Blood plasma

Blood plasma is a light amber-colored liquid that makes up about 55% of the body's total blood volume. While it lacks blood cells, it serves as a vital medium for transporting many essential substances. Plasma is the intravascular part of extracellular fluid, meaning it is the fluid located inside the blood vessels. It is composed mostly of water, which accounts for up to 95% of its volume. This liquid carries dissolved proteins, glucose, electrolytes, hormones, oxygen, and carbon dioxide. It also plays a critical role in maintaining osmotic pressure to balance electrolyte concentrations and protecting the body from infection.

To understand how plasma works, we must look at the proteins dissolved within it. The first major group is albumins, which are the most common proteins in the plasma. Produced in the liver, albumins maintain the osmotic pressure of the blood. This pressure ensures the blood has the correct viscosity, or thickness, to prevent fluid from leaking out of the capillaries. The second group is globulins. This category includes immunoglobulins, which are produced by plasma B cells to defend the body against pathogens. It also includes alpha and beta globulins. Alpha globulins help transport minerals and inhibit blood coagulation. Beta globulins, such as low-density lipoproteins (LDL), transport fat to cells for membrane synthesis. Finally, fibrinogen proteins are responsible for the process of blood clotting to prevent excessive blood loss.

Scientists can separate plasma from whole blood using a process called blood fractionation. First, an anticoagulant is added to a tube of blood to prevent it from clotting. This tube is then placed in a centrifuge, a machine that spins at very high speeds. The centrifugal force causes the heavier blood cells to fall to the bottom of the tube. The liquid plasma can then be drawn off from the top. In some medical testing, plasma is extracted through filtration or agglutination to identify specific biomarkers. It is important to distinguish plasma from serum. Serum is essentially blood plasma that has had its clotting factors removed. Because plasma still contains these factors, it can be prepared more quickly than serum, which requires a 30-minute waiting period for coagulation.

The history of plasma use is closely tied to medical advancements during wartime. In 1918, Gordon R. Ward proposed using plasma as a substitute for whole blood. During World War II, the military utilized dried plasma packages. These packages came in tin cans and included a bottle of distilled water to reconstitute the powder. This allowed the plasma to be ready for use in about three minutes. Charles Drew was a key figure in this era, serving as the medical supervisor for the "Plasma for Britain" project in 1940. Drew developed mass production techniques that transformed laboratory methods into large-scale operations. He also famously argued against policies that separated blood by the race of the donor, insisting there was no racial difference in human blood.

Another major historical milestone was the development of plasmapheresis. Dr. José Antonio Grifols Lucas pioneered this technique in 1940. In plasmapheresis, a donor's red blood cells are separated from the plasma and then immediately returned to the donor's body. This allows for the collection of plasma alone. Dr. Grifols also opened the world's first plasma donation center in 1945. This method remains a standard medical practice nearly 80 years later.

Today, plasma is a critical resource in emergency medicine. Fresh frozen plasma (FFP) is included on the WHO Model List of Essential Medicines. It is vital for treating trauma patients who have suffered significant blood loss. Because of this, FFP is stocked in hospitals, trauma centers, and ambulances. Interestingly, the rules for plasma donation are different from whole blood donation. While O- is the universal donor for red blood cells, AB is often considered the universal donor for plasma. This is because AB plasma does not contain antibodies that might react with a recipient's antigens.

Plasma levels in the body can change based on physical conditions. For instance, standing still for a long time increases transcapillary hydrostatic pressure. This can cause about 12% of the plasma volume to move into the extravascular compartment. This shift increases blood viscosity and the concentration of coagulation factors. Such changes can lead to orthostatic hypercoagulability. Understanding these shifts helps scientists and doctors manage conditions like circulatory shock, where blood pressure drops and fluid moves into the interstitium, a process known as third spacing.