Think of your body as a house, and you may begin to understand why you feel as awful as you do when the temperature and humidity soar.

As do most houses, your body has a central thermostat, located deep in your brain. Its location protects it from the elements, just as a thermostat in a house normally located on an inside wall, away from hot or cold drafts.

Your thermostat is set to maintain your body's temperature at a constant 98.6 degrees. When the temperature outside rises, the thermostat sends signals to your central air conditioning system to go to work.

There are several ways your body can cool off, the first of which is by radiation. If it is relatively cool out, say 75 or 80 degrees, your body naturally gives off heat into the surrounding environment.That is why you don't have much trouble staying comfortable when the temperature is in the low 80s, still about 15 degrees cooler than you internal organs.

As the outside temperature rises, your body begins to pump more blood to your skin for cooling, just as a house's air conditioning system will go on automatically as the heat grows more intense.

The closer the temperature outside gets to your internal temperature, the more your body works to cool you and the harder time it has doing so.

When the temperature reaches the upper 90s, it becomes impossible for your body to lose heat by radiation, and the body's relay switches turn on a back-up system sweating.

As the moisture pours from your pores, it ordinarily evaporates, a process that cools off your skin. But when the humidity reaches about 50 percent, evaporation is severely retarded because the air has become almost saturated with moisture. As a result, the perspiration lies on your skin and you feel hot and sticky.

Yesterday was one of those days: it reached a high of 90 degrees at 2:40 p.m. and the humidity was equally uncomfortable at 64 percent. The prognosis is no better for today, with temperatures to range between 88 and 93 humid degrees.

According to Dr. Michael Rolnick, director of emergency medicine at the Georgetown University Medical Center, most persons experience their greatest difficulty during the summer's first heat waves when the body has not yet had time to adjust.

Aside from making you miserably uncomfortable as well as crabby and unpleasant to be around (researchers at Johns Hopkins University have found that the heat indeed has just that effect) hot muggy weather also may cause three other physical problems: heat cramps heat exhaustion and far most serious potentially fatal heat stroke.

Heat cramps are familar to many of those crazy enough to exercise vigorously in the heat of a Washington summer. These cramps, which usually occur in the legs, are thought by some researchers to be caused by a loss during sweating of chemicals called electrolytes, such as sodium and potassium, which help regulate the electrical activity of the heart.

Heat exhaustion, says Rolnick, is "a more common thing. People feel light-headed, sweat profusely and may in fact pass out. It is thought to be cause be an electrolyte imbalance caused by sweating." The body loses sodium, potassium and other substances when it sweats.

Heat stroke is the most serious heat problem. It is caused by a malfunction of the body's thermostat. If the thermostat breaks down and no longer can keep your body from overheating, body temperature soars. Sometimes you are unable to seat and you may become disoriented. In severe cases, coma and death can follow.

Heat stroke, warns Rolnick is "a medical emergency" and should immediately be treated as such - by a physician, not by home doctoring.

Heat poses the greatest problems for persons with cardiovascular disease, particularly the elderly. "Deaths from heart attacks soar during heat waves," says Dr. Stuart Seides, formerly a senior investigator at the National Heart and Lung Institute and now chief of cardiology at Greater Southeast Community Hospital.

"The reason the hot and humid environment hears such an enormous toll on the cardiovascular system," said, "is that the body's method for removing the heat (depends upon the heart being able) to draw heat out of the internal organs to the surface."

At the same time the heart is working overtime to bring blood to the surface for cooling, blood pressure is dropping because blood vessels are being enlarged to allow for increased blood flow. These changes throw the cardiovascular system off balance even more.

"On a hot - particularly a hot and humid - day, vigorous exercise can be dangerous for somebody who's healthy," warns Seides. "But if you take somebody with cardiovascular disease, their ability to cope is that much reduced and there is enormous risk."

Patients with heart problems should remain as inactive as they can during hot humid days, he advises. They should stay where it is cool and should try to avoid excessive humidity.

To persons of normal health. Seides suggest that they, at the very least, acclimate themselves to the heat before running around in it.