Tomorrow is Valentine's Day. All week, you have been seeing heart-shaped cards. But do you know what your heart really looks like?

It's a hollow muscle about the same size as your fist. It's shaped quite a bit like a fist, too. But if you use your imagination, you might still think your heart looks like a valentine.

Your heart has a very important job. It pumps blood through your body to bring oxygen and food to your cells. You get the oxygen from the air you breathe. The oxygen keeps your cells -- and you -- alive.

Whether you're sound asleep or busy running the hundred-yard dash, your heart beats all the time.

When you put your hand on the upper left side of your chest, you can feel your heart beating. What you're feeling is the lower end of the heart, which tips toward the front of your body. Most of your heart is located in the center of your chest, protected by the bones that form your rib cage.

Your doctor uses a device called a stethoscope to listen to your heartbeat. Maybe you have had a chance to hear it, too. If not, ask your doctor to let you listen next time.

You can hear the heart by using a much simpler device than a stethoscope. Take a cardboard tube left over after you use up all the paper towels or plastic wrap in your kitchen. Ask a friend to stand still, and then place one end of the tube against the upper left side of his chest. Put your ear to the other end of the tube. You should be able to hear your friend's heart beating. That regular LUB-dub, LUB-dub means his heart muscle is busy keeping his body supplied with oxygen.

Every minute of every hour of every day of your life, your heart pumps blood. The sound you hear -- LUB-dub, LUB-dub -- is the muscle working.

Try imagining that your heart muscle is a box with four rooms inside of it -- two rooms on top, and two rooms on the bottom. The box is a kind of recycling center for the blood.

Blood coming back from the body enters the top room on the right. This blood has used up all of its oxygen. A door opens and the blood moves into the bottom room on the right, and then flows through another door to the lungs.

While the blood is in the lungs, it picks up more oxygen.

As the oxygen-carrying blood leaves the lungs, it gathers in the top left room of the heart. Once the room is full, a door opens and the blood flows into the bottom room on the left side. From this room, the oxygen-carrying blood travels to the rest of the body.

This happens about 100,000 times every day, even when you're asleep. How does it keep going?

Inside your heart is a small patch of cells that sends out electrical signals. These signals tell the heart muscle when to squeeze. Doctors call this patch of cells your pacemaker. Like your heart, it's on the job all the time. From your heart, your blood moves through your body in a very complicated network of tubes called blood vessels. You have an awful lot of blood vessels. If they were all connected and laid out in a row, they would stretch for thousands of miles. In your body, they reach every bit of you, from your brain to your toes.

You can feel your blood moving if you put your fingers on one of the blood vessels located near the surface of your body. You can feel this pulse on the inside of your wrist. You can also feel it on either side of your neck, near your jaw line. You might also be able to feel it on your temples. Each little thump that you feel is the result of your heart pumping your blood along.

Try taking your pulse. When you're sitting down and relaxed, put two fingers over your wrist. When you have found your pulse, start counting. Have someone time you for 30 seconds. Multiply the number you get by 2. That's your resting pulse. In younger children, the pulse probably will be somewhere between 90 and 120 beats a minute. As you get older and your body gets larger, it will probably be somewhere between 60 and 80 beats a minute.

If you take your pulse when you're exercising, though, you will get a higher number of beats. The heart may beat as many as 200 times a minute during really hard exercise. That's because your muscles need more oxygen when they're working hard. So the heart speeds up to keep pace with the muscles' needs.

Like the rest of your muscles, your heart and its system of blood vessels need a regular workout to keep you in good shape -- for Valentine's Day and every day of the year. Tips for Parents

Dr. Lowell Perry, vice chairman of the cardiology department at Children's Hospital, suggests that children be taught good habits for preventing heart disease later in life.

"Develop good eating habits, and follow a prudent diet like that recommended by the American Heart Association," Perry says. "Don't eat junk food. Begin a regular exercise program early in life. Get interested in a lifetime activity for cardiovascular fitness. Play tennis, volleyball or softball. Walking and hiking are very good forms of exercise, and good for weight control, too. And don't ever start smoking."