FORMED ELEMENTS

A. Erythrocytes: Erythrocytes are also called red blood cells, or RBCs. They are the most abundant formed elements in the blood (4-6 x millions/uL). Because of their presence in most tissues and organs, erythrocytes are useful to histologists and pathologists in estimating the size of other tissue and organ components (through estimates of multiples or fractions of RBC diameter).
1. Normal structure and function. RBCs are structurally and functionally specialized to transport oxygen from the lungs to other tissues. Their cytoplasm contains the oxygen binding protein hemoglobin. Their small diameter (7-8 um) and biconcave shape (in humans) help to maximize their surface-to-volume ratio, facilitating oxygen exchange. Mature RBCs lack nuclei and cytoplasmic organelles, which they lose during differentiation. Because they lack mitochondria, the energy needed to maintain the hemoglobin in a functional state must be derived from anaerobic glycolysis. Because they lack ribosomes, the glycolytic enzymes and other important proteins cannot be renewed. Mature erythrocytes therefore have a limited lifespan (120 days) in the circulation before they are removed by macrophages in the spleen and bone marrow.2. Abnormalities a. Anisocytosis refers to the presence of a high percentage of RBCs with unusually great variations in size. Those larger than 9 µm in diameter are termed macrocytes, and those smaller than 6 um are termed microcytes. b. Nuclear fragments. In some disease states, nuclear fragments, or Howell-Joliy bodies, remain in otherwise mature RBCs. When these form circular filaments they are termed Cabot rings. c. Reticulocytes. Some RBCs recently released from the bone marrow contain a small amount of residual RER and ribosomes that can be precipitated into blue, netlike structures with the vital dye brilliant cresyl blue. When these reticulocytes constitute more than about 1% of the circulating RBCs, they indicate an increased demand for oxygen carrying capacity leg, from loss of RBCs due to hemorrhage or anemia, or to recent ascent to a higher altitude).