Plasma is the pale yellow liquid that makes up about 60% of your total blood volume. It’s the fluid portion of blood, the part that remains when you remove all the red blood cells, white blood cells, and platelets. Though it might seem like just background liquid, plasma performs critical jobs: transporting nutrients and hormones, helping your blood clot, fighting infections, and keeping your body’s chemistry in balance.
What Plasma Is Made Of
Plasma is roughly 90% water. The remaining 10% is a concentrated mix of proteins, salts, sugars, fats, hormones, vitamins, and dissolved gases. That small fraction does an enormous amount of work. The proteins alone account for a wide range of essential functions, from immune defense to wound healing.
Three groups of proteins dominate the mix. Albumin makes up about 60% of all plasma protein. Globulins, which include the antibodies your immune system produces, account for around 36%. Fibrinogen, the protein responsible for forming blood clots, makes up the remaining 4% or so. Beyond proteins, plasma carries dissolved electrolytes like sodium, chloride, potassium, and calcium, along with glucose, waste products like urea, and hormones traveling from glands to their target organs.
How Plasma Moves Things Around Your Body
Plasma is your body’s primary delivery system. After you eat, digested nutrients (sugars, fats, amino acids, vitamins, and minerals) are absorbed into the blood through tiny vessels in the small intestine. Plasma carries those nutrients to cells throughout the body where they’re needed for energy, growth, and repair.
Hormones follow the same route. Glands secrete hormones directly into the bloodstream, and plasma ferries them to target organs, where they bind to receptors and trigger specific responses. Insulin traveling from the pancreas to muscle cells, thyroid hormones reaching tissues throughout the body: all of this happens through plasma.
Plasma also handles waste removal. Cells produce carbon dioxide and other metabolic byproducts as they work. These waste products enter the bloodstream and travel through plasma to the lungs (where carbon dioxide is exhaled) or to the kidneys and liver (where other waste is filtered out and excreted).
Keeping Your Blood at the Right pH
Your blood needs to stay within a very narrow pH range to function properly, and plasma plays a central role in maintaining that balance. The main mechanism is a chemical buffering system that uses two substances naturally present in plasma: bicarbonate (a weak base) and carbonic acid (a weak acid).
When your body produces too much acid, perhaps during intense exercise, the extra hydrogen ions combine with bicarbonate to form carbonic acid, which quickly converts to carbon dioxide and water. You then simply breathe out the carbon dioxide. When acid levels drop too low, the system works in reverse: carbonic acid breaks apart to release hydrogen ions and restore the balance. It’s an elegant, self-correcting loop. Plasma proteins also contribute to pH regulation, accounting for about 7% of blood’s total buffering capacity.
Plasma’s Role in Blood Clotting
When you cut yourself, a carefully choreographed chain reaction begins in your blood. Platelets arrive at the wound site first, becoming sticky and clumping together to form a temporary plug. But that plug alone isn’t stable enough. Platelets release chemical signals that activate clotting factors, a group of proteins dissolved in your plasma.
These clotting factors work in sequence, each one activating the next in a cascade. The final step converts fibrinogen (that 4% of plasma protein) into long, sticky strands of fibrin. Those strands weave through the platelet plug like reinforcing threads, forming a firm, durable clot that seals the wound while healing takes place underneath. Without adequate clotting factors in plasma, even minor injuries can cause prolonged or dangerous bleeding.
Plasma Keeps Fluid Where It Belongs
One of albumin’s most important jobs is keeping water inside your blood vessels. Plasma proteins, particularly albumin, are too large to pass through capillary walls. This creates a pulling force (called oncotic pressure, about 28 mmHg) that draws water back into blood vessels from surrounding tissues. Without enough albumin, fluid leaks out of the bloodstream and accumulates in tissues, causing the visible swelling known as edema. This is why people with severe liver disease or malnutrition, conditions that reduce albumin levels, often develop swollen legs or a distended abdomen.
Plasma vs. Serum
You’ll sometimes see “plasma” and “serum” used as if they’re interchangeable, but they’re not. Both are the liquid portion of blood without cells, but the key difference is clotting factors. Plasma is collected by adding a substance that prevents clotting, so it retains all its proteins, including fibrinogen. Serum is what’s left after blood is allowed to clot naturally and the clot is removed, meaning it contains no fibrinogen or other clotting factors.
This distinction matters in medical testing. The clotting process itself changes the sample’s chemistry. Serum tends to have higher concentrations of certain amino acids like alanine and glutamine, likely because platelets release these molecules during clotting. Even the types of tiny signaling molecules (microRNAs) differ: one comparison found 329 types in serum versus 193 in plasma. Depending on what a lab is measuring, one sample type may give more accurate results than the other.
Medical Uses of Plasma
Plasma is one of the most versatile blood products in medicine. Trauma patients and burn victims receive plasma transfusions to restore blood volume, prevent shock, and support clotting. But plasma’s value extends well beyond emergency rooms.
Proteins extracted from donated plasma are manufactured into therapies for a wide range of conditions. People with immune deficiencies receive concentrated antibodies (derived from the globulin fraction) to help their bodies fight infections. Those with bleeding disorders like von Willebrand disease get replacement clotting factors. Plasma-derived treatments also help people with hereditary conditions affecting the lungs, such as alpha-1 antitrypsin deficiency, as well as neurological disorders like certain polyneuropathies and myasthenia gravis.
Some uses are more targeted. Plasma-based therapies treat tetanus and rabies exposure. Pregnant people with Rh sensitization, a condition where the mother’s immune system could attack the baby’s blood cells, receive plasma protein therapies to protect the fetus. According to the U.S. Department of Health and Human Services, plasma donation is considered critical precisely because so many of these therapies have no synthetic alternative. The proteins must come from human donors.