The liquid portion of blood is called plasma. It makes up about 55% of your total blood volume, with the remaining 45% consisting of red blood cells, white blood cells, and platelets. Plasma itself is roughly 92% water, with the rest being a mix of dissolved proteins, sugars, hormones, electrolytes, and waste products.
What Plasma Looks Like
If you were to separate plasma from the rest of your blood, you’d see a pale yellow, slightly thick fluid. It’s a bit more viscous than water, up to about 1.6 times thicker. That yellowish tint comes from the proteins and other dissolved substances carried in it. When blood is drawn into a tube and spun in a centrifuge, the heavier red blood cells sink to the bottom while plasma rises to the top as a distinct straw-colored layer.
What Plasma Is Made Of
Beyond water, the most important components of plasma are its proteins. These make up about 7% of plasma’s volume and do most of the heavy lifting. The three major types are albumin, globulins, and fibrinogen.
Albumin is the most abundant, accounting for roughly 55% of all plasma proteins. Its main job is to keep fluid inside your blood vessels. It does this by creating osmotic pressure, essentially pulling water toward itself so it doesn’t leak out into surrounding tissues. Albumin also acts as a carrier, binding to hormones, fatty acids, and certain drugs to shuttle them through the bloodstream.
Globulins are the second largest group. The most well-known are immunoglobulins, better known as antibodies. These are the proteins your immune system produces to identify and neutralize bacteria, viruses, and other threats. During an active infection, your globulin levels typically rise as your body ramps up its immune response. Other globulins help transport metals like iron and copper.
Fibrinogen is the least abundant of the three but plays a critical role in blood clotting. When you cut yourself, fibrinogen converts into fibrin, a stringy protein that forms a mesh over the wound. This mesh traps blood cells and platelets to create a clot that stops bleeding.
Plasma also carries glucose (your cells’ primary fuel), electrolytes like sodium and potassium that keep your nerves and muscles firing properly, hormones that regulate everything from metabolism to growth, and carbon dioxide and other waste products on their way to being filtered out by your lungs and kidneys.
How Plasma Keeps Your Body in Balance
One of plasma’s less obvious but essential jobs is maintaining your body’s fluid balance. Your brain monitors the concentration of dissolved substances in your plasma with remarkable precision. A shift of just 1 to 2 units on the osmolarity scale (a measure of how concentrated a solution is) is enough to trigger a response. If your plasma gets too concentrated from dehydration, your brain signals thirst and releases a hormone that tells your kidneys to hold onto water. If you’ve had too much water, that hormone shuts off and your kidneys can flush out as much as 20 liters of dilute urine per day to bring things back to normal.
Plasma also helps maintain blood pH, keeping it in the narrow range your cells need to function. The proteins and electrolytes dissolved in plasma act as buffers, absorbing small shifts in acidity before they become a problem.
Plasma vs. Serum
You’ll sometimes see the terms “plasma” and “serum” used in medical contexts, and they’re easy to confuse. The difference comes down to one thing: fibrinogen. Plasma contains fibrinogen and all other clotting factors. Serum is what you get after blood has been allowed to clot. The clotting process uses up fibrinogen, converting it into fibrin, so serum is essentially plasma minus the clotting proteins. When your doctor orders blood work, the lab chooses plasma or serum depending on what they’re testing for.
Medical Uses of Donated Plasma
Plasma has significant value in medicine. One well-established approach is convalescent plasma therapy, which uses plasma from people who have recovered from a specific infection. Their plasma contains antibodies against that illness, which can help patients whose immune systems aren’t responding fast enough on their own. This technique has been used against pandemic influenza, Ebola, Lassa virus, and most recently COVID-19. In December 2024, the FDA approved convalescent plasma for treating COVID-19 in people with weakened immune systems.
Beyond convalescent therapy, donated plasma is used to manufacture treatments for hemophilia (which requires clotting factors extracted from plasma), immune deficiencies (which require concentrated immunoglobulins), and severe burns or trauma where patients need help maintaining blood volume and pressure. Because plasma can be frozen and stored, it’s a staple of hospital blood banks worldwide.