Blood plasma is mostly water, about 92% by weight. The remaining 8% is a mix of proteins (roughly 7%) and a final 1% made up of salts, hormones, vitamins, enzymes, dissolved gases, and waste products. That simple breakdown doesn’t capture how much work each component does, though. Plasma makes up more than half your total blood volume, and every substance dissolved in it plays a specific role in keeping you alive.
Water: The 92% That Makes Everything Else Work
Water is the solvent that allows plasma to function as a transport system. It dissolves nutrients from digested food, carries hormones from glands to distant organs, and moves waste products toward the kidneys and liver for disposal. Water also gives blood its fluid volume, which is what keeps blood pressure stable as your heart pumps. Lose too much plasma volume, whether from severe bleeding, burns, or dehydration, and blood pressure drops dangerously.
Proteins: The Functional Core
Proteins account for about 7% of plasma, but they do the heaviest lifting. Three major types dominate.
Albumin is the most abundant plasma protein. It acts like a sponge that holds water inside your blood vessels through osmotic pressure. Without enough albumin, fluid leaks out of blood vessels and into surrounding tissues, causing swelling. Albumin also shuttles fatty acids, hormones, and certain drugs through the bloodstream.
Globulins include antibodies (also called immunoglobulins), the proteins your immune system produces to neutralize bacteria, viruses, and other invaders. Other globulins help transport metals like iron and copper.
Fibrinogen and other clotting factors are what allow blood to clot when you’re injured. When a blood vessel is damaged, a chain reaction converts fibrinogen into fibrin, a mesh-like protein that forms the structural scaffold of a blood clot. This is actually the key difference between plasma and serum: serum is what’s left after plasma has clotted, so it contains the same components minus fibrinogen and the other clotting proteins.
Electrolytes and Mineral Salts
Dissolved salts, or electrolytes, make up a small fraction of plasma by weight but are critical for nerve signaling, muscle contraction, and fluid balance. Sodium is the dominant electrolyte in plasma, typically present at concentrations of 136 to 146 milliequivalents per liter. Chloride runs close behind at 95 to 105. Potassium levels are much lower, between 3.5 and 5.0, but even small shifts in potassium can disrupt heart rhythm. Bicarbonate, at 22 to 28, plays a central role in keeping your blood at the right pH.
How Plasma Controls Blood pH
Your blood needs to stay in a narrow pH range (around 7.35 to 7.45) for enzymes and cells to function properly. Plasma manages this primarily through the bicarbonate buffer system. When your blood starts to become too acidic, bicarbonate ions neutralize the excess acid, converting it into carbonic acid, a weak acid that quickly breaks down into carbon dioxide and water. You then exhale the carbon dioxide through your lungs. When blood trends too alkaline, carbonic acid donates hydrogen ions to bring the pH back down.
Under normal conditions, plasma maintains a ratio of about 20 parts bicarbonate to 1 part carbonic acid. That heavy tilt toward bicarbonate means the system is especially good at absorbing acid, which is useful because normal metabolism constantly produces acidic byproducts. Your kidneys fine-tune the system on the other end by controlling how much bicarbonate gets reabsorbed back into the blood versus excreted in urine.
Dissolved Gases
While red blood cells carry the vast majority of oxygen, a small amount dissolves directly in plasma. This dissolved fraction is tiny compared to the oxygen bound to hemoglobin, but it matters because dissolved oxygen is what’s immediately available to cells. Carbon dioxide also dissolves in plasma, at roughly 20 times the rate of oxygen. Most CO₂ is eventually converted to bicarbonate for transport back to the lungs, connecting gas exchange directly to the pH buffering system described above.
Nutrients, Hormones, and Waste
Plasma is the delivery route for glucose, amino acids, fatty acids, and vitamins absorbed from your gut. It also carries hormones like insulin, thyroid hormones, and cortisol from the glands that produce them to the tissues that need them.
At the same time, plasma hauls metabolic waste in the opposite direction. Urea, a byproduct of protein breakdown, and creatinine, a byproduct of muscle metabolism, both travel through plasma to the kidneys for filtration. Doctors use plasma levels of urea and creatinine as a window into kidney function: when the kidneys start to fail, they can no longer filter efficiently, and concentrations of both waste products rise in the blood. Bilirubin, a yellow compound produced when old red blood cells are broken down, also circulates in plasma on its way to the liver for processing. Elevated bilirubin is what causes the yellowing of skin and eyes known as jaundice.
Why Plasma Is Used in Medicine
Plasma can be separated from whole blood, frozen, and later transfused into patients. The most common reason for a plasma transfusion is a deficiency in clotting factors that’s causing, or threatening to cause, dangerous bleeding. People on blood-thinning medications who need emergency surgery sometimes receive frozen plasma to restore clotting ability quickly. Patients with massive blood loss from trauma get plasma alongside red blood cells to replace both volume and clotting proteins simultaneously.
Plasma transfusions are also used to treat certain rare blood disorders. In one condition called thrombotic thrombocytopenic purpura, the body lacks an enzyme needed to prevent abnormal clotting. Donor plasma supplies the missing enzyme directly. Beyond transfusion, individual plasma proteins can be isolated and concentrated into specific therapies: immunoglobulin preparations from pooled donor plasma, for example, are used to treat immune deficiencies and autoimmune diseases.