The Formed Elements of Blood

The formed elements of blood include two types of blood cells: erythrocytes, or red blood cells, and leukocytes, or white blood cells. Erythrocytes are by far the more numerous of the two. A cubic millimeter of blood contains 5.1 million to 5.8 million erythrocytes in males and 4.3 million to 5.2 million erythrocytes in females. The same volume of blood, by contrast, contains only 5,000 to 9,000 leukocytes.

Erythrocytes

Erythrocytes are flattened, biconcave discs, about 7 |im in diameter and 2.2 |im thick. Their unique shape relates to their function of transporting oxygen; it provides an increased surface area through which gas can diffuse (fig. 13.2). Erythrocytes lack nuclei and mitochondria (they obtain energy through anaerobic respiration). Partly because of these deficiencies, erythrocytes have a relatively short circulating life span of only about 120 days. Older erythrocytes are removed from the circulation by phagocytic cells in the liver, spleen, and bone marrow.

Each erythrocyte contains approximately 280 million hemoglobin molecules, which give blood its red color. Each hemoglobin molecule consists of four protein chains called globins, each of which is bound to one heme, a red-pigmented molecule that contains iron. The iron group of heme is able to combine with oxygen in the lungs and release oxygen in the tissues.

Heart and Circulation

Heart and Circulation

■ Figure 13.2 A scanning electron micrograph of red blood cells. As seen here, they are clinging to a hypodermic needle. Notice the shape of the red blood cells, sometimes described as a "biconcave disc."

Anemia refers to any condition in which there is an abnormally low hemoglobin concentration and/or red blood cell count. The most common type is iron-deficiency anemia, caused by a deficiency of iron, which is an essential component of the hemoglobin molecule. In pernicious anemia there is an inadequate amount of vitamin B12, which is needed for red blood cell production. This is usually due to atrophy of the glandular mucosa of the stomach, which normally secretes a protein called intrinsic factor. In the absence of intrinsic factor, the vitamin B|2 obtained in the diet cannot be absorbed by intestinal cells. Aplastic anemia is anemia due to destruction of the bone marrow, which may be caused by chemicals (including benzene and arsenic) or by radiation.

Leukocytes

Leukocytes differ from erythrocytes in several respects. Leukocytes contain nuclei and mitochondria and can move in an amoeboid fashion. Because of their amoeboid ability, leukocytes can squeeze through pores in capillary walls and move to a site of infection, whereas erythrocytes usually remain confined within blood vessels. The movement of leukocytes through capillary walls is referred to as diapedesis or extravasation.

White blood cells are almost invisible under the microscope unless they are stained; therefore, they are classified according to their staining properties. Those leukocytes that have granules in their cytoplasm are called granular leukocytes; those without clearly visible granules are called agranular (or nongranular) leukocytes.

The stain used to identify white blood cells is usually a mixture of a pink-to-red stain called eosin and a blue-to-purple stain called a "basic stain." Granular leukocytes with pink-staining granules are therefore called eosinophils, and those with blue-staining granules are called basophils. Those with granules that have little affinity for either stain are neutrophils (fig. 13.3). Neutrophils are the most abundant type of leukocyte, accounting for 50% to 70% of the leukocytes in the blood. Immature neutrophils have sausage-shaped nuclei and are called band cells. As the band cells mature, their nuclei become lobulated, with two to five lobes connected by thin strands. At this stage, the neutrophils are also known as polymorphonuclear leukocytes (PMNs).

There are two types of agranular leukocytes: lymphocytes and monocytes. Lymphocytes are usually the second most numerous type of leukocyte; they are small cells with round nuclei and little cytoplasm. Monocytes, by contrast, are the largest of the leukocytes and generally have kidney- or horseshoe-shaped nuclei. In addition to these two cell types, there are smaller numbers of plasma cells, which are derived from lymphocytes. Plasma cells produce and secrete large amounts of antibodies. The immune functions of the different white blood cells are described in more detail in chapter 15.

Blood cell counts are an important source of information in assessing a person's health. An abnormal increase in erythrocytes, for example, is termed polycythemia and is indicative of several dysfunctions. As previously mentioned, an abnormally low red blood cell count is termed anemia. (Polycythemia and anemia are described in detail in chapter 16.) An elevated leukocyte count, called leukocytosis, is often associated with infection (see chapter 15). A large number of immature leukocytes in a blood sample is diagnostic of the disease leukemia. A low white blood cell count, called leukopenia, may be due to a variety of factors; low numbers of lymphocytes, for example, may result from poor nutrition or from whole-body irradiation treatment for cancer.

Platelets

Platelets, or thrombocytes, are the smallest of the formed elements and are actually fragments of large cells called megakary-ocytes, which are found in bone marrow. (This is why the term formed elements is used instead of blood cells to describe erythrocytes, leukocytes, and platelets.) The fragments that enter the circulation as platelets lack nuclei but, like leukocytes, are capable of amoeboid movement. The platelet count per cubic millimeter of blood ranges from 130,000 to 400,000, but this count can vary greatly under different physiological conditions. Platelets survive for about 5 to 9 days before being destroyed by the spleen and liver.

Platelets play an important role in blood clotting. They constitute most of the mass of the clot, and phospholipids in their cell membranes activate the clotting factors in plasma that result in threads of fibrin, which reinforce the platelet plug. Platelets that attach together in a blood clot release serotonin, a chemical that stimulates constriction of the blood vessels, thus

Fox: Human Physiology, I 13. Heart and Circulation I Text I © The McGraw-Hill

Eighth Edition Companies, 2003

Chapter Thirteen

Neutrophils

Lymphocytes

Eosinophils

Basophils

Monocytes

Platelets

Erythrocytes

■ Figure 13.3 The blood cells and platelets. The white blood cells depicted above are granular leukocytes; the lymphocytes and monocytes are nongranular leukocytes.

Table 13.2

Formed Elements of the Blood

Component

Description

Number Present

Function

Erythrocyte (red

Biconcave disc without nucleus;

4,000,000 to 6,000,000 / mm3

Transports oxygen and carbon

blood cell)

contains hemoglobin; survives 100

dioxide

to 120 days

Leukocytes (white

5,000 to 10,000 / mm3

Aid in defense against infections by microorganisms

blood cells)

Granulocytes

About twice the size of red blood cells;

cytoplasmic granules present; survive

12 hours to 3 days

1. Neutrophil

Nucleus with 2 to 5 lobes; cytoplasmic

54% to 62% of white

Phagocytic

granules stain slightly pink

cells present

2. Eosinophil

Nucleus bilobed; cytoplasmic granules

1% to 3% of white

Helps to detoxify foreign substances; secretes enzymes

stain red in eosin stain

cells present

that dissolve clots; fights parasitic infections

3. Basophil

Nucleus lobed; cytoplasmic granules

Less than 1% of white

Releases anticoagulant heparin

stain blue in hematoxylin stain

cells present

Agranulocytes

Cytoplasmic granules not visible; survive

100 to 300 days (some much longer)

1. Monocyte

2 to 3 times larger than red blood cell;

3% to 9% of white cells

Phagocytic

nuclear shape varies from round to lobed

present

2. Lymphocyte

Only slightly larger than red blood cell;

25% to 33% of white

Provides specific immune response

nucleus nearly fits cell

cells present

(including antibodies)

Platelet (thrombocyte)

Cytoplasmic fragment; survives

130,000 to 400,000 / mm3

Enables clotting; releases serotonin,

5 to 9 days

which causes vasoconstriction

reducing the flow of blood to the injured area. Platelets also secrete growth factors (autocrine regulators—see chapter 11), which are important in maintaining the integrity of blood vessels. These regulators also may be involved in the development of atherosclerosis, as described in a later section.

The formed elements of the blood are illustrated in figure 13.3, and their characteristics are summarized in table 13.2.

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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