The Blood Cells
The formed elements contain blood cells and platelets. In the adult, the formed elements are produced continuously in the red bone marrow of the skull, ribs, and vertebrae, the iliac crests, and the ends of long bones.
The process by which formed elements are made is called hematopoiesis (Fig. 11.2). A stem cell is capable of dividing and producing new cells that go on to become particular types of cells. Stem cells in red bone marrow produce cells that mature into the various types of formed elements. At the top of Figure 11.2 is a multipotent stem cell that divides, producing two other types of stem cells. The myeloid stem cell gives rise to the cells that go through a number of stages to become red blood cells, platelets, granular leukocytes, and monocytes. The lymphatic stem cell produces the lymphocytes.
Many scientists are very interested in developing ways to use blood stem cells, as well as stem cells from other adult tissues, to regenerate the body’s tissues in the laboratory. If all goes well, embryos will not be needed as a source of stem cells to generate tissues for various illnesses.
Red Blood Cells
Red blood cells (RBCs, or erythrocytes) are small, biconcave disks that lack a nucleus when mature. They occur in great quantity; there are 4 to 6 million red blood cells per mm3 of whole blood. Red blood cells transport oxygen, and each contains about 200 million molecules of hemoglobin, the respiratory pigment. If this much hemoglobin were suspended within the plasma rather than enclosed within the cells, blood would be so viscous that the heart would have difficulty pumping it.
Figure 11.2 Hematopoiesis. Multipotent stem cells give rise to two specialized stem cells. The myeloid stem cell gives rise to still other cells, which become red blood cells, platelets, and all the whole blood cells except lymphocytes. The lymphatic stem cell gives rise to lymphoblasts, which become lymphocytes.
In a molecule of hemoglobin, each of four polypeptide chains making up globin has an iron-containing heme group in the center. Oxygen combines loosely with iron when hemoglobin is oxygenated:
In this equation, the hemoglobin on the right, which is combined with oxygen, is called oxyhemoglobin. Oxyhemoglobin forms in lung capillaries, and has a bright red color. The hemoglobin on the left, which is not combined with oxygen, is called deoxyhemoglobin. Deoxyhemoglobin forms in tissue capillaries, and has a dark maroon color.
Hemoglobin is remarkably adapted to its function of picking up oxygen in lung capillaries and releasing it in the tissues. Hemoglobin alone can be used as a blood substitute. The higher concentration of oxygen, plus the slightly cooler temperature and slightly higher pH within lung capillaries, causes hemoglobin to take up oxygen. The lower concentration of oxygen, plus the slightly warmer temperature and slightly lower pH within tissue capillaries, causes hemoglobin to give up its oxygen.
Production of Red Blood Cells
Erythrocytes are formed from red bone marrow stem cells (see Fig. 11.2): A multipotent stem cell descendant, called a myeloid stem cell, gives rise to erythroblasts, which divide many times. During maturation, these cells lose their nucleus and other organelles. As they mature, they gain many molecules of hemoglobin and lose their nucleus and most of their organelles. Possibly because mature red blood cells lack a nucleus, they live only about 120 days. It is estimated that about 2 million red blood cells are destroyed per second; therefore, an equal number must be produced to keep the red blood cell count in balance.
Whenever blood carries a reduced amount of oxygen, as happens when an individual first takes up residence at a high altitude, loses red blood cells, or has impaired lung function, the kidneys accelerate their release of erythropoietin (Fig. 11.3).
This hormone stimulates stem cells and speeds up the maturation of red blood cells. The liver and other tissues also produce erythropoietin. Erythropoietin, now mass-produced through biotechnology, is sometimes abused by athletes in order to raise their red blood cell counts and thereby increase the oxygen-carrying capacity of their blood.
Destruction of Red Blood Cells
With age, red blood cells are destroyed in the liver and spleen, where they are engulfed by macrophages.
Figure 11.3 Action of erythropoietin. The kidneys release increased amounts of erythropoietin whenever the oxygen capacity of the blood is reduced. Erythropoietin stimulates the red bone marrow to speed up its production of red blood cells, which carry oxygen. Once the oxygen-carrying capacity of the blood is sufficient to support normal cellular activity, the kidneys cut back on their production of erythropoietin.
When red blood cells are broken down, hemoglobin is released. The globin portion of the hemoglobin is broken down into its component amino acids, which are recycled by the body. The iron is recovered and returned to the bone marrow for reuse. The heme portion of the molecule undergoes chemical degradation and is excreted as bile pigments by the liver into the bile. These bile pigments are bilirubin and biliverdin, which contribute to the color of feces. Chemical breakdown of heme is also what causes a bruise on the skin to change color from red/purple to blue to green to yellow.
Abnormal Red Blood Cell Counts
Anemia is an illness in which the patient has a tired, run-down feeling. The cells are not getting enough oxygen due to a reduction in the amount of hemoglobin or the number of red blood cells. Hemolysis (bursting of red blood cells) can also cause anemia.
White Blood Cells
White blood cells (WBCs, or leukocytes) differ from red blood cells in that they are usually larger, have a nucleus, lack hemoglobin, and are translucent unless stained. White blood cells are not as numerous as red blood cells; there are only 5,000-11,000 per mm3 of blood. White blood cells fight infection and in this way are important contributors to homeostasis. This function of white blood cells concerns immunity.
White blood cells are derived from stem cells in the red bone marrow, and they, too, undergo several maturation stages (see Fig. 11.2). Each type of white blood cell is apparently capable of producing a specific growth factor that circulates back to the bone marrow to stimulate its own production.
Red blood cells are confined to the blood, but white blood cells are able to squeeze through pores in the capillary wall (Fig. 11.4). Therefore, they are found in tissue fluid and lymph (the fluid within lymphatic vessels) and in lymphatic organs. When an infection is present, white blood cells greatly increase in number. Many white blood cells live only a few days-they probably die while engaging pathogens. Others live months or even years.
Figure 11.4 Mobility of white blood cells. White blood cells can squeeze between the cells of a capillary wall and enter the tissues of the body.
Types of White Blood Cells
White blood cells are classified into the granular leukocytes and the agranular leukocytes. Both types of cells have granules in the cytoplasm surrounding the nucleus, but the granules are more visible upon staining in granular leukocytes. (The white cells in Figure 11.2 have been stained with Wright stain.) The granules contain various enzymes and proteins, which help white blood cells defend the body. There are three types of granular leukocytes and two types of agranular leukocytes. They differ somewhat by the size of the cell and the shape of the nucleus (see Fig. 11.1), and they also differ in their functions.
Granular Leukocytes Neutrophils are the most abundant of the white blood cells. They have a multilobed nucleus joined by nuclear threads; therefore, they are also called polymorphonuclear. Some of their granules take up acid stain, and some take up basic stain (creating an overall lilac color). Neutrophils are the first type of white blood cell to respond to an infection, and they engulf pathogens during phagocytosis.
Eosinophils have a bilobed nucleus, and their large, abundant granules take up eosin and become a red color. (This accounts for their name, eosinophil.) Among several functions, they increase in number in the event of a parasitic worm infection. Eosinophils also lessen an allergic reaction by phagocytizing antigen-antibody complexes involved in an allergic attack.
Basophils have a U-shaped or lobed nucleus. Their granules take up the basic stain and become dark blue in color. (This accounts for their name, basophil.) In the connective tissues, basophils, as well as a similar type of cells called mast cells, release histamine and heparin.
Histamine, which is associated with allergic reactions, dilates blood vessels and causes contraction of smooth muscle. Heparin prevents clotting and promotes blood flow.
Agranular Leukocytes The agranular leukocytes include lymphocytes, which have a spherical nucleus, and monocytes, which have a kidney-shaped nucleus. Lymphocytes are responsible for specific immunity to particular pathogens and their toxins (poisonous substances). Lymphocytes are of two types, B lymphocytes and T lymphocytes. Pathogens have antigens, surface molecules that the immune system can recognize as foreign. When an antigen is recognized as foreign, B lymphocytes will form antibodies against it. Antibodies are proteins that neutralize antigens. T lymphocytes, on the other hand, directly attack and destroy any cell, such as a pathogen that has foreign antigens.
Monocytes are the largest of the white blood cells, and after taking up residence in the tissues, they differentiate into even larger macrophages. Macrophages phagocytize pathogens, old cells, and cellular debris. They also stimulate other white blood cells, including lymphocytes, to defend the body.