When your red blood cells can't carry enough oxygen to other parts of your body, you're said to have anemia. It can occur because you have fewer red blood cells than normal; you have a lack of oxygen-transporting hemoglobin (HEE-moe-glow-bin) in your blood; or your red cells have decreased in volume.

Requirements for blood donors vary somewhat from one state to another, but there are general guidelines established by the Red Cross. In most instances donors should be 17 or older, weigh at least 110 pounds, and be in good health.

It's becoming increasingly common to donate your own blood to be used during surgery, when your operation is scheduled far enough in advance. This method is known as autologous (aw-TOL-oh-guss) donation.

Hypoglycemia (hi-poe-gly-SEE-me-uh) is the medical term for low blood sugar. To some degree, you've probably experienced hypoglycemia at one time or another, when you've gone too long between meals.

Most commonly afflicting African-Americans, and people with ancestors in regions like Greece, Italy, or India, sickle cell anemia is an inherited disease that can be painful and destructive.

Transfusions are procedures where blood or blood products are pumped into your bloodstream. They can help replace blood lost during accidents or operations; boost red cell count in anemic patients; supply clotting factor for hemophiliacs, so their blood will thicken when needed; and increase white blood cells in those receiving chemotherapy.