Go with the flow.

Students at Calhoun High School in Merrick, New York, gave their community a great gift last holiday season: the gift of life. Just before Thanksgiving, they turned their gym into a blood-donor center. In six hours, they collected 100 units (pints) of blood from classmates, teachers, and parents. Once the blood is screened for diseases and separated into its various components, it can be used to save as many as 500 lives.

"All you feel is a pinch [from the needle], but it goes away," says Colleen Bird, 17, who donated blood for the third time this year.

"One of my relatives had to have a blood transfusion," adds Jaclyn Markel, 17. "So I know from my family how important this is."

Need more facts about blood before you donate? Turn the page.

Why is it so important to donate blood?

Nine out of 10 of us will need a transfusion (a blood "refill") at some point in our lives. You or someone you know might lose blood in a traumatic accident, during surgery, or as the result of a serious illness, like leukemia. But fewer than 5 percent of people who can donate blood take the time to do it.

What makes blood "life giving"?

Blood carries out essential life functions. Red blood cells carry oxygen from the lungs to body tissues and transport carbon dioxide, a waste product, back to the lungs to be exhaled. White blood cells fight off invading bacteria and viruses. Cell fragments called platelets help blood clot. Plasma, the liquid part of blood, transports salts, nutrients, hormones, and other materials throughout the body. Each blood component can be used to save lives.

What's involved in giving blood? Does in hurt?

First, you'll answer several questions about your medical history. (No one who is sick can give blood. Blood centers want to make sure you and the blood supply stay safe.) A nurse will test a drop of blood from your fingertip to check your iron level. (Iron carries oxygen in red blood cells. If your iron level is low, you can't donate.) The nurse will also check your vital signs (like your pulse, blood pressure, and temperature).

If everything checks out, you're ready to donate (see photo, above). All you feel is the pinch of a needle. Within 10 minutes, you could fill a one-pint (500ml) bag. Don't worry, you'll still have plenty of blood left--8 to 12 pints. But to prevent dizziness, you'll be asked to rest. Then you can help yourself to free snacks and juice to begin replacing lost fluid. Within six to eight weeks, your body will regenerate all your missing blood cells.

Can I catch a disease--like AIDS--from donating blood?

Though viruses like the one that causes AIDS can live in blood, you can't catch any diseases from donating blood. That's because nurses use nonreusable, sterile needles every time they draw blood. No one else's germs ever come in contact with your bloodstream.

What if my blood carries a disease? Will I pass it on to patients who receive my blood?

Lab workers test all donated blood for many disease-causing microbes, including HIV, the virus that causes AIDS. (That test became available in 1985.) They reject any samples that test "positive" for a disease.

What happens to "healthy" blood?

Healthy blood is separated into its components and stored for use. Platelets last five days. Red cells can be refrigerated for up to 42 days and plasma can be frozen for up to a year.

Labs also test each blood sample's "type." Blood type is determined by which antigens (substances that trigger an immune response) are present on the surfaces of your blood cells. The blood types are A, B, AB (for both antigens), and O (for neither antigen). Blood transfusions must "match" a recipient's blood type so the transfusion doesn't trigger an immune response. Transfusing the "wrong" blood type can trigger the recipient's immune system to attack the foreign blood, causing it to clump--a potentially lethal reaction.

Have scientists created any blood substitutes to help prevent a shortage?

Not yet, but they're working on it. An ideal blood sub would not have different "types" that need to be matched; it would be free of disease-causing pathogens; it would last longer in storage than real blood; and it could allow transfusions to take place at the site of an emergency, where real blood isn't usually available.

Who's eligible to donate blood?

Anyone in good health between the ages of 17 and 75 and weighing at least 110 pounds can donate blood. (In some states, teens under 17 can donate if they get parental consent.)

What else can I do if I don't meet those criteria?

Like the students at Calhoun High, you can help run a blood drive. Encourage your parents, siblings, and friends to donate. Start by calling the American Red Cross or a Community Blood Center.