The day after tomorrow, Karen's taking her brother Luke down to Kenilworth Park on Anacostia Avenue in Washington. Luke's a preschooler who loves to run and jump. Karen would tell you that he's so full of energy that he's hard to keep up with -- even though he's only 4 years old.

Luke likes to go to the park any day. But this Friday is a special occasion. It's the D.C. Department of Recreation's annual Supertots Day. This year, the theme of the celebration is "Physical Fitness Is a Happy, Healthy Heart." The recreation department wants to remind kids that exercise is an important part of keeping your heart in good shape. The American Heart Association, a nationwide group dedicated to fighting heart disease, also wants to promote healthy hearts in young people. The association will be giving away T-shirts to all the kids who take part in Supertots Day.

The D.C. Department of Recreation expects 400 kids age 3 to 5 to take part in running, tumbling, hula-hooping and Maypole dancing at Supertots Day. As they demonstrate these skills, the kids will also be getting healthful cardiovascular exercise.

The cardiovascular system includes the heart and the network of vessels that carries blood through the body to nourish all the cells. Running and jumping put greater demand on the heart to circulate blood. If you run and jump every day, the heart and blood vessels learn to circulate blood without working as hard.

When Luke takes part in the races at Kenilworth Park on Friday, he'll feel like he's playing. In the meantime, his body will be doing a tough job. His leg muscles will work hard, burning up energy as they go. His heart muscle will squeeze harder than usual to pump blood through his body. As he runs, his heart will pump several times as much blood each minute as it does when he's resting.

When an exerciser keeps up an activity like running, swimming, fast walking or biking for several minutes at a stretch, he or she is doing aerobic exercise. Aerobic means using oxygen. This kind of activity requires deep breathing to bring in extra oxygen for hard-working muscles. Regular aerobic exercise helps keep you fit and strengthens your heart.

Dr. William Strong, a heart specialist, works with children at the University of Georgia to find out how to help them avoid heart trouble later in life. He emphasizes the importance of good nutrition and healthy exercise. He says that making these practices a regular part of your life is important no matter how old you are. But if you're young, healthy heart habits give you a good start at avoiding problems like high blood pressure or certain kinds of heart disease.

Dr. Strong says that kids should exercise regularly by taking part in a five-times-a-week physical education classes at school, and staying active during vacation times. "And kids should eat a good mixed diet that's nutritionally sound," he adds. That doesn't mean you have to give up every single cookie or French fry forever, though. Instead, Dr. Strong says, "Use moderation." That means be sensible about what you eat, and don't overdo it. "And the most important thing of all," he says, "is to never, ever pick up a cigarette."

For the little kids at Kenilworth Park on Friday, exercise is fun. They probably don't make excuses not to go outside to play the way adults sometimes do. You probably enjoy getting exercise, too. But sometimes an afternoon TV show may look like more fun than going for a run, or playing a game of kickball with the kids in your neighborhood. If you feel that way, think again. Remember that your heart has to pump 8,000 gallons of blood 12,000 miles through your body every day. You can make its job easier by getting regular exercise. Next Week: Learning to save lives. Tips for Parents

For more information about the eighth annual Supertots Day, scheduled for May 23 from 10 a.m. to 1:30 p.m. at Kenilworth Parkside Recreation Center, 4300 Anacostia Ave. NE, call the D.C. Recreation Department's day care office at 576-7226.

Most government-run day care centers use the American Heart Association's "Heart Treasure Chest" in their curriculums. This program, packaged in a cardboard treasure box, contains such materials as a real stethoscope, a board game, and wall charts to teach preschoolers about the heart. Your child's preschool can get more information about this public service program by calling the nearest American Heart Association office.