Blood cells

There are four major types of blood cells: red blood cells, platelets, lymphocytes, and phagocytic cells. Collectively, the lymphocytes and phagocytic cells constitute the white blood cells. Each type of blood cell has a specialized function: red cells take up oxygen from the lungs and deliver it to the tissues; platelets participate in forming blood clots; lymphocytes are involved with immunity; and phagocytic cells occur in two varieties—granulocytes and monocytes—and ingest and break down microorganisms and foreign particles (see video of macrophage consuming bacteria). The circulating blood functions as a conduit, bringing the various kinds of cells to the regions of the body in which they are needed: red cells to tissues requiring oxygen, platelets to seal over points of injury, lymphocytes to areas of infection, and phagocytic cells to sites of microbial invasion and inflammation. Each type of blood cell is described in detail below.

The process of blood cell formation (hematopoiesis) takes place in hematopoietic tissue. In the developing embryo, blood cell formation occurs in the liver, but, as the fetus develops, hematopoiesis shifts to the bone marrow, a dark red, gelatinous tissue in the central cavities of the bones. In young children, hematopoietic bone marrow fills most of the skeleton, whereas in adults the marrow is located mainly in the central bones (ribs, sternum, vertebrae, and pelvic bones). Bone marrow is a rich mixture of developing and mature blood cells, as well as fat cells and other cells that provide nutrition and an architectural framework upon which the blood-forming elements arrange themselves. The weight of the marrow of a normal adult is 1,600 to 3,700 grams and contains over 1,000,000,000,000 hematopoietic cells (18 ? 109 cells per kilogram).Nourishment of this large mass of cells comes from the blood itself. Arteries pierce the outer walls of the bones, enter the marrow, and divide into fine branches, which ultimately coalesceinto large venous sacs (sinusoids) through which blood flows sluggishly. In the surrounding hematopoietic tissue, newly formed blood cells enter the general circulation by penetrating the walls of the sinusoids.

All blood cells arise from primordial cells called multipotent hematopoietic stem cells. By dividing and differentiating, these precursor cells give rise to the four major blood cell lineages: red cells, phagocytic cells, megakaryocytes, and lymphocytes. The cells of the marrow are under complex controls that regulate their formation and adjust their production to the changing demands of the body. When marrow stem cells are cultured outside the body they form tiny clusters of cells (colonies), which correspond to red cells, phagocytic cells, andmegakaryocytes. The formation of these individual colonies depends on hormonal sugar-containing proteins (glycoproteins), referred to collectively as colony-stimulating factors (CSFs). These factors are produced throughout the body. Even in minute amounts, CSFs can stimulate the division and differentiation of precursor cells into mature blood cells and thus exert powerful regulatory influences over the production of blood cells. A master colony-stimulating factor (multi-CSF), also called interleukin-3, stimulates the most ancestral hematopoietic stem cell. Further differentiation of this stem cell into specialized descendantsrequires particular kinds of colony-stimulating factors; for example, the CSF erythropoietin is needed for the maturation of red cells, and granulocyte colony-stimulating factor controls theproduction of granulocytes. These glycoproteins, as well as other colony-stimulating factors, serve as signals from the tissues to the marrow. For instance, a decrease in the oxygen content of the blood stimulates the kidney to increase its production of erythropoietin, thus ultimately raising the number of oxygen-carrying red cells in the blood. Certain bacterial components accelerate the formation of granulocyte colony-stimulating factor, thereby leading to an increased production of phagocytic granulocytes by the bone marrow during infection.