Blood is a vital fluid in the human body that is composed of several components, including PLASMA, SERUM, and sera. PLASMA is the liquid part of blood that contains proteins, salts, and other substances. SERUM is the clear, yellowish liquid that is left after the blood has been allowed to clot. Sera is a type of SERUM that is used in medical research and diagnosis. All three components are essential for the proper functioning of the human body.