Blood supply refers to the blood resources in blood banks and hospitals that are available for use by the health care community. The blood supply consists of donated blood units (in pints) that are used in blood transfusions. A blood transfusion is a procedure whereby blood is administered through a needle into the vein of a person or animal. Such transfusions are usually performed in order to replace blood lost due to injury or surgery. Blood banks are institutions that store blood to be distributed to local hospitals and medical centers. There are a number of blood banks in every country of the world. Together they contain most of the nation's supply of donated blood. Many blood banks are run by the Red Cross, an organization that also conducts frequent blood drives throughout the world. In fact, the American Red Cross gathers half the blood used in the United States. The blood supply must be replenished constantly to meet the needs of hospitals and trauma units as well as to replace blood components that have a short shelf life.
Donation of blood by volunteers is critical in maintaining the supply of blood in blood banks. Beginning in the late 1990s, blood donations in the United States began to increase by 2 to 3 percent per year. But at the same time, the demand for blood increased by 6 to 8 percent. In 2000, about 13 million units of blood were used in the United States. Blood is collected from a donor by inserting a needle attached to a thin plastic tube into a vein of the arm. Blood flows through the tube and into a sterile plastic bag. The body of an average adult human contains approximately 6 quarts (5.6 micro liters) of blood, and the removal of one pint usually has little effect (although some people—especially those with low body weights—may experience temporary dizziness, nausea, or headache). Healthy donors can make blood donations about every eight weeks without causing harm to their bodies.
The collected blood of the donor is tested for hepatitis (a disease of the liver), syphilis (an STD, or sexually transmitted disease), human immunodeficiency virus (HIV; the virus believed to cause AIDS), and related viruses. It is also classified according to blood type and the presence of Rh, or Rhesus, factor. (Blood types are A, B, AB, and O. Rhesus factor is a substance, called an antigen, in the blood of most people.) It is extremely important that blood be marked correctly. Patients receiving donated blood that is incompatible with their own may suffer serious reactions to it. After being collected and classified, whole donated blood is refrigerated.
Most donated blood is separated into its components—plasma, red blood cells, white blood cells, and platelets—before being stored. This allows the blood of a single donor to be used for several patients who have different needs. Blood is separated by means of centrifugation, a process in which the blood is rapidly spun so that the heavier blood cells and platelets separate out from the lighter plasma. Plasma, the liquid part of blood, can be dried into a powder or frozen. Fresh frozen plasma and freeze-dried preparations containing clotting factors are used to treat patients with hemophilia. Hemophilia is an inherited disorder in which certain clotting factors are missing in the blood, resulting in excessive bleeding. Concentrated red blood cells are used to transfuse patients with anemia, a condition in which the blood contains an insufficient number of red blood cells. White blood cells and platelets are used for transfusions in patients who have a deficiency of these components in their blood.
When AIDS (acquired immunodeficiency syndrome) began to appear in hemophiliacs and surgical patients in the early 1980s, it was determined that these patients had contracted the disease through donated blood. In 1985, a test was developed to detect HIV—the virus believed to cause AIDS—in blood. Donors are now carefully screened to eliminate any who may be at risk for carrying the AIDS virus. Although the risk of contracting HIV from blood transfusions is remote, some patients who are scheduled to undergo surgery may choose to donate their own blood beforehand, in case a transfusion is necessary. It is important to know that a blood donor cannot contract AIDS or any other disease by donating blood. The equipment used to collect donated blood is used only once and then discarded. The restrictions on who can donate blood have been expanding steadily to reduce the risk of introducing infections into blood supplies. Almost all countries run a number of diagnostic tests to screen for viruses and other contaminants before shipping blood. Once-rare procedures, like stripping out white blood cells from donated blood to reduce the side effects from transfusions, is now commonplace.
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