The liquid portion of blood is called plasma. It makes up about 55% of your total blood volume, with the remaining 45% consisting of red blood cells, white blood cells, and platelets. Plasma itself is roughly 90 to 92% water, with proteins, salts, sugars, fats, hormones, and dissolved gases making up the rest.
What Plasma Looks and Contains
Fresh plasma is a pale, straw-colored liquid. Its yellow tint comes mainly from bilirubin, a pigment produced when old red blood cells break down. In people with conditions that raise bilirubin levels, plasma can appear bright yellow to greenish-brown.
Water is the dominant ingredient, but the 8 to 10% of plasma that isn’t water does most of the heavy lifting. That fraction includes proteins, electrolytes like sodium and potassium, dissolved nutrients like glucose and fatty acids, hormones, enzymes, and metabolic waste products like urea and carbon dioxide waiting to be filtered out by the kidneys or lungs.
The Proteins That Make Plasma Work
Proteins account for 6 to 8% of plasma by weight, and they fall into three major groups: albumin, globulins, and fibrinogen. Each group handles a different job.
Albumin is the most abundant, making up about 55% of all plasma proteins. Its primary role is maintaining osmotic pressure, the force that keeps fluid inside your blood vessels instead of leaking into surrounding tissues. When albumin levels drop significantly, fluid seeps out and causes swelling, particularly in the legs and abdomen.
Globulins are a broad family. The most important subgroup for everyday health is gamma globulins, better known as antibodies. These are the proteins your immune system produces to tag and neutralize bacteria, viruses, and other invaders. Other globulins help transport metals like iron and copper through the bloodstream.
Fibrinogen is the key clotting protein. When you cut yourself, a chain reaction converts fibrinogen into long, sticky strands called fibrin. Those strands weave together into a mesh that traps platelets and forms a solid clot, stopping the bleeding. Plasma also carries several other clotting factors that work alongside fibrinogen in this process.
What Plasma Does in the Body
Plasma is essentially the body’s delivery and pickup service. It carries nutrients from digested food, hormones from glands, and oxygen-support proteins to every cell that needs them. At the same time, cells dump their metabolic waste into plasma, which routes it to the kidneys, liver, or lungs for disposal.
Beyond transport, plasma helps regulate body temperature. Because it’s mostly water, it absorbs and distributes heat efficiently, moving warmth from active muscles toward the skin where it can dissipate.
Plasma also acts as a chemical buffer that keeps your blood’s pH in a very narrow range, roughly 7.35 to 7.45. The bicarbonate dissolved in plasma is the primary buffering system: when blood becomes too acidic, bicarbonate neutralizes the excess acid, and the lungs and kidneys adjust how much bicarbonate and carbon dioxide stay in circulation. Plasma proteins themselves contribute about 20% of total buffering capacity, providing a secondary safety net.
Plasma vs. Serum
You’ll sometimes see the words “plasma” and “serum” used as though they’re interchangeable, but they aren’t. The difference comes down to clotting factors. Plasma is collected by adding an anticoagulant to a blood sample so that clotting never happens, which means fibrinogen and all other clotting proteins remain in the liquid. Serum is what’s left after blood is allowed to clot naturally. The clot traps fibrinogen and red blood cells into a solid mass, leaving behind a clear liquid that contains everything plasma has except those clotting factors.
In medical labs, the choice between plasma and serum depends on which test is being run. Some tests need clotting factors present, others need them removed.
Medical Uses of Plasma
Donated plasma is one of the most versatile blood products in medicine. It can be transfused directly into patients who have lost large amounts of blood or whose livers aren’t producing enough clotting factors on their own. People undergoing heart surgery or liver transplants often receive plasma transfusions to prevent dangerous bleeding.
Plasma can also be separated into its individual components for targeted therapies. Concentrated antibodies extracted from donated plasma are used to treat immune deficiencies. Clotting factor concentrates go to patients with hemophilia or other bleeding disorders. In a procedure called plasma exchange, a patient’s own plasma is removed and replaced with donor plasma to treat conditions where harmful antibodies are attacking the body’s own tissues.
Because plasma can be frozen and stored for extended periods, unlike whole blood or red blood cells which have shorter shelf lives, plasma donation programs play a critical role in maintaining hospital supplies. Donors can give plasma more frequently than whole blood, since the body replenishes its liquid and protein components faster than it replaces red blood cells.