Blood plasma is roughly 91% to 92% water. The remaining 8% to 9% is a complex mix of proteins, electrolytes, nutrients, hormones, waste products, and dissolved gases that keep your body functioning. In an average adult with about 5 liters of blood, plasma makes up around 60% of that volume, or about 3 liters.
Water: The Base of Plasma
Water is by far the largest component, and it serves as the solvent that carries everything else. It absorbs and distributes heat throughout your body, helping regulate temperature. It also creates the fluid pressure inside blood vessels that keeps blood moving and prevents tissues from collapsing. Without that water content, none of the dissolved substances in plasma could reach the cells that need them.
Plasma Proteins
Proteins are the most abundant solutes in plasma, and they fall into three main groups: albumin, globulins, and clotting factors. A healthy plasma sample contains slightly more albumin than globulins, reflected in a normal albumin-to-globulin ratio just above 1.
Albumin is the single most plentiful protein. It acts like a molecular sponge, pulling water into blood vessels through osmotic pressure. This prevents fluid from leaking out into surrounding tissues, which is why low albumin levels often cause swelling. Albumin also serves as a carrier, binding to fatty acids, hormones, and certain drugs to shuttle them through the bloodstream.
Globulins come in several subtypes. Some are antibodies (also called immunoglobulins) that target bacteria, viruses, and other foreign invaders. Others act as transport proteins, ferrying iron, copper, and fat-soluble vitamins to where they’re needed.
Clotting factors, particularly fibrinogen, are the proteins responsible for stopping bleeding. When a blood vessel is damaged, fibrinogen converts into long, sticky strands that form a mesh over the wound. This is actually what distinguishes plasma from serum: serum is plasma with the clotting factors removed.
Electrolytes
Dissolved minerals carry electrical charges in plasma, which is why they’re called electrolytes. The major ones include sodium (normally 135 to 145 mEq/L), potassium (3.5 to 5 mEq/L), and bicarbonate (22 to 26 mEq/L). Chloride, calcium, magnesium, and phosphate round out the group.
Sodium is the dominant electrolyte in plasma and plays the biggest role in controlling how much water stays in your blood versus leaking into cells. Potassium, though present in much smaller amounts, is critical for nerve signaling and heart rhythm. Bicarbonate acts as a buffer, neutralizing excess acid to keep blood pH in the narrow range (7.35 to 7.45) your cells require to function.
Nutrients in Transit
Plasma carries the fuel your cells run on. Glucose is the primary energy source, and a normal fasting level sits at or below 99 mg/dL. After a meal, plasma glucose rises temporarily before insulin moves it into cells for use or storage.
Fats travel through plasma as well, though they can’t dissolve in water on their own. Instead, they’re packaged into protein-coated particles called lipoproteins. In a healthy person, total cholesterol averages around 192 mg/dL, with LDL (“bad”) cholesterol around 111 mg/dL, HDL (“good”) cholesterol around 62 mg/dL, and triglycerides near 92 mg/dL. These lipids serve as building blocks for cell membranes, as raw material for hormone production, and as a concentrated energy reserve.
Amino acids, the building blocks of protein, also circulate in plasma after your digestive system breaks down dietary protein. Cells pull these amino acids from the bloodstream to build enzymes, repair tissue, and produce signaling molecules.
Hormones and Signaling Molecules
Your body produces more than 50 different hormones, and plasma is the highway that carries them from the glands that make them to the distant cells that respond to them. These chemical messengers fall into a few broad categories.
Steroid hormones, including testosterone, estrogen, progesterone, and cortisol, are all built from cholesterol. Because they dissolve in fat rather than water, they typically hitch a ride on carrier proteins like albumin as they travel through plasma. Thyroid hormone and vitamin D follow a similar pattern.
Peptide hormones are the largest and most diverse group. These are small chains of amino acids, and they include insulin (which lowers blood sugar), glucagon (which raises it), growth hormone, and many others produced by the pituitary gland. Unlike steroid hormones, peptide hormones dissolve readily in plasma’s watery environment and don’t always need a carrier.
Waste Products
Plasma doesn’t just deliver useful substances. It also collects metabolic waste and carries it to the organs that eliminate it. The two most commonly measured waste products are urea and creatinine, both of which your kidneys filter out into urine.
Urea forms when your liver breaks down proteins. A normal blood urea nitrogen level falls between 6 and 24 mg/dL. Creatinine is a byproduct of normal muscle activity. Doctors often measure both together because rising levels of either one can signal that the kidneys aren’t filtering efficiently. Bilirubin, a yellowish waste product from the breakdown of old red blood cells, also circulates in plasma until the liver processes and excretes it through bile.
Dissolved Gases
While red blood cells carry the bulk of oxygen, a small amount dissolves directly in plasma. Carbon dioxide, the waste gas produced by cellular metabolism, is more soluble and travels in plasma in three forms: dissolved gas, bound to proteins, and converted to bicarbonate (which doubles as the pH buffer mentioned earlier). This dissolved gas exchange in plasma helps fine-tune how much oxygen reaches your tissues and how quickly carbon dioxide is cleared.
How Plasma Differs Between People
The exact composition of your plasma shifts throughout the day and varies based on hydration, diet, age, sex, and overall health. Women tend to have a lower total blood volume than men, which means a somewhat smaller total plasma volume. Eating a meal temporarily raises glucose, amino acids, and triglycerides in plasma. Dehydration concentrates everything, making electrolyte and protein levels appear artificially high on a blood test. Chronic conditions like kidney disease, liver disease, or diabetes alter plasma composition in characteristic ways, which is exactly why a standard blood panel, drawn from plasma, can reveal so much about what’s happening inside your body.