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''Percentage of leukocytes.

''Percentage of leukocytes.

the huffy coat. As Table 9.1 indicates, there are nearly 1000 times more erythrocytes (~5 X 1012 cells/L of blood) than leukocytes (~7 X 107L of blood).

s? plasma

Although the blood cells are the major objects of interest in histology, a brief examination of plasma is also useful. The composition of plasma is summarized in Table 9.2. More than 90% of plasma by weight is water, which serves as the solvent for a variety of solutes, including proteins, dissolved gases, electrolytes, nutrients, regulatory substances, and waste materials.

Plasma proteins consist primarily of albumin, globulins, and fibrinogen

Albumin is the main protein constituent of the plasma, accounting for approximately half of the total plasma proteins. It is the smallest plasma protein (about 70 kDa) and is made in the liver. Albumin is responsible for exerting the concentration gradient between blood and extracellular tissue fluid. This major osmotic pressure on the blood vessel wall, called the colloid osmotic pressure, maintains the correct proportion of blood to tissue fluid volume. If a significant amount of albumin leaks out of the blood vessels into the loose connective tissue or is lost from the blood to the urine in the kidneys, the colloid osmotic pressure of the blood decreases, and fluid accumulates in the tissues. (This increase in tissue fluid is most readily noted by swelling of the ankles at the end of a day.) Albumin also acts as a carrier protein; it binds and transports hormones (thyroxine), metabolites (bilirubin), and drugs (barbiturates).

Component %

Water 91 -92

Protein (albumin, globulins, fibrinogen) 7-8

Other solutes: 1-2

Electrolytes (Na+, K',Ca2'-, Mg2+, CI", HC03~, PO,3", SO„21

• Nonprotein nitrogen substances (urea, uric acid, creatine, creatinine, ammonium salts)

• Nutrients (glucose, lipids, amino acids)

• Blood gases (oxygen, carbon dioxide, nitrogen)

• Regulatory substances (hormones, enzymes)

TABLE 9.2. Composition of Blood Plasma

Globulins include the immunoglobulins (y-globulins), the largest component of the globulin fraction, and nonimmune globulins (a- and fi-globulins). The immunoglobulins are antibodies, a class of functional immune-system molecules secreted by plasma cells. (Antibodies are discussed in Chapter 13.)

Nonimmune globulins are secreted by the liver. They help maintain the osmotic pressure within the vascular system and also serve as carrier proteins for various substances such as copper (by ceruloplasmin), iron (by transferrin), and hemoglobin (by haptoglobin). Nonimmune globulins also include fibronectin, lipoproteins, coagulation factors, and other molecules that may exchange between the blood and the extravascular connective tissue.

Fibrinogen, the largest protein (340 kDa), is made in the liver. In cascade reactions with other coagulation factors, fibrinogen is transformed into fibrin, which forms an insoluble clot that stops blood flow in the event of damage to the blood vessel.

Aside from these large proteins and the regulatory substances, which are small proteins or polypeptides, most of the other plasma constituents are small enough to pass through the blood vessel wall into the extracellular spaces of the adjacent connective tissue.

In general, plasma proteins react with common fixatives; they are often retained within the blood vessels in tissue sections. Plasma proteins do not possess structural form above the molecular level; thus, when they are retained in blood vessels in the tissue block, they appear as a homogeneous substance that stains evenly with eosin in hematoxylin and eosin (H&E)-stained sections.

The interstitial fluid of connective tissues is derived from blood plasma

Interstitial fluid, not surprisingly, has an electrolyte composition that reflects that of blood plasma, from which it is derived. The composition of interstitial fluid in non-connective tissues, however, is subject to considerable modification by the absorptive and secretory activities of epithelia. Epithelia may create special microenvironments conducive to their function. For example, a blood-brain barrier exists between the blood and nerve tissue. Barriers also exist between the blood and the parenchymal tissue in the testis, thymus gland, eye, and other epithelial compartments. Fluids, barriers, and their functions are discussed in subsequent chapters that describe these particular organs.

Examination of blood cells requires special preparation and staining

The preparation method that best displays the cell types of peripheral blood is the blood smear. This method differs from the usual preparation seen in the histology laboratory in that the specimen is not embedded in paraffin and sectioned. Rather, a drop of blood is placed directly on a slide and spread thinly over the surface of the slide, i.e., "pulled" with the edge of another slide, to produce a monolayer of cells (Fig. 9.1a). The preparation is then air-dried and stained. Another difference in the preparation of a blood smear is that instead of H&E, special mixtures of dyes are used to stain the blood cells. The resulting prepa-

a

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