Plasma is the liquid portion of your blood. It makes up about 60% of your total blood volume, while red blood cells, white blood cells, and platelets account for the remaining 40%. If you’ve ever seen a tube of blood that’s been left to settle, the pale yellow fluid sitting on top is plasma. It serves as the river system of your body, carrying cells, nutrients, hormones, and waste products to wherever they need to go.
What Plasma Is Made Of
Plasma is mostly water, about 91% to 92%. The remaining 8% to 9% is a mix of dissolved solids that keep your body functioning. Those solids include proteins (the most important group), electrolytes like sodium and potassium, dissolved gases, hormones, vitamins, and waste products on their way to being filtered out.
The three major protein groups in plasma each serve a distinct purpose. Albumin is the most abundant, making up roughly half of all plasma protein. It acts like a sponge that holds water inside your blood vessels, preventing fluid from leaking into surrounding tissues. Globulins include antibodies (also called immunoglobulins) that recognize and fight infections. Fibrinogen is the protein responsible for blood clotting. When you cut yourself, fibrinogen converts into a mesh-like substance called fibrin that forms a stable clot to stop the bleeding.
Plasma also carries electrolytes, including sodium (normally 135 to 145 mEq/L) and potassium (3.5 to 5 mEq/L), along with bicarbonate, chloride, and calcium. These minerals help maintain your blood’s pH between 7.35 and 7.45, a narrow range your body works hard to protect. Even small shifts outside that window can disrupt how your cells function.
What Plasma Does in Your Body
Plasma’s primary job is transport. It picks up nutrients from your digestive system and delivers them to cells throughout your body. It carries hormones from the glands that produce them to the organs that respond to them. And it works in reverse, too: cells dump their metabolic waste into the plasma, which carries it to the kidneys and liver for removal.
Beyond transport, plasma is what moves every blood cell through your circulatory system. Red blood cells, white blood cells, and platelets are all suspended in plasma. Without it, those cells would have no way to travel from your heart to your fingertips or your brain.
Albumin’s role deserves a closer look because it’s so central to keeping you healthy. By pulling water into your blood vessels through osmotic pressure, albumin prevents the swelling (edema) that happens when fluid builds up in tissues. People with liver disease or severe malnutrition often have low albumin levels, and one of the first visible signs is swollen ankles and legs from fluid that has escaped the bloodstream.
Plasma vs. Serum
You’ll sometimes hear “plasma” and “serum” used as though they’re interchangeable, but they’re not. The key difference is clotting factors. Plasma contains fibrinogen and other clotting proteins. Serum is what’s left after blood has been allowed to clot and those proteins have been used up. In practical terms, when a lab draws your blood into a tube with an anticoagulant (something that prevents clotting), the liquid they separate out is plasma. When they draw blood into a plain tube and let it clot naturally, the leftover liquid is serum.
This distinction matters in medicine. Some lab tests require plasma, others require serum, depending on whether the presence or absence of clotting factors would interfere with the measurement.
How Plasma Is Used in Medicine
Plasma donation and transfusion are a significant part of modern healthcare. Fresh frozen plasma is used to treat patients who are bleeding and lack sufficient clotting factors, whether from liver failure, massive blood loss, or genetic conditions. Plasma-derived products are also manufactured into specific therapies: concentrated clotting factors for people with hemophilia, immunoglobulin preparations for patients with weakened immune systems, and albumin solutions for people in critical care.
Plasma exchange is another medical procedure where a patient’s plasma is removed and replaced with donor plasma or a substitute fluid. This is used for conditions where harmful antibodies or other substances circulating in the plasma are causing damage. In acquired hemophilia A, for example, the body produces antibodies that attack its own clotting factors. Removing and replacing the plasma can help eliminate those antibodies and restore normal clotting ability.
Donating plasma works differently from donating whole blood. During a plasma donation, blood is drawn, the plasma is separated out by a machine, and the red blood cells and platelets are returned to the donor. Because you’re keeping your cells, plasma can be donated more frequently than whole blood.
Why Plasma Color and Appearance Vary
Healthy plasma is a clear, straw-yellow color. The yellow tint comes mainly from bilirubin, a pigment produced when old red blood cells break down. Plasma can appear darker or more amber in people who are dehydrated, and it may look milky or cloudy after a high-fat meal because of elevated lipids temporarily circulating in the blood. A greenish tint can occasionally appear in people taking certain medications. These color variations are usually harmless and temporary, though unusually dark or cloudy plasma during a donation may be flagged for further evaluation.