Blood: What It’s Made Of, Blood Types and Blood Tests

Blood is part liquid and part solid.

• The liquid part, plasma, is water, salts and protein, and makes up more than half of the blood.
• The solid part is made of cells:
• • red blood cells carry oxygen from the lungs to the tissues and organs;
  • white blood cells fight infection as part of the immune system; and
  • platelets help blood clot when you have a cut or wound.

Always being replaced

Blood cells die all the time, and bone marrow — the spongy material inside bones — makes new ones. Red blood cells live about 120 days and platelets about 6 days; some white blood cells live less than a day, while others last much longer.

Blood types

There are four blood types: A, B, AB and O. Blood is also either Rh-positive or Rh-negative — so type A blood, for example, is either A positive or A negative.

Your type matters if you need a blood transfusion. Your Rh factor can matter in pregnancy: an incompatibility between a mother’s type and her baby’s can cause problems.

Blood tests and blood problems

Blood tests, such as blood counts, help doctors check for certain diseases and conditions, see how well organs are working, and track how well treatments work.

Blood problems include bleeding disorders, excessive clotting and platelet disorders. Someone who loses too much blood may need a transfusion.

Sources

Based on "Blood," MedlinePlus, National Library of Medicine, with information from the National Heart, Lung, and Blood Institute; a work of the United States government in the public domain.