Formed Elements of the Blood

the solid fraction of the blood, consisting of erythrocytes, leukocytes, and thrombocytes. The formed elements constitute up to 40 percent of the volume of the blood in lower vertebrates (fishes, amphibians, and reptiles) and up to 54 percent in higher vertebrates (birds and mammals, including man).

The most numerous among the formed elements of the blood are the erythrocytes, which contain hemoglobin. They transfer oxygen from the respiratory organs to the tissues and help transfer carbon dioxide in the opposite direction. They are responsible for the blood’s red color. The colorless leukocytes are highly diverse in size and function; they aid in the blood’s defensive functions. The thrombocytes, and the blood platelets that correspond to them in mammals and man, are responsible for the coagulation of blood.