Blood is an organ essential for human life.
While its most critical function is the delivery of oxygen to organs and tissues, it also transports nutrients and important chemical messages and waste products from the cells. Components of Blood Whole blood comprises many cellular components suspended in the protein-rich fluid called plasma . In response to chemical messages, they proliferate and become specialized for a particular function. The red cells circulate for about 120 days, picking up oxygen as they flow through tiny blood vessels in the lung and delivering it to the tissues. Also called leukocytes, the white cells perform an assortment of janitorial and protective functions depending on their lineage.