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2 Formed elements The cells of the blood include the red blood cells (eryth- rocytes) and the white blood cells (leukocytes) [Fig. 2]. In all vertebrates, except nearly all mammals, the red blood cells or corpuscles contain a nucleus and cytoplasm rich in hemoglo- bin. In contrast, in nearly all mammals, the nucleus has been extruded during the developmental stages. Importantly, there are genetically determined markers on the surface of the cellu- lar blood elements; these markers determine the various blood types or groups. Hematopoiesis is the process by which the cellular elements of the blood are formed. Some of the blood cells are terminal cells, in the sense that they are fully differentiated and have little or no capacity for division. These include the erythrocytes and the granulocytes. The agranular leukocytes may develop extravascularly into other cell types (macrophages, fibroblasts, or plasma cells). All blood cells are formed in the hematopoi- etic organs (chiefly lymph nodes and spleen for agranular leukocytes, and bone marrow for erythrocytes, platelets, and granular leukocytes). Although blood cells occur in all vertebrates, they differ in density, relative proportions, morphology (to some extent), and their sites of origin. For example, the red blood cells of fish are nucleated, are ovoid or spindle-shaped, and may be larger than in any other vertebrate. They are also large in frogs, where they are somewhat less oval than in birds. There also are notable differences in the internal structure of the leukocytes, especially with regard to the size, shape, and staining proper- ties of the granular leukocytes. Erythrocytes In normal adult men and women, the blood contains ap- proximately 5,000,000 and 4,500,000 erythrocytes per cubic mm, respectively. Human erythrocytes measure about 8 μm in diameter and about 2 μm at their thickest, and they have a biconcave shape. They contain hemoglobin, which imparts to them their color, and possess an envelope, which appears highly structured when viewed with an electron microscope. The hemoglobin is involved in the transport of oxygen and carbon dioxide, and it plays a role in maintaining a constant pH in the blood. When circulating in the blood vessels, the red blood cells are not evenly dispersed. In the narrowest vessels (the capillaries), the erythrocytes are often distorted. In certain conditions, they may be densely aggregated; this is known as a sludge. The erythrocytes respond to changes in osmotic pres- sure of the surrounding fluid by swelling in hypotonic fluids and by shrinking irregularly in hypertonic fluids. Shrunken red blood cells are referred to as crenated cells. The average life of a mature red blood cell is surprisingly long, spanning about 120 days. Sickle cell disease is an inherited disorder that causes erythrocytes to assume a sickle shape, impairing their function (Fig. 3). Leukocytes In humans, the white blood cells in the blood are fewer in number, amounting to about 5000 – 9000 per cubic mm. In general, there are two varieties: agranular and granular leuko- cytes. The agranular cells include the small, medium, and large lymphocytes and the monocytes. The small lymphocytes are spherical, measuring about the diameter of erythrocytes or a Blood (continued) + ward ' s science Fig. 2: Colorized scanning electron micrograph of (left) a human red blood cell (erythrocyte; colorized red) and (right) a human white blood cell (leukocyte; colorized blue). (Credit: Science Photo Library/Alamy Stock Photo) Fig. 1: Formed elements of blood. (Copyright © McGraw-Hill Education)