Erythrocytes Are flattened, thin bi-concave discs of an average diameter of 7.2 to 7.8, and are non-nucleated in their normal adult condition. They are very elastic so that they can pass readily through the small capillaries. Their active life is supposed to be quite short, their function is the carrying of oxygen from the lungs to the tissues throughout the body. In abnormal conditions these cells may vary greatly in size, shape and hemoglobin content. Microcytes are those averaging 3.5 in diameter. These are observed in all severe anemias and as a rule stain deeply and uniformly. Macrocytes are those varying from 10 to 20 . They indicate a severe and chronic anemia, such as pernicious anemia. Poikilocytes are those which show marked alteration in shape, usually pear-formed. They may be large or small and when occurring in large numbers, the condition is known as poikilo-cytosis. It is indicative of severe anemia with degenerative changes in the red cells. At times such cells may be nucleated poikiloblasts.

Nucleated red cells are always pathological when found in the circulation with the exception of the first few days following birth. Normoblasts are nucleated red cells similar in size, shape and color to the normal cell. The nucleus is about one-third the diameter of the cell, stains densely, is homogeneous, sharply defined and spheroidal in shape, without any definite chromatin network. It is usually situated near the periphery of the cell. At times the nucleus may be protruded partially or even escape entirely from the erythrocyte. Occasionally the nucleus may show indications of multiplication. The presence of these cells in the blood indicates an increased activity of the blood-making organs, especially of the bone-marrow, an attempt at rapid regeneration. They are commonly found in the milder forms of anemia, chlorosis and acute anemia from hemorrhage. In severe types of anemia they are associated with the megaloblasts.