Plasma is the liquid portion of your blood, and it does a lot more than simply carry red blood cells around. It makes up about 60% of your total blood volume and serves as the body’s primary transport system, delivering nutrients, hormones, and proteins to cells while hauling waste products away. It also regulates blood pressure, fights infections, stops bleeding, and keeps your body’s pH in a narrow safe range.
What Plasma Is Made Of
Plasma is 91% to 92% water. The remaining 8% to 9% is a mix of dissolved solids: proteins, electrolytes, hormones, vitamins, and enzymes. That small solid fraction is what gives plasma its remarkable range of functions. The three most important protein groups are albumin (which controls fluid balance), globulins (which fight infection), and fibrinogen (which enables clotting). Electrolytes like sodium, potassium, calcium, bicarbonate, and chloride round out the mix, helping maintain blood pH and electrical signaling between cells.
Keeping Fluid Where It Belongs
One of plasma’s most critical jobs is preventing fluid from leaking out of your blood vessels and pooling in surrounding tissues. Albumin, the most abundant plasma protein, is the key player here. Its large molecular size and negative electrical charge pull water and positively charged particles into the bloodstream, creating what’s called oncotic pressure. This inward pull counterbalances the outward push of blood pressure at the capillary level, keeping the right amount of fluid inside your vessels.
When albumin levels drop, whether from liver disease, severe malnutrition, or kidney problems, fluid escapes into tissues. This is why people with low albumin develop swelling in their legs, abdomen, or lungs. About 30% to 40% of albumin stays in the bloodstream at any given time, with the rest distributed in the spaces between cells, where it continues to manage fluid movement.
Fighting Infection
Immunoglobulins, also called antibodies, make up roughly 20% of total plasma protein. These Y-shaped molecules are produced by specialized white blood cells called plasma cells, and each type handles a different aspect of immune defense.
- IgM is the first antibody your body produces when it encounters a new pathogen. It’s large (a five-unit structure) and effective at activating the complement system, a cascade of proteins that punch holes in bacterial cell walls.
- IgG dominates the secondary immune response, meaning it kicks in after your body has already seen a particular threat. It’s the most abundant antibody in blood and provides long-lasting protection.
- IgA guards the mucous membranes lining your digestive, respiratory, and urinary tracts. It carries a protective component that prevents digestive enzymes from breaking it down.
- IgE targets parasites but is also responsible for allergic reactions. It binds to mast cells and triggers histamine release when it encounters an allergen.
Together, these antibodies neutralize toxins, tag bacteria for destruction, block viruses from entering cells, and prevent microbes from attaching to tissue surfaces.
Stopping Bleeding
When you cut yourself, plasma proteins orchestrate the clotting process. Fibrinogen, a soluble protein circulating in plasma, gets converted into insoluble fibrin strands by an enzyme called thrombin. These fibrin strands weave together into a mesh that traps platelets and red blood cells, forming a stable clot over the wound.
The structure of the resulting clot depends on how much thrombin is present. High thrombin concentrations produce dense, tightly branched networks that resist being broken down. Low concentrations yield looser, coarser networks that dissolve more easily. The body also crosslinks additional proteins into the fibrin mesh to reinforce it, including molecules that prevent the clot from being prematurely dissolved. This entire system is carefully balanced: strong enough to stop bleeding, but not so aggressive that clots form where they shouldn’t.
Transporting Nutrients and Waste
Plasma functions as the body’s highway system. It carries glucose, amino acids, fatty acids, and vitamins from the digestive tract to cells that need them. It also transports hormones from the glands that produce them to distant target organs. Albumin alone acts as a carrier for a wide range of substances, including hormones and medications, shuttling them through the bloodstream to their destinations.
On the return trip, plasma picks up metabolic waste. Carbon dioxide produced by cells dissolves into plasma (some also binds to hemoglobin in red blood cells) and travels back to the lungs, where it’s exhaled. Other waste products, like urea from protein metabolism, travel through plasma to the kidneys for filtration and excretion. Without this constant circulation, toxic byproducts would accumulate in tissues within minutes.
Regulating Blood pH
Your blood must stay within a very narrow pH range of 7.35 to 7.45 to keep enzymes and cellular processes functioning properly. Plasma plays a central role in maintaining this balance through its bicarbonate buffer system. Carbon dioxide from cellular metabolism dissolves in plasma and combines with water to form carbonic acid, which then splits into bicarbonate and a hydrogen ion. This reaction works in both directions, absorbing excess acid or releasing it as needed to keep pH stable.
The system connects directly to the lungs and kidneys. When blood becomes too acidic, you breathe faster to expel more carbon dioxide, shifting the reaction to consume hydrogen ions and raise pH. The kidneys contribute by reabsorbing bicarbonate or excreting acid into urine. Plasma electrolytes, particularly bicarbonate, are essential partners in this process, acting as a chemical sponge that absorbs pH swings before they become dangerous.
Medical Uses of Plasma
Because plasma carries so many essential proteins, it has significant therapeutic value. Donated plasma is used to treat people whose own plasma is missing critical components or contains harmful substances. In a procedure called therapeutic plasma exchange, a patient’s plasma is removed and replaced with healthy donor plasma. This approach works in two ways: it can flush out damaging antibodies, toxins, or inflammatory molecules that are driving a disease, or it can replenish protective proteins the patient lacks.
Conditions treated with plasma exchange span a wide range. Guillain-Barré syndrome and myasthenia gravis, both autoimmune disorders affecting the nervous system, respond to removal of the rogue antibodies attacking nerve tissue. A rare but life-threatening blood clotting disorder called thrombotic thrombocytopenic purpura requires donor plasma to replace a missing enzyme that normally prevents dangerous clot formation. Plasma exchange is also used after organ transplants to remove antibodies that might cause rejection, and in certain cases of acute liver failure, vasculitis, and severe infections.
Plasma-derived products are equally important. Immunoglobulin therapies, made from pooled donor plasma, provide antibodies to people with immune deficiencies. Clotting factor concentrates extracted from plasma treat hemophilia and other bleeding disorders. Albumin preparations are used to restore blood volume in trauma patients and people with severe burns.