Blood plasma is the liquid component of blood, consisting of around half of the total blood volume. Plasma itself is around 90% water, with the 10% remainder including proteins, minerals, waste products, clotting factors, hormones, and immunoglobins. Without plasma, blood cells would have no medium to travel on as they moved through the body, and plasma also performs a number of other useful functions in the body.

Separating blood plasma from the blood itself is very easy. Blood can be drawn from the patient and then run in a centrifuge. As the blood spins, the heavier blood cells settle to the bottom, and the plasma rises to the top. Plasma is usually straw colored, although it can be cloudy or grayish, depending on the health and diet of the plasma's host. Tests can be performed on the plasma to learn more about the health of the donor, and the blood cells can also be analyzed for information.

As plasma circulates through the body, it acts like a milkman making deliveries. The plasma drops off various substances to the cells of the body, and collects waste products for processing. Blood plasma flows constantly, and the components of plasma are constantly being renewed. In addition to providing nutrition and waste cleanup, blood plasma also harbors immune system cells which attack infections in the body, and it is used to deliver hormones and clotting factors to areas where they are needed.

Doctors sometimes use infusions of plasma to treat a variety of medical conditions. Pure plasma contains clotting factors which increase the rate at which blood clots, making it useful in surgery and in the treatment of hemophilia. Frozen, plasma can keep for up to 10 years, making it an extremely stable blood product, and plasma can also be packaged in dried form for reconstitution, a technique which was developed for military applications. For hemophiliacs, packages of plasma combining clotting products from hundreds or thousands of donors are used to compensate for the clotting factors that the hemophiliac lacks.

In a medical process called plasmapheresis, plasma can be pulled out of the blood, treated, and returned to the patient to treat certain medical conditions. Plasmapheresis can also be used in plasma donation, allowing people to donate just plasma, without any blood cells. Since plasma is often in high demand, plasma donation is an excellent donation option for people who wish to contribute blood products to people in need. Plasma donation takes a little bit longer than regular blood donation, but the blood plasma replenishes itself within 48 hours, making for a very rapid recovery.