Plasma is the liquid portion of your blood, making up about 55% of total blood volume. It carries nutrients, hormones, waste products, and immune proteins to virtually every tissue in your body. While red and white blood cells get most of the attention, plasma is what keeps them moving and performs a surprising number of critical jobs on its own, from preventing your tissues from swelling with excess fluid to stopping bleeding when you’re injured.
Plasma is about 90% water, with the remaining 10% consisting of proteins, salts, fats, hormones, and dissolved gases. That small protein fraction does an outsized amount of work.
Transporting Nutrients, Hormones, and Waste
Plasma is your body’s delivery and removal service. After you eat, nutrients like glucose, amino acids, and fatty acids are absorbed through the digestive tract and enter plasma, which carries them to cells throughout the body for energy and repair. Hormones produced by glands like the thyroid, pituitary, and adrenals also travel through plasma to reach their target tissues, regulating everything from metabolism to growth.
At the same time, plasma picks up metabolic waste. Urea and creatinine (byproducts of protein breakdown and muscle activity) travel through plasma to the kidneys for filtering. Bilirubin, a waste product from old red blood cells, is carried to the liver for processing. Carbon dioxide produced by cells dissolves into plasma and is transported back to the lungs for exhaling. Without this constant two-way traffic, cells would starve and toxins would accumulate.
Keeping Fluid Inside Your Blood Vessels
One of plasma’s most important jobs is preventing fluid from leaking out of your bloodstream and pooling in surrounding tissues. This comes down to a protein called albumin, which makes up about half of all plasma protein by weight. Albumin generates what’s called oncotic pressure, a pulling force that draws water back into blood vessels and counterbalances the outward push of blood pressure.
Albumin is responsible for roughly 80% of this pulling force. It works partly through sheer numbers (albumin molecules vastly outnumber other plasma proteins) and partly through an electrical effect: the albumin molecule carries a negative charge that attracts positively charged ions like sodium into the bloodstream, further increasing the inward pull of water. The normal oncotic pressure in human plasma is about 28 mmHg, with about two-thirds coming from dissolved proteins and the rest from this ion-attracting effect.
When albumin levels drop, whether from liver disease, kidney problems, or severe malnutrition, fluid escapes into tissues and causes swelling (edema). This is why puffy ankles or a swollen abdomen can signal a protein problem in the blood.
Stopping Bleeding With Clotting Proteins
Plasma contains fibrinogen and a cascade of other clotting factors that work together to seal wounds. When a blood vessel is damaged, an enzyme called thrombin converts fibrinogen into fibrin, long sticky strands that weave together to form a mesh over the injury site. Platelets get trapped in this mesh, creating a stable clot that stops bleeding.
This system is precisely balanced. Too little fibrinogen or other clotting factors leads to excessive bleeding; too much clotting activity can cause dangerous blood clots. Plasma carries the proteins on both sides of that equation, including factors that break clots down once healing is underway.
Supporting Your Immune System
Plasma carries three major types of globulin proteins (alpha, beta, and gamma), each with distinct roles. The gamma globulins are better known as antibodies, or immunoglobulins. These are produced by white blood cells in response to infections and vaccinations, and they circulate in plasma ready to latch onto bacteria, viruses, and toxins to neutralize them.
Beyond antibodies, plasma contains the complement system: a group of proteins that amplify the immune response by helping immune cells engulf pathogens, triggering inflammation at infection sites, and directly punching holes in bacterial cell walls. Plasma also transports signaling molecules called cytokines, which coordinate immune activity across different tissues. If an infection starts in your lungs, cytokines traveling through plasma can mobilize immune defenses elsewhere in the body within minutes.
Regulating Blood pH
Your blood needs to stay within a very narrow pH range, right around 7.4, for enzymes and cellular processes to function properly. Plasma maintains this through a buffering system built from carbonic acid and bicarbonate ions.
When something acidic enters your bloodstream (during intense exercise, for example, or from certain foods), bicarbonate ions neutralize the extra acid by converting it into carbonic acid and water, both of which are already normal components of blood. When something basic enters the bloodstream, carbonic acid neutralizes it by producing bicarbonate ions and water. This two-way system continuously adjusts to keep pH stable, preventing the dangerous swings that would otherwise occur with every meal, workout, or breath.
Maintaining Electrolyte Balance
Plasma contains dissolved electrolytes, including sodium, potassium, calcium, chloride, bicarbonate, and magnesium. These ions regulate nerve signaling, muscle contraction, heart rhythm, and fluid distribution between cells and the bloodstream. Your body keeps them within tight ranges: sodium typically runs between 136 and 146 millimoles per liter, potassium between 3.5 and 5.0, and calcium between 8.4 and 10.2 mg/dL. Even small deviations from these ranges can cause symptoms like muscle cramps, irregular heartbeat, or confusion.
How Plasma Is Used in Medicine
Because plasma is so rich in clotting factors and other proteins, it’s used in transfusions for patients with severe bleeding, especially when their own clotting system is overwhelmed. Massive trauma, major surgery, and liver failure (since the liver produces most clotting factors) are common scenarios. For one rare clotting factor deficiency, factor V deficiency, plasma transfusion is the only treatment because no concentrated replacement exists.
Plasma is also used in a procedure called therapeutic plasma exchange, where a patient’s own plasma is removed and replaced with donor plasma. This is a first-line treatment for several serious conditions, including a dangerous blood clotting disorder called thrombotic thrombocytopenic purpura, acute liver failure, and certain autoimmune diseases where harmful antibodies need to be physically removed from circulation.
One important distinction: plasma is not used simply to replace lost fluid volume. Saline or other intravenous solutions handle that job. Plasma transfusions are specifically for replacing the proteins and clotting factors that only plasma contains.
Plasma vs. Serum
You’ll sometimes see “plasma” and “serum” used in medical contexts, and the difference is straightforward. Plasma is the liquid you get when you prevent blood from clotting (by adding an anticoagulant to a blood sample). Serum is the liquid left over after blood has been allowed to clot. They contain nearly identical components: water, albumin, globulins, hormones, nutrients, and waste products. The key difference is that plasma still contains fibrinogen and other clotting factors, while serum does not, because those proteins were consumed during the clotting process. In lab tests, doctors choose one or the other depending on what they need to measure.