The pale yellow liquid part of blood that carries cells and nutrients is plasma.
Plasma makes up about 55% of total blood volume and is mostly water—up to roughly 95% by volume. Suspended in it are blood cells, along with dissolved glucose, electrolytes, hormones, carbon dioxide, oxygen, and proteins such as albumin, globulins, and fibrinogen. This watery transport medium helps distribute materials throughout the body.
Plasma is normally light yellow or amber because of substances including bilirubin, carotenoids, hemoglobin, and transferrin. It also helps maintain osmotic balance and supports clotting and immune defenses. When blood is spun in a centrifuge, the heavier cells settle while plasma remains above them.
A common mix-up is serum. Serum is plasma after clotting has removed fibrinogen and other clotting factors. Plasma donation, or plasmapheresis, separates plasma and returns many blood cells to the donor.