Blood consists of red blood cells that carry oxygen, white blood cells that fight infections, platelets that function in clotting, and plasma, the liquid that carries all the other components of blood. The plasma makes up more than half the total volume of blood. In addition to transporting blood cells and platelets, the various components of plasma serve several critical functions of their own.
Components
Plasma is mainly water, providing a liquid medium to distribute the cellular components of blood. In addition to water, plasma also contains proteins like albumin, an important binding factor that keeps other components from diffusing out of the blood into the tissues, as well as vital electrolytes like sodium and potassium. Plasma also contains clotting factors, hormones, vitamins and sugars, including glucose.
Purpose
Plasma provides a medium to transport needed blood components --- like the red blood cells that carry oxygen, and the white blood cells for defense against infection --- to all the tissues of the body. Plasma also provides bulk to keep the blood vessels properly dilated, and a constant flow of liquid to keep them flushed out. When the tissues are dehydrated and need more water, plasma can readily provide it. Conversely, when the tissues need to rid themselves of water, they can dump it into the plasma. According to the Merck Manual, plasma also helps the extremities remain warm because it carries heat from the body's core out to peripheral areas like fingers and toes.
Donation
In addition to donating whole blood, people can donate plasma alone, without the other components of blood. Hospitals use plasma transfusions for patients with serious infections; for burn victims, to combat infection and dehydration; and for patients with liver failure. Blood donors can specifically give plasma, without loss of red or white blood cells, through a process called plasmapheresis. As with normal blood donation, a needle in the vein at the inside bend of the elbow channels blood out of the body. In the case of plasma donation, the blood flows into a machine that separates out the blood cells from the plasma. The plasma flows into a sterile collecting bag, while the blood cells are rerouted to return to the body through the same needle and vein. The process takes about 40 minutes, according to the National Institutes of Health Blood Bank.
Transfusion
The process for a plasma transfusion is no different than that for transfusing whole blood. The contents of a sterile bag of plasma run into the body through a needle inserted into a vein. Because plasma contains no blood cells, it looks like a clear yellow liquid in its transfusion bag.
Pharmaceuticals
In addition to hospitals, drug companies are a major consumer of plasma. The companies use bulk quantities of donated plasma to extract components for people with blood disorders; for example, drugs to treat hemophilia come from clotting factors extracted from donated plasma.
