Blood plasma is roughly 91% to 92% water. The remaining 8% to 9% is a complex mix of proteins, nutrients, waste products, dissolved gases, salts, and hormones that keep your body functioning. Despite being mostly water, that small dissolved fraction does enormous work: transporting nutrients, fighting infections, clotting wounds, and maintaining the chemical balance your cells depend on.
Water and Dissolved Solids
Plasma is the liquid portion of blood, the pale yellow fluid that remains after you remove red blood cells, white blood cells, and platelets. It makes up about 55% of your total blood volume. Its high water content serves as the solvent that carries everything else through your circulatory system, from the sugar your muscles burn to the antibodies that neutralize viruses.
The 8% to 9% of dissolved solids includes proteins (the largest share by weight), electrolytes like sodium and potassium, nutrients like glucose and fats, waste products headed for your kidneys, and signaling molecules like hormones. Each of these plays a distinct role, and their concentrations stay remarkably stable in a healthy person.
Proteins: The Heaviest Component
Proteins account for the bulk of plasma’s dissolved material, totaling about 7.8 grams per deciliter. They fall into three main categories: albumin, globulins, and fibrinogen.
Albumin makes up roughly 55% of all plasma protein. A normal blood level ranges from 3.5 to 5.5 grams per deciliter. Albumin’s main jobs are maintaining osmotic pressure (which keeps fluid from leaking out of your blood vessels into surrounding tissue) and ferrying substances that can’t dissolve in water on their own, including certain hormones, fatty acids, and medications.
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 threats. Other globulins help transport iron, copper, and fats through the bloodstream.
Fibrinogen is the protein responsible for blood clotting. When you cut yourself, fibrinogen converts into long, sticky strands called fibrin that form a mesh over the wound. This is actually the key difference between plasma and serum: serum is what you get after plasma has clotted and the fibrinogen has been removed. If a lab draws your blood into a tube with an anticoagulant, they get plasma. If they let it clot first, they get serum.
Nutrients in Transit
Plasma acts as a delivery system for the fuel and building materials your cells need. Glucose, your body’s primary energy source, circulates in plasma at a normal fasting level of about 4.1 to 6.4 mmol/L (roughly 74 to 115 mg/dL). After a meal, those numbers rise temporarily before insulin helps shuttle glucose into cells.
Fats travel through plasma packaged inside protein shells called lipoproteins, because fat and water don’t mix. A healthy fasting total cholesterol level is under 200 mg/dL, while normal triglycerides fall below 150 mg/dL. LDL cholesterol (the type linked to artery disease) should ideally stay below 100 mg/dL, and HDL cholesterol (the protective type) should be above 40 mg/dL, with 60 mg/dL or higher offering meaningful protection against heart disease. Amino acids, vitamins, and minerals also dissolve in plasma on their way to tissues that need them.
Electrolytes and pH Balance
Dissolved salts, primarily sodium, potassium, calcium, chloride, and bicarbonate, give plasma its electrical properties and regulate how much water moves in and out of cells. These electrolytes also help maintain plasma’s pH, which stays in a very tight range of 7.35 to 7.45. Even a small shift outside this range can disrupt the chemical reactions your cells rely on.
The body’s most important pH buffering system is the bicarbonate system, where bicarbonate ions neutralize excess acid in the blood. This system is both the most abundant buffer in the body and an “open” system, meaning the lungs can adjust it in real time by breathing out more or less carbon dioxide. Plasma proteins, especially albumin, also contribute to buffering.
Dissolved Gases
Oxygen and carbon dioxide both dissolve in plasma, though plasma is not the primary carrier of either. Only about 2% of the oxygen in your bloodstream is dissolved directly in plasma; the other 98% rides inside red blood cells, bound to hemoglobin. Plasma’s small dissolved oxygen fraction still matters, because it’s the form that cells can actually pull from nearby capillaries.
Carbon dioxide, the main waste product of metabolism, travels back to the lungs through three routes. Most of it gets converted into bicarbonate inside red blood cells and then released into the plasma. A smaller portion binds directly to hemoglobin, and a small amount dissolves in plasma as gas. At the lungs, these reactions reverse and carbon dioxide is exhaled.
Waste Products
Plasma carries metabolic waste to the kidneys and liver for disposal. The main nitrogen-containing waste products are urea (produced when the liver breaks down protein), creatinine (a byproduct of normal muscle activity), and uric acid (generated when your body recycles old cells). Normal plasma urea ranges from about 2.1 to 7.1 mmol/L, while creatinine typically runs between 44 and 97 µmol/L depending on sex. Doctors measure these levels to assess kidney function, since rising concentrations suggest the kidneys aren’t filtering effectively.
Hormones and Signaling Molecules
Plasma carries hormones from the glands that produce them to the distant tissues where they act. Insulin travels from the pancreas to muscle and fat cells. Thyroid hormones circulate bound to carrier proteins until they reach cells that need them. Cortisol, growth hormone, and reproductive hormones all use plasma as their highway. The concentrations are tiny compared to proteins or glucose, but their effects are enormous, regulating everything from metabolism to mood.
Why Plasma Matters in Medicine
Because plasma contains clotting factors, antibodies, and other proteins that are difficult to manufacture synthetically, donated plasma is a critical medical resource. Frozen plasma transfusions are used for patients with clotting disorders, active bleeding, or conditions where multiple clotting factors are depleted at once, such as severe liver disease or a complication called disseminated intravascular coagulation, where the body’s clotting system goes into overdrive and exhausts its supply of clotting proteins.
Plasma transfusions also play a role in reversing the effects of blood-thinning medications during emergencies, managing massive blood loss from trauma or surgery, and treating a rare blood disorder called thrombotic thrombocytopenic purpura, where a missing enzyme causes dangerous clots in small blood vessels. Beyond whole plasma, pharmaceutical companies fractionate donated plasma to extract specific products like concentrated antibodies for people with immune deficiencies, and clotting factor concentrates for hemophilia.
The composition of your plasma also gives doctors a detailed snapshot of your health. Nearly every routine blood test, from cholesterol panels to kidney function markers to hormone levels, is ultimately measuring something dissolved in plasma.