Blood Plasma: What It Is (and Plasma vs Serum)

Blood plasma is the liquid part of your blood — the pale-yellow fluid that carries everything else along. It makes up a little more than half of your blood's volume and acts as a delivery system for cells, nutrients, hormones, waste, and dozens of important proteins.¹ Unlike red blood cells, white blood cells, and platelets, plasma is not a cell, and it is not a lab marker you read off a report — it is a component of blood. This guide explains what blood plasma is, what's in it, the often-confused difference between plasma and serum, what plasma does in the body, and how plasma donation and plasma-derived medicines work in the U.S.

Key takeaways

• Plasma is the liquid fraction of blood — roughly 55% of blood volume — and it is what remains once you remove the red cells, white cells, and platelets.²
• It is about 90–92% water, with the rest being proteins (albumin, antibodies, clotting factors like fibrinogen), plus electrolytes, nutrients, hormones, and waste products.³
• Albumin is the most abundant plasma protein: it holds water inside your vessels (oncotic pressure) and ferries hormones, calcium, and drugs — see the albumin blood test.⁴
• Plasma vs serum: serum is plasma with the clotting factors removed (including fibrinogen). Serum is what's left after blood has been allowed to clot; plasma is collected in an anticoagulant tube so it still contains those factors.⁵
• The tube your blood is drawn into (its additive and cap color) decides whether the lab gets plasma or serum — which is why the blood tube colors matter for each test.⁵
• Plasma can be donated and used as fresh frozen plasma or as raw material for plasma-derived medicines (albumin, clotting factors, immune globulins).⁶
• There is no "plasma level" on a blood test — plasma is the medium. What labs measure are its components (albumin, fibrinogen, electrolytes, glucose, and so on).

What is blood plasma?

Blood has two broad parts: the cells (red blood cells, white blood cells, and platelets) and the fluid they float in, which is the plasma. If you spin a tube of blood in a centrifuge, the heavier cells pack down at the bottom and the plasma rises to the top as a clear, straw-colored (pale yellow) liquid. That fluid accounts for about 55% of your total blood volume, while the cells make up the other ~45%.¹

Think of plasma as the body's transport medium. It circulates nutrients (glucose, amino acids, fats), hormones, dissolved gases, and metabolic waste (such as urea) between the organs that produce, use, or eliminate them, and it helps distribute body heat.³ Because plasma is mostly water, it also gives blood the volume and fluidity it needs to flow through your vessels and reach every tissue.⁷

That pale-yellow color is normal — it comes largely from small amounts of pigments such as bilirubin. The shade isn't fixed, though: after a fatty meal, plasma can look cloudy or milky (a state called lipemia), and certain conditions can tint it. Because appearance can hint at what's dissolved in the sample, labs sometimes note whether a specimen is clear, hemolyzed (reddish), icteric (deep yellow), or lipemic before they run it.¹

What's in plasma?

By weight, plasma is overwhelmingly water, but the dissolved and suspended solutes are what make it biologically interesting.

The plasma proteins are the part clinicians pay closest attention to:³

• Albumin — the most abundant plasma protein. It generates the oncotic pressure that keeps water inside your blood vessels, and it works as a near-universal carrier for hormones, calcium, bilirubin, fatty acids, and many medications. Albumin is made by the liver, so a low level can point toward liver disease, inflammation, kidney loss, or poor nutrition.⁴
• Globulins, including the antibodies (immunoglobulins) that drive much of your immune defense.
• Fibrinogen and the other clotting factors, which are essential for forming a blood clot — the same fibrinogen measured on a coagulation panel.

Dissolved alongside the proteins are electrolytes (sodium, potassium, chloride, bicarbonate, calcium), nutrients (glucose, lipids, amino acids), hormones, and waste products on their way to the kidneys or liver.²

Plasma vs serum (the key difference)

This is one of the most common points of confusion, and it matters in the lab. Both plasma and serum are the liquid part of blood with the cells removed — but they are collected differently, and one contains something the other does not:⁵

• Plasma is obtained when blood is drawn into a tube containing an anticoagulant (which prevents clotting) and then spun. Because clotting was blocked, plasma still contains fibrinogen and the other clotting factors.
• Serum is obtained by letting the blood clot first, then spinning it. The clot traps fibrinogen and the clotting factors, so the fluid left behind — the serum — is missing those proteins.

In one line: serum = plasma − clotting factors (including fibrinogen). Everything else — albumin, electrolytes, glucose, hormones, antibodies — is present in both.

Why does it matter? Because the choice of collection tube determines which one the lab gets, and different tests require one or the other. Coagulation tests (like fibrinogen) need plasma and a specific anticoagulant, while many chemistry and serology tests use serum. That is exactly what the blood tube colors signal — each cap color corresponds to a different additive. It is also why a poorly collected sample (an underfilled tube, the wrong additive, or hemolysis from a difficult draw) can skew results and sometimes force a repeat draw.⁵ If you want to picture what happens at the chair, see how a blood draw works.

What plasma does in the body

Plasma is not a passive fluid — its water and proteins carry out several jobs at once:

• Transport. It moves nutrients, hormones, dissolved gases, and waste between organs, and it distributes heat around the body.³
• Maintaining blood volume and pressure. Albumin's oncotic pressure pulls and holds water inside the vessels; when albumin drops sharply, fluid leaks into tissues and causes edema (swelling).⁴
• Clotting. Fibrinogen and the clotting factors let blood form a clot and stop bleeding. When they are massively consumed — as in disseminated intravascular coagulation — plasma fibrinogen can collapse.³
• Immune defense. Circulating antibodies (immunoglobulins) travel in plasma and help neutralize pathogens.⁷
• Balance. Plasma helps hold the body's pH and its electrolyte concentrations within a narrow, stable range.²

Because plasma carries so many things at once, it is where clinicians go looking when they want to measure almost anything in the blood — but they measure the passengers, not the fluid itself.

Plasma donation and plasma-derived medicines

Plasma has real therapeutic value. It can be donated through a process called plasmapheresis, in which a machine draws your blood, separates out the plasma, and returns your red cells and platelets to you — which is why plasma can be donated more often than whole blood.⁶ To learn about eligibility and the donation experience more broadly, see the guide on blood donation.

In the U.S., plasma is collected in two overlapping ways. Recovered plasma is separated from whole-blood donations at blood centers, while source plasma is collected directly by plasmapheresis, often at dedicated donation centers, and is destined mainly for manufacturing medicines. Because the red cells go back to the donor, source-plasma donation can be done considerably more frequently than a whole-blood donation, and centers may compensate donors for their time.⁶

Donated plasma has two main uses:

• Transfusion. Fresh frozen plasma (FFP) is transfused into patients who are missing clotting factors — for example, during major bleeding or certain liver problems. Modern guidance stresses a careful benefit–risk assessment and discourages "prophylactic" plasma transfusions that aren't clearly justified.⁸
• Plasma-derived medicines. Plasma is the raw material for purified products such as albumin, clotting factor concentrates, and immune globulins (antibodies) used to treat bleeding disorders, immune deficiencies, and other conditions.⁶

In some diseases, doctors perform therapeutic plasma exchange (TPE, or plasmapheresis) — removing a patient's plasma to strip out harmful substances (such as pathogenic antibodies) and replacing it with a substitute fluid like albumin or donor plasma. It is an invasive procedure reserved for specific, evidence-based indications (thrombotic thrombocytopenic purpura is a classic example) and is carried out by specialized teams.⁹

You may also have heard of convalescent plasma — plasma collected from people who have recovered from an infection, given in the hope that their antibodies help a current patient. It drew a lot of attention during COVID-19, but the evidence is mixed: a large living systematic review found that convalescent plasma does not reduce deaths in hospitalized patients with COVID-19 and provided little to no benefit for most, so its role is limited and situation-specific.¹⁰¹¹

Get your results interpreted by AI DiagMe

Plasma carries dozens of parameters — albumin, proteins, electrolytes, glucose, lipids — that only make sense together and in your context. It's that cross-referencing that matters, not a single value read in isolation.

👉 AI DiagMe interprets your lab results — blood, urine, or stool — in plain language, taking your whole profile into account. An informational service that does not provide a diagnosis and complements, never replaces, your physician.

Frequently asked questions

Bottom line

Blood plasma is the liquid fraction of blood — about 55% of its volume — made of roughly 90% water plus the proteins that do the heavy lifting: albumin (oncotic pressure and transport), antibodies (immunity), and fibrinogen (clotting). The distinction to remember is plasma vs serum: serum is simply plasma with the clotting factors removed, which is why the collection tube determines what the lab measures. Plasma isn't a result you read on a report — it's the medium your body uses to move nearly everything, and it has genuine therapeutic uses through donation, transfusion, and plasma exchange. To make sense of the many parameters plasma carries, it's their combination and your context that count — which is exactly what AI DiagMe helps with, alongside your physician.

Sources

Official U.S. sources and peer-reviewed publications (PubMed) used for this guide: