ACS :: Blood and Its Components

Blood is the vital mixture of cells and liquid pumped by the heart through the arteries and veins. It reaches all of the cells in the body, bringing them oxygen and nutrients and taking away carbon dioxide (CO2) and other waste products. Blood is made up of many parts (components), such as red blood cells, white blood cells, platelets, plasma, clotting factors, and small proteins. Each of these components has a different job.

Whole blood (what people usually donate) contains all parts of the blood. Each donated unit is about a pint (which is around 475 milliliters.) After it is taken, the whole blood is usually separated into several blood products, such as red blood cells, plasma, platelets, and/or cryoprecipitate. This allows doctors to give patients only what they need and also helps to get the most out of the donated blood supply. People rarely need whole blood transfusions.

Red blood cells (RBCs) give blood its color. Their job is to take oxygen through the bloodstream to every part of the body and bring carbon dioxide (CO2) back to the lungs, where it is removed from the body when we exhale.

Red blood cells (and other blood cells) are normally made in the bone marrow, the soft inner part of certain bones. The production of RBCs in the body is controlled by the kidneys. When the kidneys sense that there aren't enough RBCs in the blood, they release a hormone called erythropoietin that causes the bone marrow to make more.

When people need red blood cells, they get "packed red blood cells" (PRBCs). This blood component has much of the plasma and other cells removed from it.

Plasma, the clear pale-yellow liquid portion of blood, contains some clotting factors -- proteins that help make blood clot. This is important when the body is injured because clots are needed to help seal blood vessels and stop bleeding. Plasma also has other proteins, such as antibodies, which help fight infection. Once plasma is separated from the red blood cells, it can be frozen and kept for up to a year until it is needed. Once thawed, it is called fresh frozen plasma (FFP).

Plasma can be donated by itself in a different process called apheresis, or sometimes called plasmapheresis. The donor is hooked up to a machine that removes blood, then separates the plasma and puts it into a different container. The machine then returns the red cells and other parts of the blood back to the donor's bloodstream.

Platelets are fragments of cells in blood that are another important part of the clotting process. They work with the clotting factors in plasma to help prevent unwanted bleeding. Platelets come from special cells called megakaryocytes. Like other early (immature) forms of blood cells, megakaryocytes are mainly found in the bone marrow.

Platelets are usually found in the plasma, but they can be separated from it. A unit of whole blood contains only a small volume of platelets. It takes several units of platelets (from different donors) to help keep a person from bleeding. A unit of platelets is defined as the amount that can be separated from a unit of whole blood. Typically, 6 to 10 units of these random donor platelets are usually combined and given to adult patients at one time.

Platelets can also be collected by apheresis. This is sometimes called plateletpheresis. In this procedure, a donor is hooked up to a machine that removes the blood, and keeps just the platelets. Then the rest of the blood cells and plasma are returned to the donor. Apheresis can collect enough platelets so that they don't have to be pooled with platelets from other donors. Platelets collected in this way are called single donor platelets. (More information about this is available in the section on blood donation.)

Cryoprecipitate is the name given to the small fraction of plasma that separates out (precipitates) when plasma is frozen and then thawed in the refrigerator. It contains several clotting factors found in plasma, but they are now concentrated in a smaller amount of liquid. A unit of whole blood contains only a small amount of cryoprecipitate, so about 10 units of cryoprecipitate are usually pooled together for one transfusion.

Granulocytes are types of white blood cells (WBCs or leukocytes) that help the body fight infection. As with other types of blood cells, granulocytes are made in the bone marrow. When more of these WBCs are needed, the body normally makes substances called colony stimulating factors (CSFs), which cause the bone marrow to make more granulocytes.

Like platelets and plasma, granulocytes can be collected by apheresis, sometimes called leukopheresis. Granulocytes can also be separated out of a unit of whole blood from the plasma and RBCs into a product called buffy coat, which can then be transfused. Granulocytes are mainly used when people have very low white blood cell counts and have serious infections. Since man-made versions of colony stimulating factors are now available, along with better antibiotics, granulocyte transfusions have become rare.

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