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Blood
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Blood Heart Circulation

Blood

Blood is composed of liquid plasma and solid components including red blood cells, white blood cells, and platelets, all of which play vital roles in maintaining health.

Overview

Your blood is made up of liquid and solids. The liquid part, called plasma, is made of water, salts, and protein. Over half of your blood is plasma. The solid part of your blood contains red blood cells, white blood cells, and platelets.

Blood Cell Functions

Red blood cells (RBC) deliver oxygen from your lungs to your tissues and organs. White blood cells (WBC) fight infection and are part of your immune system. Platelets help blood to clot when you have a cut or wound. Bone marrow, the spongy material inside your bones, makes new blood cells. Blood cells constantly die and your body makes new ones. Red blood cells live about 120 days, and platelets live about 6 days. Some white blood cells live less than a day, but others live much longer.

Blood Types and Transfusions

There are four blood types: A, B, AB, or O. Also, blood is either Rh-positive or Rh-negative. So if you have type A blood, it's either A positive or A negative. Which type you are is important if you need a blood transfusion. And your Rh factor could be important if you become pregnant - an incompatibility between your type and the baby's could create problems.

Monitoring Blood Health

Blood tests such as blood count tests help doctors check for certain diseases and conditions. They also help check the function of your organs and show how well treatments are working. Problems with your blood may include bleeding disorders, excessive clotting and platelet disorders. If you lose too much blood, you may need a transfusion.

Frequently Asked Questions

Dehydration reduces plasma volume, making blood more concentrated. This can falsely elevate hemoglobin, hematocrit, and electrolyte levels, and may mask anemia or kidney issues. Rehydrating before blood draws helps ensure accurate test results.
Your doctor will perform an indirect Coombs test early in pregnancy to detect antibodies. If you’re Rh‑negative, you’ll receive Rh immunoglobulin (Rho(D) immune globulin) around weeks 28‑32 and within 72 hours after any event that could mix fetal and maternal blood (e.g., miscarriage, amniocentesis). This prevents your immune system from forming antibodies that could harm future Rh‑positive babies.
Your blood type (ABO and Rh) is genetically determined and does not change throughout life. Routine re‑testing is only needed before surgeries, transfusions, or pregnancy if the type is unknown.
Warning signs include easy bruising, frequent nosebleeds, bleeding gums, petechiae (tiny red spots) on the skin, and prolonged bleeding from cuts. If you notice these, especially with heavy or uncontrolled bleeding, seek medical care promptly.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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