When you go to the doctor for your physical, you probably have to get a blood test. Sometimes, the test is a simple finger stick -- one quick ouch, and it's all over. Sometimes, your doctor needs to get more and uses a syringe to draw a test tube of blood from your arm.

Have you ever wondered what happens to the blood after the doctor or nurse takes it from your body? Experts can tell a lot of things about your health by examining your blood.

That's not so surprising when you consider all the important jobs blood does in the body. It carries oxygen to your cells to give you energy. It fights infection and illness. It does the repair work when you get a cut or a bruise. And it carries waste materials away from your cells before they cause trouble.

Your blood's red cells carry oxygen. Its white cells fight infection. And cells called platelets protect you by forming clots and promoting healing when an injury breaks your skin and blood vessels.

Surrounding all those parts, there's a liquid called plasma that contains lots of chemicals that get carried from place to place in the body as they are needed. It's important that these substances be in just the right mixture to keep you well. That's why doctors like to check on your blood chemistry from time to time.

When a doctor or technician looks at someone's blood under a powerful microscope, there are a number of things to see.

The number of red blood cells can be counted. Of course, they're not all counted -- 35 trillion red blood cells. Even one drop of blood usually contains about 4 million red cells! To do a red-cell count, the technician only has to count the number of cells in one tiny smear of blood. If there aren't enough red cells in the blood, the patient may be anemic.

Simple anemia is a blood problem that happens when someone isn't getting enough iron. It's easy to treat by giving the anemic person iron supplements to take.

There are also other, more serious kinds of anemia that doctors can find out about from blood tests. It's possible to examine white blood cells during a blood test. The technician counts the cells and looks at their shape. Because the body makes extra white cells when it's fighting an infection, a high white-blood-cell count can tell doctors that something is wrong somewhere.

Platelets and plasma can also be checked to find out if all the body's systems are operating smoothly. For good health, it's essential that a person's blood be in good condition.

Your kidneys and liver filter your blood and maintain the right balance of chemicals in it. Your bone marrow -- a spongy substance in the middle of your bones -- has the job of making red blood cells to replace the ones that grow old and die.

Your spleen is assigned to cleanup duty. It produces cells that destroy old red blood cells when your body can't use them anymore. Your spleen also stores extra red cells for use in emergencies.

Most of the time, all these organs do their jobs well and your blood stays healthy. But certain illnesses can hurt the organs that keep blood in good shape. Kidney disease is one of them.

Your kidneys are like washing machines for your blood. Each minute, about a quart of blood passes through the kidneys and comes out clean. In a lifetime, the kidneys wash the blood over and over again. But when they stop working, it's serious.

Sometimes, patients with kidney disease receive a treatment called dialysis to keep their blood free from the poisons that build up when the kidneys stop working. Dialysis rinses and cleans the blood by passing it through an artificial kidney. Some patients must have this process done five hours a day, three days a week. The patient is hooked up to a machine in the hospital or special dialysis center. It takes a lot of time, but it means the difference between life and death.

A new method of dialysis allows a patient to have the procedure done at home, but it's still time-consuming. It's hard to have a normal life when you have to have dialysis.

For some patients, a kidney transplant can cure their illness and free them from having dialysis done each week. Next week, read about how organ transplants work. :: Tips for Parents

Life as a dialysis patient can be particularly trying for children, since they miss a lot of school, eat differently, are often smaller than their peers and must monitor their condition almost hourly. At the University of Maryland School of Medicine, a new pediatric kidney-dialysis center -- funded in part from money raised by schoolchildren throughout the state -- opened last month to serve the special needs of these children.

Patients range in age from birth to 19. "Our job is to make them feel as normal as possible," says Dr. Charles Medani, director of pediatric nephrology.

While on dialysis, the children have a tutor who works with teachers to ensure that the dialysis patients keep up. Social workers and dietitians work to help the child and family manage the limitations imposed by the disease. For more information, contact: Division of Pediatric Nephrology, The University of Maryland School of Medicine, 22 S. Greene St., Baltimore, Md. 21201; phone (301) 328-5303.

Catherine O'Neill is a free-lance children's writer in Baltimore.