Blood

Blood is the fluid that circulates through the heart, arteries, veins and capillaries of the body, carrying oxygen, nutrients, hormones and waste products. It is made up of cells suspended in a pale yellow liquid called plasma. The main cellular components are red blood cells, which transport oxygen using the iron-rich protein haemoglobin; white blood cells, which defend the body against infection; and platelets, small fragments that help blood to clot and seal wounds. Plasma itself is mostly water and carries dissolved proteins, salts and other substances.

Human blood is classified into groups according to molecules called antigens on the surface of red cells. The best-known is the ABO system, which gives the blood types A, B, AB and O depending on which antigens are present, determined by a person's inherited genotype. A second important system is the Rhesus factor, which makes blood either Rh positive or Rh negative. These groups matter greatly for blood transfusions, because mixing incompatible types can trigger a harmful immune reaction. Knowing a person's ABO and Rhesus type ensures that donated blood is safely matched.

Blood tests are a common medical tool, used to examine the numbers and condition of blood cells, measure chemicals and proteins, and detect disease. The study of blood and its disorders is called haematology. The body also regulates blood production through signals such as the hormone erythropoietin (EPO), which stimulates the formation of red blood cells when more oxygen-carrying capacity is needed.

Frequently asked questions

What are the main components of blood?

Blood consists of plasma, the liquid part, plus red blood cells that carry oxygen, white blood cells that fight infection, and platelets that help blood clot.

What are blood groups?

Blood groups classify blood by antigens on red cells. The ABO system gives types A, B, AB and O, and the Rhesus system adds positive or negative, which together guide safe transfusions.

Why are blood tests done?

Blood tests measure cells, chemicals and proteins to assess health, diagnose disease, monitor conditions and check how organs such as the liver and kidneys are functioning.