Plasma is the liquid portion of your blood, a pale yellow fluid that makes up about 55% of your total blood volume. The remaining 45% consists of red blood cells, white blood cells, and platelets, all of which are suspended in plasma. Despite being easy to overlook next to those more familiar blood cells, plasma performs an extraordinary range of jobs: transporting nutrients, fighting infections, regulating temperature, and keeping your blood chemistry in balance.
What Plasma Is Made Of
Plasma is roughly 92% water and 8% dissolved substances. That 8% does a lot of heavy lifting. Proteins are the largest solid component, making up about 7% of plasma’s total volume. The remaining 1% includes electrolytes like sodium and potassium, dissolved gases like oxygen and carbon dioxide, glucose, fats, hormones, and metabolic waste products such as urea and creatinine.
In physical terms, plasma is slightly thicker and stickier than water. Its pale yellow color comes from dissolved proteins and a small amount of bilirubin, a pigment produced when old red blood cells break down. When you see a tube of blood that has been allowed to settle or been spun in a centrifuge, the straw-colored layer sitting on top is plasma.
The Three Major Plasma Proteins
Three groups of proteins do most of the work inside plasma: albumin, globulins, and fibrinogen. A healthy adult carries a total protein concentration of about 6 to 8 grams per deciliter of plasma.
Albumin is the most abundant, accounting for roughly 55% of all plasma proteins. A 70-kilogram (154-pound) person has about 300 grams of it circulating at any given time, produced entirely by the liver. Albumin acts as a molecular taxi service, carrying hormones, fatty acids, calcium, bilirubin, and even medications to where they need to go. It also plays a major role in keeping fluid inside your blood vessels. Because albumin molecules are large and carry a negative charge, they create what’s called oncotic pressure, a pulling force that draws water back into capillaries. Without enough albumin, fluid leaks into surrounding tissues and causes swelling.
Globulins come in several subtypes. The most well-known are gamma globulins, which are antibodies your immune system produces to fight infections. Other globulins help transport metals like iron and copper or assist with inflammation responses.
Fibrinogen is present in smaller amounts but is essential for blood clotting. When you’re injured, fibrinogen converts into fibrin, forming a mesh-like network that traps platelets and red blood cells to seal the wound. If blood is allowed to clot in a test tube and the clot is removed, the remaining liquid (called serum) is essentially plasma minus the fibrinogen.
How Plasma Moves Things Around
Nearly every substance your body needs to distribute travels through plasma. Glucose from the food you digest enters plasma and is delivered to cells for energy. Amino acids travel the same way, reaching muscles and organs that need them for repair. Lipids, including cholesterol, are packaged into protein-coated particles and shuttled through plasma to tissues throughout the body.
Hormones rely on plasma transport as well. Thyroid hormones and steroids, for instance, often bind to albumin or specialized carrier proteins for the ride to their target organs. Even dissolved gases use plasma as a highway: while most oxygen travels inside red blood cells, a small fraction dissolves directly in plasma, and carbon dioxide (a waste product of metabolism) dissolves in plasma on its way to the lungs for removal.
Waste products follow the same route in reverse. Urea and creatinine, both byproducts of protein and muscle metabolism, dissolve in plasma and are filtered out by the kidneys.
Keeping Your Body Chemistry Stable
Your blood needs to stay within a very narrow pH range, around 7.35 to 7.45, for your cells to function properly. Plasma contains three buffer systems that prevent dangerous swings in acidity: proteins, phosphate compounds, and the bicarbonate-carbonic acid system.
Protein buffering is the most powerful of the three, accounting for about two-thirds of the blood’s total buffering capacity. Because proteins contain both positively and negatively charged regions, they can absorb excess hydrogen ions (which would make blood too acidic) or release them (if blood becomes too alkaline). Albumin, with its negative charge, is particularly effective at this.
The bicarbonate system works alongside proteins. In healthy blood, bicarbonate ions outnumber carbonic acid by a ratio of about 20 to 1, making this system especially good at neutralizing acids. Your kidneys and lungs fine-tune the balance continuously: the lungs blow off carbon dioxide (which would form carbonic acid), while the kidneys adjust how much bicarbonate they retain or excrete.
Electrolytes Dissolved in Plasma
Plasma carries a carefully regulated mix of minerals that control nerve signaling, muscle contraction, and fluid balance. Sodium is the most abundant electrolyte in plasma, normally ranging from 135 to 145 millimoles per liter. It plays the central role in determining how much water your body retains. Potassium, present at much lower levels (3.6 to 5.5 mmol/L), is critical for heart rhythm and muscle function. Calcium (8.8 to 10.7 mg/dL in adults) supports bone health, clotting, and nerve transmission, while magnesium (1.5 to 2.6 mg/dL) contributes to hundreds of enzyme reactions.
Even small deviations in these electrolyte levels can cause noticeable symptoms. Low sodium can lead to confusion and nausea, while abnormal potassium levels can trigger dangerous heart rhythms. Because plasma is constantly filtered by the kidneys, your body has a real-time feedback loop that adjusts electrolyte concentrations minute by minute.
Plasma’s Role in Temperature Control
Plasma, as the liquid base of blood, helps distribute heat generated by your muscles and organs. A control center in the brain monitors blood temperature and receives nerve signals from the skin. When you’re overheating, blood vessels near the skin surface widen, allowing more warm plasma to flow close to the surface where heat can escape. When you’re cold, those vessels narrow, keeping warm blood closer to your core organs.
This is one reason staying hydrated matters for temperature regulation. Adequate fluid intake maintains blood volume, which allows plasma to flow efficiently to the skin when cooling is needed. Dehydration reduces that capacity and makes it harder for your body to shed excess heat.
Medical Uses of Donated Plasma
Plasma donations are used both as direct transfusions and as raw material for manufactured therapies. Trauma patients and burn victims receive plasma transfusions to restore blood volume and support clotting, which helps prevent shock. Beyond emergency settings, plasma-derived products treat a wide range of chronic and genetic conditions, including immune deficiencies, hemophilia, and certain neurological diseases.
The scale of need is striking. Treating one person with primary immunodeficiency for a single year requires about 130 plasma donations. Someone with hemophilia needs approximately 1,200 donations per year. A person with alpha-1 antitrypsin deficiency, a genetic condition that damages the lungs, requires around 900 donations annually.
Plasma antibodies also have targeted uses. People immunized against tetanus or rabies carry protective antibodies in their plasma that can be extracted and given to infected patients. In pregnancy, plasma-derived therapies protect babies at risk from Rh sensitization, a condition where a mother’s immune system attacks fetal red blood cells, potentially causing brain damage or death if untreated.