Plasma is the liquid portion of your blood, and it does a lot of heavy lifting: transporting nutrients and hormones, fighting infections, helping blood clot, and keeping fluid balanced between your blood vessels and tissues. It makes up about 55% of your total blood volume and is roughly 90% water, with the remaining 10% consisting of proteins, electrolytes, vitamins, and dissolved gases.
Transporting Nutrients, Hormones, and Waste
Plasma serves as the body’s main delivery system. Glucose, amino acids, vitamins, and electrolytes all dissolve in plasma and travel through the bloodstream to reach cells that need them. Hormones released by glands like the thyroid or adrenal glands also hitch a ride through plasma to reach their target organs.
The return trip matters just as much. Cells produce waste products as they work, and plasma picks those up for disposal. Carbon dioxide, for example, is absorbed from cells into the plasma and carried back to the lungs, where you exhale it. Other metabolic waste travels through plasma to the liver and kidneys, which filter it out of the body.
Three Key Proteins and What They Do
Plasma proteins are where much of the action happens. Healthy adults carry between 5.5 and 9.0 grams of total protein per deciliter of blood, and these proteins fall into three major categories.
Albumin is the most abundant plasma protein. Your liver produces it, and it does double duty. First, it acts as a molecular taxi, binding to and carrying substances like hormones, fatty acids, and medications to where they’re needed. Second, it controls something called oncotic pressure, which is the force that keeps fluid inside your blood vessels rather than leaking into surrounding tissues. Albumin’s large size and negative electrical charge pull water and positively charged particles into the bloodstream. When albumin levels drop too low, fluid seeps out of blood vessels and into tissues, causing swelling (edema). Normal albumin levels range from 3.5 to 5.5 grams per deciliter.
Globulins, particularly immunoglobulins, are your plasma’s defense force. These are antibodies that recognize and attack bacteria, viruses, fungi, and parasites. Different types of immunoglobulins handle different threats: some respond to new infections, while others provide long-term immunity after you’ve recovered from an illness or received a vaccine.
Fibrinogen is the protein responsible for blood clotting. When you cut yourself, fibrinogen works alongside other clotting factors to form a mesh of fibrin strands that plugs the wound and stops bleeding. Fibrinogen also rises during inflammation, which is why doctors sometimes use related blood tests as a marker for inflammatory conditions.
Keeping Fluid in the Right Places
One of plasma’s less obvious but critical jobs is maintaining the balance of fluid between your blood vessels and the tissues around them. Two opposing forces are constantly at play: blood pressure pushes fluid out of capillaries into tissues, while the oncotic pressure created by plasma proteins (mainly albumin) pulls fluid back in. In a healthy person, plasma proteins maintain an oncotic pressure of about 25 mmHg, which is enough to counterbalance the outward push and prevent tissues from becoming waterlogged.
This is why conditions that damage the liver (which makes albumin) or the kidneys (which can leak albumin into urine) often cause visible swelling in the legs, ankles, or abdomen. The drop in albumin means less pulling force to keep fluid where it belongs.
Buffering Your Blood’s pH
Your blood needs to stay within a narrow pH range of 7.35 to 7.45 to function properly. Even small shifts outside that window can disrupt the chemical reactions your cells depend on. Plasma helps maintain this balance through buffer systems, chemical pairs that absorb excess acid or base to prevent dangerous swings.
The most important of these is the bicarbonate system, which is the most plentiful buffer in the body. At a normal blood pH of 7.4, bicarbonate outnumbers carbonic acid by a ratio of 5,000 to 1, giving the body a large reserve to neutralize acids produced during exercise, digestion, or illness. Plasma proteins themselves also act as buffers, adding another layer of protection. Together, these systems give the body a total buffering capacity of about 75 millimoles per liter at normal pH.
How Plasma Differs From Serum
You’ll sometimes see “plasma” and “serum” used in medical contexts, and they’re not the same thing. Plasma is collected from blood that has been treated with an anticoagulant to prevent clotting, so it retains all its proteins, including fibrinogen and other clotting factors. Serum, on the other hand, is what’s left after blood is allowed to clot and the clot is removed. Serum contains most of the same components as plasma minus fibrinogen and several clotting factors. Which one a lab uses depends on what’s being tested.
Medical Uses of Plasma
Plasma’s rich mix of proteins and antibodies makes it valuable in medicine. Plasma transfusions are used for patients who have lost large amounts of blood or who have clotting disorders. Donated plasma is also processed to extract specific proteins, like immunoglobulins for patients with immune deficiencies or clotting factors for people with hemophilia.
A procedure called therapeutic plasma exchange takes things a step further. In this treatment, a patient’s plasma is removed and replaced with donor plasma or a substitute solution. The goal is to physically strip out harmful substances circulating in the blood: autoantibodies, immune complexes, or inflammatory molecules that are driving disease. This approach is a frontline therapy for several serious conditions, including Guillain-Barré syndrome, myasthenia gravis, and a rare clotting disorder called thrombotic thrombocytopenic purpura. It’s also used during organ transplants to reduce the risk of antibody-mediated rejection, in acute liver failure, and even to remove certain toxins or venoms from the bloodstream.
Plasma exchange works for metabolic conditions too. People with inherited high cholesterol that doesn’t respond adequately to medication can undergo regular plasma exchange sessions to physically remove excess cholesterol from their blood.