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Understanding blood
Audience: patients-public, students-learners ยท Review state: published
What it is
Blood is the body's transport system: a fluid tissue that circulates through the heart and blood vessels, delivering oxygen and nutrients to every cell and carrying away waste. An average adult has about five liters of it.
It is made of plasma, the liquid portion, plus three main cell types, each with a distinct job.
The three cell types
Red blood cells carry oxygen from the lungs to the tissues using hemoglobin, the iron-rich protein that makes blood red. White blood cells are the immune system's soldiers, fighting infection. Platelets are cell fragments that clump at injury sites to start clotting and stop bleeding.
Plasma itself carries hormones, nutrients, clotting proteins, and waste, and keeps blood pressure and volume stable.
Blood groups
Red cells carry markers that define blood groups, mainly the ABO and Rh systems: A, B, AB, or O, each Rh positive or negative. Matching matters for transfusions, because the immune system attacks mismatched blood.
Group O negative can be given to almost anyone in an emergency, and group AB positive can receive from almost anyone. Blood group also matters in pregnancy, when a mother's Rh-negative status needs monitoring.
When blood goes wrong
Too few red cells or too little hemoglobin is anemia, causing fatigue and breathlessness. Problems with white cells range from infection-fighting failure to leukemias. Clotting problems cause either excessive bleeding, as in hemophilia, or dangerous clots in veins and arteries.
Blood tests are among the most common investigations in medicine because blood reflects the health of the whole body. Donating blood is safe for eligible donors and saves lives - a single donation can help several patients.
Limitations
General educational overview, not medical advice and not a diagnosis. Blood test interpretation belongs with a clinician.