Plasma is the liquid portion of your blood. It makes up about 55% of your total blood volume, and it’s mostly water, roughly 90 to 92%. The remaining 8 to 10% is a mix of proteins, salts, sugars, fats, hormones, and dissolved gases that keep your body running. If you’ve ever seen blood settle in a tube, the pale yellow fluid sitting on top of the red blood cells is plasma.
What Plasma Is Made Of
Water forms the base, but the dissolved components are what make plasma so critical. The most important of these are three groups of proteins, each with a distinct job.
Albumin is the most abundant plasma protein. It controls oncotic pressure, which is the force that keeps fluid inside your blood vessels rather than leaking into surrounding tissue. Albumin also acts as a transport vehicle, carrying hormones, fatty acids, and even medications through your bloodstream. Normal albumin levels fall between 35 and 50 grams per liter of blood.
Globulins come in several subtypes. The gamma globulins, better known as antibodies, are your immune system’s targeting system. They latch onto bacteria, viruses, fungi, and parasites and mark them for destruction. Another subtype, alpha-1 antitrypsin, protects your lungs by neutralizing enzymes that immune cells release during inflammation. Like albumin, globulins also help maintain the fluid balance between your blood vessels and tissues.
Fibrinogen is the protein responsible for blood clotting. When you cut yourself, fibrinogen converts into fibrin strands that form a mesh over the wound. Fibrinogen also rises during infections or injuries as part of your body’s inflammatory response. Healthy total protein in plasma (albumin, globulins, and fibrinogen combined) ranges from 60 to 80 grams per liter.
Beyond proteins, plasma carries electrolytes like sodium and potassium, glucose for energy, dissolved carbon dioxide headed back to the lungs, and metabolic waste products that the kidneys and liver filter out.
What Plasma Does in Your Body
Plasma is essentially your body’s delivery and removal service. Every cell in your body depends on it to receive nutrients and oxygen and to get rid of waste. Hormones produced in one organ travel through plasma to reach their target tissues elsewhere. Carbon dioxide, a byproduct of cell metabolism, dissolves into plasma (and partially binds to red blood cells) and travels back to the lungs, where you exhale it.
Plasma also moves heat. Your internal organs generate a lot of thermal energy, and plasma carries that heat from your body’s core to blood vessels near the skin surface, where it radiates away. This is one of the main ways your body regulates its temperature.
How Plasma Defends Against Infection
Your plasma contains an entire defense network called the complement system: a collection of proteins that work together to fight pathogens in three distinct ways. First, complement proteins coat the surface of bacteria and viruses, tagging them so that immune cells can recognize and swallow them more efficiently. Second, small fragments of complement proteins act as chemical signals that attract more immune cells to the site of an infection and activate them once they arrive. Third, complement proteins can assemble into a ring-shaped structure that punches a hole directly through a bacterial cell membrane, killing the bacterium by destroying its ability to maintain its internal environment.
This complement system works hand in hand with antibodies. When an antibody binds to a pathogen, it triggers the complement cascade, amplifying the immune response. The complement system was actually named because early researchers found it “complemented” the bacteria-killing ability of antibodies.
Plasma vs. Serum
You’ll sometimes see “plasma” and “serum” used in medical contexts, and they’re not the same thing. Both come from blood, but the difference is clotting. Plasma is collected by adding an anticoagulant to blood before spinning it in a centrifuge, so it retains fibrinogen and other clotting factors. Serum is collected by letting blood clot first, then spinning it. The clotting process consumes fibrinogen, so serum is essentially plasma minus the clotting proteins. Depending on what a lab is testing for, one may be preferred over the other.
How Plasma Donation Works
Donating plasma is different from donating whole blood. The process, called plasmapheresis, uses a specialized machine connected to a vein in your arm. The machine draws whole blood, separates out the plasma, and returns the red blood cells, white blood cells, and platelets back into your body. You’re typically given saline during the process to maintain your fluid levels. The whole procedure takes about an hour, compared to roughly 10 minutes for a standard blood draw.
Because your blood cells are returned to you, your body recovers faster than after whole blood donation. Most plasma centers allow donors to give more frequently than blood donors can.
Medical Uses of Donated Plasma
Donated plasma is processed into several life-saving treatments. One of the most widely used is concentrated immunoglobulin, pooled antibodies from thousands of donors given intravenously to people whose immune systems can’t produce enough antibodies on their own. This treatment is used for a range of immune deficiencies and autoimmune conditions, including chronic inflammatory nerve disorders.
Plasma is also the source of clotting factor concentrates for people with hemophilia. Patients with hemophilia A lack a specific clotting factor, and patients with hemophilia B lack a different one. Both are extracted from donated plasma and given as infusions. Another plasma product is used to urgently reverse the effects of blood-thinning medications before emergency surgery or during severe bleeding.
Because plasma contains such a complex mix of functional proteins, it remains one of the most versatile biological materials in medicine. Synthetic alternatives exist for some of these products, but for many patients, plasma-derived treatments have no substitute.