■ Chemical energy is the form stored in the bonds of chemical substances. When chemical reactions occur that rearrange the atoms of the chemicals in a certain way, the potential energy is unleashed and becomes kinetic energy, or energy in action.

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.

Anatomy and physiology textbooks typically describe two sets of internal body cavities called the dorsal and ventral body cavities. These cavities are closed to the outside and provide different degrees of protection to the organs contained within them. Because these two cavities differ in their mode of embryonic development, and their lining membranes, the dorsal body cavity is not recognized as such in many anatomical references. However, the idea of two sets of internal body cavities is a useful learning concept and is used here.

Homeostasis is so important that most disease can be regarded as a result of its disturbance, a condition called homeostatic imbalance. As we age, our
body’s control systems become less efficient, and our internal environment becomes less and less stable. These events increase our risk for illness and produce the changes we associate with aging.

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.

In positive feedback mechanisms, the result or response enhances the original stimulus so that the activity (output) is accelerated. This feedback mechanism is “positive” because the change that occurs proceeds in the same direction as the initial disturbance, causing the variable to deviate further and further from its original value or range. In contrast to negative feedback controls, which maintain some physiological function or keep blood chemicals within narrow ranges, positive feedback mechanisms usually control infrequent events that do not require continuous adjustments. Typically, they set off a series of events that may be self-perpetuating and that, once initiated, have an amplifying or waterfall effect. Because of these characteristics, positive feedback mechanisms are often referred to as cascades (from the Italian word meaning “to fall”). Positive feedback mechanisms are likely to race out of control, and so they are rarely used to promote the moment-to-moment well-being of the body. However, two familiar examples of their use as homeostatic mechanisms are the enhancement of labor contractions during birth and blood clotting.

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.