Blood plasma is roughly 92% water, 7% proteins, and 1% everything else: electrolytes, hormones, vitamins, dissolved gases, nutrients, and waste products. That simple breakdown, though, hides a remarkably complex fluid. Plasma makes up about 55% of your total blood volume, and every component in it serves a specific purpose, from fighting infections to keeping your blood pressure stable.
Water: The Base of Plasma
Water makes up the vast majority of plasma and acts as the solvent that keeps everything else dissolved and flowing. This high water content is what allows plasma to carry substances throughout your body efficiently. It also plays a direct role in regulating body temperature and maintaining blood volume. When you’re dehydrated, plasma volume drops, which is one reason your blood pressure can fall and your heart rate rises.
The Three Major Plasma Proteins
Proteins account for about 7% of plasma by weight, but they do most of the heavy lifting. There are three main types, each with a distinct job.
Albumin is the most abundant, making up roughly 55% of all plasma proteins (about 4.5 grams per deciliter of blood). Its primary role is maintaining what’s called oncotic pressure: it keeps fluid inside your blood vessels instead of letting it leak into surrounding tissues. Without enough albumin, fluid seeps out and causes swelling. Albumin also acts as a transport shuttle, binding to fatty acids, hormones, and certain drugs to carry them through the bloodstream.
Globulins are the second largest group and come in several subtypes. The most familiar are gamma globulins, which are your antibodies. These are the immune system’s frontline defenders, latching onto bacteria, viruses, and other foreign invaders to neutralize them. Other globulins handle fat transport. High-density lipoproteins (HDL) and low-density lipoproteins (LDL) are both globulin subtypes that carry cholesterol and other fats to and from your cells.
Fibrinogen is the least abundant of the three but essential for survival. When you get a cut, fibrinogen converts into fibrin, which forms the mesh-like structure of a blood clot. This is also what separates plasma from serum: serum is the fluid left over after blood has clotted and the fibrinogen has been used up. Plasma contains fibrinogen and other clotting factors; serum does not.
Electrolytes That Keep Cells Working
Dissolved minerals in plasma carry electrical charges that regulate nerve signals, muscle contractions, and the balance of fluids inside and outside your cells. The major electrolytes in plasma include:
- Sodium (135 to 145 mmol/L): the main electrolyte controlling fluid balance and blood pressure
- Potassium (3.6 to 5.5 mmol/L): critical for heart rhythm and muscle function
- Calcium (8.8 to 10.7 mg/dL): involved in bone health, blood clotting, and nerve signaling
- Bicarbonate (23 to 30 mmol/L): acts as a buffer to keep blood pH in a narrow, safe range
Even small shifts outside these ranges can cause serious problems. Too much potassium, for instance, can trigger dangerous heart rhythms. Too little sodium can cause confusion and seizures. Your kidneys constantly adjust these levels, and plasma is the medium they work in.
Nutrients and Dissolved Gases
Plasma is the body’s delivery system for fuel. After you eat, nutrients absorbed through your intestines enter the bloodstream and travel through plasma to reach cells everywhere. This includes glucose (your cells’ primary energy source), amino acids (the building blocks of protein), fatty acids, and vitamins and minerals.
Plasma also carries dissolved gases, though in relatively small amounts. Most oxygen travels attached to hemoglobin inside red blood cells, but a small fraction dissolves directly in plasma. Carbon dioxide, the waste product of cellular metabolism, dissolves more readily in plasma and is carried to the lungs for removal. A small amount of nitrogen gas is dissolved in plasma as well, which normally has no effect but becomes relevant in situations like deep-sea diving, where pressure changes can cause nitrogen to form bubbles.
Hormones and Signaling Molecules
Your endocrine glands release hormones directly into the bloodstream, and plasma is what carries them to their target tissues. Insulin travels through plasma from the pancreas to cells throughout the body. Thyroid hormones, cortisol, estrogen, testosterone, and dozens of other chemical messengers all depend on plasma as their transport medium. Many of these hormones hitch a ride on albumin or specific binding proteins, which protects them from being broken down too quickly and helps regulate how much of the hormone is active at any given time.
Metabolic Waste Products
Plasma doesn’t just deliver useful substances. It also picks up the garbage. Cells produce waste as they break down proteins and other molecules, and plasma carries these byproducts to the organs that dispose of them. Urea and creatinine, both generated from protein metabolism, travel through plasma to the kidneys for filtration. Bilirubin, a yellowish compound produced when old red blood cells are broken down, is carried by albumin in plasma to the liver, where it’s processed and eventually excreted in bile. When the liver can’t keep up with bilirubin clearance, it accumulates and causes the yellowing of skin and eyes known as jaundice.
How Much Protein Is Normal
If you’ve had blood work done, you may have seen a “total protein” result. Current reference ranges put normal total plasma protein between 5.9 and 8.3 g/dL, with albumin specifically falling between 3.6 and 5.1 g/dL. Levels outside these ranges can signal a variety of conditions. Low albumin often points to liver disease, kidney problems, or malnutrition. High globulin levels can indicate chronic inflammation or certain immune disorders. These numbers give your doctor a snapshot of how well your liver is producing proteins and how your immune system is functioning.
Plasma is, in essence, a water-based solution packed with proteins, salts, nutrients, hormones, gases, and waste products, all held in a precise balance that your body constantly monitors and adjusts. Every component has a role, and the composition shifts in response to what you eat, how hydrated you are, whether you’re fighting an infection, and how well your organs are functioning.