequence of events that finally forms a clot.