plasma

Plasma is the liquid part of the blood, and it is the larger part by volume. It is mostly water, and what makes it more than water is what it carries dissolved: nutrients from the digestive tract, hormones from the glands, wastes on their way to the kidneys, the proteins, including the clotting proteins that finish the plug the platelets start, and carbon dioxide, most of which travels as bicarbonate in the plasma rather than on the red cells. The blood's cells — red blood cells, white blood cells and platelets — ride in the plasma rather than making it up. That division of labor is what separates the four components of blood on the exam: plasma carries cells, dissolved substances and the clotting proteins; red cells carry oxygen bound to hemoglobin; white cells defend; platelets clot. Oxygen is the one cargo plasma does not carry in a useful quantity; hemoglobin carries it.