Disorders that affect the blood are called blood disorders or hematologic disorders. There are many blood disorders, and they can affect the quantity as well as the function of the cells in the blood (blood cells) or proteins in the blood clotting system or immune system.
A blood transfusion is the transfer of blood or a blood component from one healthy person (a donor) to a sick person (a recipient). Transfusions are given to increase the blood's ability to carry oxygen, restore the amount of blood in the body (blood volume), and correct clotting problems.
Iron is essential for life, so the body usually tightly controls iron absorption from food and recycles the iron from red blood cells. People lose small amounts of iron every day, and even a healthy diet contains only a small amount of iron. Thus, people rarely have too much iron in their body. Causes of excess iron in the body (iron overload) include the following:
In myeloproliferative neoplasms (myelo = bone marrow; proliferative = rapid multiplication; and neoplasm = new abnormal growth), the blood-producing cells in the bone marrow (precursor cells, also called stem cells) develop and reproduce excessively or are crowded out by an overgrowth of fibrous tissue. Sometimes, blood-producing cells appear and reproduce in the spleen and liver. Myeloproliferative neoplasms are caused by genetic mutations. Typically the mutations are acquired and not inherited, although rarely there are families in which several members have these disorders.
Plasma cell disorders are uncommon. They begin when a single plasma cell multiplies excessively. The resulting group of genetically identical cells (called a clone) produces a large quantity of a single type of antibody (immunoglobulin). Plasma cells develop from B cells (B lymphocytes), a type of white blood cell that normally produces antibodies. These proteins help the body fight infection.
Platelets (sometimes called thrombocytes) are cell fragments that circulate in the bloodstream and help blood to clot. Thrombopoietin, primarily produced in the liver, stimulates the bone marrow to make large cells (megakaryocytes), which in turn make platelets from their cytoplasm. Platelets that are not used in clots circulate for 7 to 10 days and are then destroyed. About one third are always stored in the spleen.
The spleen, a spongy, soft organ about as big as a person’s fist, is located in the upper left part of the abdomen, just under the rib cage. The splenic artery brings blood to the spleen from the heart. Blood leaves the spleen through the splenic vein, which drains into a larger vein (the portal vein) that carries the blood to the liver. The spleen has a covering of fibrous tissue (the splenic capsule) that supports its blood vessels and lymphatic vessels.
White blood cells (leukocytes) are an important part of the body’s defense against infectious organisms and foreign substances (the immune system). To defend the body adequately, a sufficient number of white blood cells must receive a message that an infectious organism or foreign substance has invaded the body, get to where they are needed, and then kill and digest the harmful organism or substance (see figure ).