Formed Elements — RBC, WBC and Platelets

Suspended within blood plasma are three structurally and functionally distinct kinds of formed element, and each is worth examining in turn, because in every case its structure is closely matched to the one job it performs.

Red blood cells, or erythrocytes, are by far the most numerous formed element, numbering roughly five to five-and-a-half million per cubic millimetre of blood in an adult male (somewhat fewer in an adult female). A mature mammalian red blood cell has an unusual structure among vertebrate blood cells: it is a small, biconcave disc — thinner in the centre than at the rim — and, uniquely among mammalian cells, it has actually lost its nucleus by the time it enters circulation, along with most of its other organelles. Both features serve the red blood cell's single function, the transport of respiratory gases: the biconcave shape gives the cell a large surface area relative to its volume, which speeds the diffusion of oxygen and carbon dioxide across the cell membrane, while the absence of a nucleus leaves more internal space to be packed almost entirely with haemoglobin, the iron-containing red pigment that reversibly binds oxygen in the lungs and releases it in the tissues, and that also assists in carrying a portion of the blood's carbon dioxide. Red blood cells are produced continuously in the red bone marrow by a process called erythropoiesis, live for about a hundred and twenty days in circulation, and are finally broken down chiefly in the spleen, which for this reason is sometimes called the 'graveyard' of red blood cells.

White blood cells, or leukocytes, are far less numerous — normally around six thousand to eight thousand per cubic millimetre — but unlike red blood cells they retain a nucleus and are colourless, without haemoglobin. They fall into two broad categories based on the appearance of their cytoplasm under the microscope. The granulocytes have visibly granular cytoplasm and an irregularly lobed nucleus, and include three types: neutrophils, the most abundant white blood cell of all (typically sixty to sixty-five percent of the total white cell count), which are actively phagocytic and are usually the first immune cells to arrive at a site of bacterial infection, engulfing and destroying invading microbes; eosinophils, a much smaller fraction, which are particularly important in resisting parasitic infections and in mediating allergic reactions; and basophils, the least numerous of all, which secrete histamine and heparin and are involved in inflammatory and allergic responses. The agranulocytes, by contrast, have cytoplasm that appears clear or only faintly granular, and include lymphocytes (roughly twenty to twenty-five percent of the total), which are the cornerstone of the body's specific immune response — B-lymphocytes differentiate into antibody-producing plasma cells, mediating what is called humoral immunity, while T-lymphocytes directly attack infected or abnormal cells, mediating cell-mediated immunity — and monocytes, the largest of all white blood cells, which are also phagocytic and, on migrating out of the bloodstream into the tissues, mature into macrophages that continue to engulf pathogens and debris there. …