What is the liquid part of blood called?

• A. Serum
• B. Red blood cells
• C. White blood cells
• D. Plasma

Answer: (D)

Plasma is the liquid part of blood. It is a pale yellow fluid that makes up about 55% of blood and is mostly water, with dissolved proteins, electrolytes, nutrients, and wastes. It carries cells and other substances through the circulatory system and helps maintain blood volume and pressure. The proteins in plasma include albumin, which helps keep fluid in the vessels; fibrinogen, which is involved in clotting; and various antibodies. If blood is allowed to clot, the liquid that remains is serum, which lacks fibrinogen. Red blood cells and white blood cells are the solid components floating in plasma, not the liquid portion.