The Blood Formed Elements
All of the blood’s formed elements are produced in red bone marrow, which is located in the ends of long bones and in the inner mass of all other bones. The ancestors of all the blood cells are called hematopoietic (blood-forming) stem cells. These cells have the potential to develop into any of the blood cell types produced within the red marrow. In comparison with other cells, most of those in the blood are short lived. The need for constant blood cell replacement means that normal activity of the red bone marrow is absolutely essential to life.
Erythrocytes Erythrocytes, the red blood cells (RBCs, or red cells), measure about 7 nm in diameter. They are disk-shaped bodies with a depression on both sides. This biconcave shape creates a central area that is thinner than the edges (Fig. 9-3).
Figure 9-3 Red blood cells as seen under a scanning electron microscope. This type of microscope provides a threedimensional view of the cells.
Erythrocytes are different from other cells in that the mature form found in the circulating blood lacks a nucleus (is anuclear) and also lacks most of the other organelles commonly found in cells. As red cells mature, these components are lost, providing more space for the cells to carry oxygen. This vital gas is bound in the red cells to hemoglobin, a protein that contains iron (see Box 9-1, Hemoglobin: Door to Door Oxygen Delivery). Hemoglobin, combined with oxygen, gives the blood its characteristic red color. The more oxygen carried by the hemoglobin, the brighter is the red color of the blood. Therefore, the blood that goes from the lungs to the tissues is a bright red because it carries a great supply of oxygen; in contrast, the blood that returns to the lungs is a much darker red because it has given up much of its oxygen to the tissues. Hemoglobin has two lesser functions in addition to the transport of oxygen. Hemoglobin that has given up its oxygen is able to carry hydrogen ions. In this way, hemoglobin acts as a buffer and plays an important role in acid-base balance. Hemoglobin also carries some carbon dioxide from the tissues to the lungs for elimination.
The carbon dioxide is bound to a different part of the molecule than the part that holds oxygen, so that it does not interfere with oxygen transport. Hemoglobin’s ability to carry oxygen can be blocked by carbon monoxide. This odorless and colorless but harmful gas combines with hemoglobin to form a stable compound that can severely restrict the erythrocytes’ ability to carry oxygen. Carbon monoxide is a byproduct of the incomplete burning of fuels, such as gasoline and other petroleum products and coal, wood, and other carbon- containing materials. It also occurs in cigarette smoke and automobile exhaust. Erythrocytes are by far the most numerous of the blood cells, averaging from 4.5 to 5 million per microliter (mL) of blood. (A microliter is one millionth of a liter. It is equal to a cubic millimeter or mm3). Because mature red cells have no nucleus and cannot divide, they must be replaced constantly. After leaving the bone marrow, they circulate in the bloodstream for about 120 days before their membranes deteriorate and they are destroyed by the liver and spleen. Red cell production is stimulated by the hormone erythropoietin (EPO), which is released from the kidney in response to a decrease in its oxygen supply. The constant production of red cells requires an adequate supply of nutrients, particularly protein, the B vitamins B12 and folic acid, required for the production of DNA, and the minerals iron and copper for the production of hemoglobin. Vitamin C is also important for the proper absorption of iron from the small intestine.
Leukocytes The leukocytes, or white blood cells (WBCs, or white cells), are different from the erythrocytes in appearance, quantity, and function. The cells themselves are round, but they contain prominent nuclei of varying shapes and sizes. Occurring at a concentration of 5,000 to 10,000 per cubic millimeter of blood, leukocytes are outnumbered by red cells by about 700 to 1. Although the red cells have a definite color, the leukocytes tend to be colorless. The different types of white cells are identified by their size, the shape of the nucleus, and the appearance of granules in the cytoplasm when the cells are stained. The stain commonly used for blood is Wright stain, which is a mixture of dyes that differentiates the various blood cells. The “granules” in the white cells are actually lysosomes and other secretory vesicles. They are present in all white blood cells, but they are more easily stained and more visible in some cells than in others. The relative percentage of the different types of leukocytes is a valuable clue in arriving at a medical diagnosis
The granular leukocytes, or granulocytes, are so named because they show visible granules in the cytoplasm when stained (see Fig. 9-4 A-C). Each has a very distinctive, highly segmented nucleus. The different types of granulocytes are named for the type of dyes they take up when stained. They include the following:
* Neutrophils stain with either acidic or basic dyes and show lavender granules;
* Eosinophils stain with acidic dyes (eosin is one) and have beadlike, bright pink granules;
* Basophils stain with basic dyes and have large, dark blue granules that often obscure the nucleus.
The neutrophils are the most numerous of the white cells, constituting approximately 60% of all leukocytes. Because the nuclei of the neutrophils have various shapes, these cells are also called polymorphs (meaning “many forms”) or simply polys. Other nicknames are segs, referring to the segmented nucleus, and PMNs, an abbreviation of polymorphonuclear neutrophils. Before reaching full maturity and becoming segmented, the nucleus of the neutrophil looks like a thick, curved band (Fig. 9-5). An increase in the number of these band cells (also called stab or staff cells) is a sign of infection and the active production of neutrophils. The eosinophils and basophils make up a small percentage of the white cells but increase in number during allergic reactions. The agranular leukocytes, or agranulocytes, are so named because they lack easily visible granules (see Fig. 9-4 D, E). Their nuclei are round or curved and are not segmented. There are two types of agranular leukocytes:
Figure 9-4 Granulocytes (A-C) and agranulocytes (D, E). (A) The neutrophil has a large, segmented nucleus. (B) The eosinophil has many bright pink-staining granules. (C) The basophil has large dark blue-staining granules. (D) The lymphocyte has a large undivided nucleus. (E) The monocyte is the largest of the leukocytes.
Figure 9-5 Stages in neutrophil development. (A) A mature neutrophil has a segmented nucleus. (B) An immature neutrophil is called a band cell because the nucleus is shaped like a thick, curved band. (x1325)
* Lymphocytes are the second most numerous of the white cells. Although lymphocytes originate in the red bone marrow, they develop to maturity in lymphoid tissue and can multiply in this tissue as well. They circulate in the lymphatic system and are active in immunity.
* Monocytes are the largest in size. They average about 5% of the leukocytes.
Function of Leukocytes Leukocytes clear the body of foreign material and cellular debris. Most importantly, they destroy pathogens that may invade the body. Neutrophils and monocytes engage in phagocytosis, the engulfing of foreign matter (Fig. 9-6). Whenever pathogens enter the tissues, as through a wound, they are attracted to the area. They squeeze between the cells of the capillary walls and proceed by ameboid, or amebalike, motion to the area of infection where they engulf the invaders. Lysosomes in the cytoplasm then digest the foreign organisms and the cells eliminate the waste products. When foreign organisms invade, the bone marrow and lymphoid tissue go into emergency production of white cells, and their number increases enormously as a result. Detection of an abnormally large number of white cells in the blood is an indication of infection. In battling pathogens, leukocytes themselves may be destroyed. A mixture of dead and living
Figure 9-6 Phagocytosis. (A) A phagocytic leukocyte (white blood cell) squeezes through a capillary wall in the region of an infection and engulfs a bacterium. (B) The bacterium is enclosed in a vesicle and digested by a lysosome.
bacteria, together with dead and living leukocytes, forms
pus. A collection of pus localized in one area is known as an abscess. Some monocytes enter the tissues, enlarge, and mature into macrophages, which are highly active in disposing of invaders and foreign material. Although most circulating lymphocytes live only 6 to 8 hours, those that enter the tissues may survive for longer periods-days, months, or even years. Some lymphocytes become plasma cells, active in the production of circulating antibodies needed for immunity.
Platelets The blood platelets (thrombocytes) are the smallest of all the formed elements (Fig. 9-7 A). These tiny structures are not cells in themselves but rather fragments constantly released from giant bone marrow cells called megakaryocytes (Fig. 9-7 B). Platelets do not have nuclei or DNA, but they do contain active enzymes and mitochondria. The number of platelets in the circulating blood has been estimated to range from 150,000 to 450,000 per L (mm3). They have a life-span of about 10 days. Platelets are essential to blood coagulation (clotting). When blood comes in contact with any tissue other than the smooth lining of the blood vessels, as in the case of injury, the platelets stick together and form a plug that seals the wound. The platelets then release chemicals that participate in the formation of a clot to stop blood loss. More details on these reactions follow.
Figure 9-7 Platelets (thrombocytes). (A) Platelets in a blood smear. (B) A megakaryocyte releases platelets.