Blood plasma is roughly 90% water, with the remaining 10% made up of proteins, electrolytes, nutrients, waste products, hormones, and dissolved gases. This straw-colored fluid accounts for about 55% of your total blood volume and serves as the body’s main transport medium, carrying everything your cells need to function and ferrying away what they discard.
Water and Proteins
Water is by far the largest component, giving plasma its liquid consistency and acting as a solvent for nearly everything else. Dissolved in that water are three major types of protein: albumin, globulins, and fibrinogen. Together, proteins make up about 8% of plasma’s total volume.
Albumin is the most abundant, circulating at concentrations of 35 to 45 grams per liter. Its main job is maintaining osmotic pressure, the force that keeps fluid inside your blood vessels instead of leaking into surrounding tissues. When albumin drops too low (below roughly 5 grams per 100 milliliters of total protein), fluid can seep out and cause swelling known as edema. Albumin also acts as a carrier, binding to hormones, fatty acids, and certain drugs to shuttle them through the bloodstream.
Globulins account for about 36% of total plasma protein. This group includes antibodies (immunoglobulins) that target infections, plus transport proteins that move iron, copper, and other substances. Fibrinogen makes up a smaller share, around 4% of plasma protein, but plays a critical role in blood clotting. When you cut yourself, fibrinogen converts into fibrin strands that form the mesh of a clot. Fibrinogen is actually what distinguishes plasma from serum: when blood is allowed to clot in a lab tube, the fibrinogen gets used up, and the leftover fluid is serum rather than plasma.
Electrolytes and Minerals
Inorganic salts make up about 0.9% of plasma. That small fraction is tightly regulated because even slight shifts can disrupt nerve signaling, muscle contraction, and heart rhythm. The major electrolytes and their normal ranges are:
- Sodium: 136 to 146 mEq/L, the primary electrolyte controlling fluid balance
- Chloride: 95 to 105 mEq/L, which partners with sodium to regulate fluid movement
- Potassium: 3.5 to 5.0 mEq/L, essential for normal heart rhythm and muscle function
- Calcium: 8.4 to 10.2 mg/dL, involved in bone health, clotting, and nerve transmission
Smaller amounts of magnesium, phosphate, and bicarbonate also circulate in plasma. Bicarbonate is particularly important as a buffer, helping keep plasma at its narrow healthy pH of 7.38 to 7.42. Even a small drift outside that range can be life-threatening, so the body uses bicarbonate along with the lungs and kidneys to constantly fine-tune acidity.
Nutrients in Transit
Plasma is the delivery route for the fuel your cells burn and the building blocks they use for repair. Glucose circulates at roughly 100 mg per 100 milliliters in a fasting state, serving as the body’s primary energy source. Total lipids (fats) are present at much higher concentrations, around 500 mg per 100 milliliters, including cholesterol in the range of 150 to 250 mg per 100 milliliters. These fats travel bundled with proteins as lipoproteins, the familiar HDL and LDL particles that show up on cholesterol panels.
Amino acids, the building blocks of protein, circulate at about 40 mg per 100 milliliters. Your cells pull these from the plasma to build enzymes, repair tissue, and produce signaling molecules. Vitamins and trace elements are present in much smaller quantities, with individual vitamins ranging from 0.0001 to 2.5 mg per 100 milliliters and trace elements like zinc and copper at 0.001 to 0.3 mg per 100 milliliters. The concentrations are tiny, but they’re enough to support the countless biochemical reactions that depend on them.
Waste Products
Just as plasma delivers nutrients, it also picks up metabolic waste and carries it to the kidneys and liver for disposal. The main waste products are nitrogenous compounds, leftovers from the breakdown of proteins and nucleic acids.
Urea is the most abundant, circulating at 20 to 45 mg per deciliter. It forms in the liver when the body breaks down amino acids and is filtered out by the kidneys. Uric acid, a byproduct of breaking down certain molecules in DNA, runs between 5.4 and 6.7 mg per deciliter. Creatinine, produced at a fairly steady rate from normal muscle metabolism, sits around 0.9 mg per deciliter. Ammonia is present only in trace amounts (0.039 to 0.09 mg per deciliter) because it is highly toxic and gets rapidly converted to urea in the liver.
When kidney function declines, these waste products accumulate in the plasma. Rising creatinine and urea levels are, in fact, two of the earliest lab markers doctors use to detect kidney problems.
Dissolved Gases
Oxygen and carbon dioxide both dissolve directly in plasma, though in relatively small amounts. Only about 1.5% of the oxygen in your blood travels this way; the vast majority rides inside red blood cells, bound to hemoglobin. Carbon dioxide is slightly more soluble, with 7 to 10% of it dissolving directly in the plasma. The rest is either carried by hemoglobin or converted to bicarbonate for transport.
Even though the dissolved fraction is small, it matters. The partial pressure of these gases in the plasma is what drives oxygen into tissues and carbon dioxide out. At the lungs, high oxygen pressure pushes oxygen into the blood. At the tissues, where cells are consuming oxygen and producing carbon dioxide, the pressure gradients reverse, and the exchange goes the other way.
Hormones and Signaling Molecules
Plasma carries hormones from the glands that produce them to the distant organs they act on. Insulin travels from the pancreas to muscle and fat cells. Thyroid hormones circulate from the thyroid gland to regulate metabolism throughout the body. Cortisol, adrenaline, estrogen, testosterone, and dozens of other chemical messengers all use plasma as their highway. Most of these are present in extremely low concentrations, measured in nanograms or even picograms per milliliter, but their effects are powerful because cells are highly sensitive to them.
Plasma also carries clotting factors beyond fibrinogen, complement proteins that assist the immune system, and cytokines that coordinate inflammation. This mix of signaling molecules makes plasma far more than passive fluid. It is an active communication network connecting every organ in the body.
How Plasma Is Separated in the Lab
If you’ve ever had blood drawn, the lab likely spun your sample in a centrifuge to isolate the plasma. The process takes about 10 to 15 minutes at high speed in a refrigerated centrifuge. Red blood cells, white blood cells, and platelets are heavier and settle to the bottom, leaving the pale yellow plasma on top. Collecting blood into a tube that contains an anticoagulant prevents clotting, which is what preserves the fibrinogen and other clotting factors in the sample. If the blood is instead allowed to clot before spinning, the result is serum, which is chemically similar to plasma but lacks fibrinogen and a few other proteins consumed during the clotting process.