For laboratory purposes, samples of blood are often drawn from a vein (the procedure called venipuncture). When blood is removed from the circulation, it immediately clots. A blood clot consists mostly of erythrocytes entangled in a network of fine fibers composed of fibrin. To prevent clotting of a blood sample, an anticoagulant such as citrate or heparin is added to the blood specimen as it is obtained. Citrate binds calcium ions, which are essential for triggering the cascade of coagulation reactions; heparin deactivates the clotting factors in the plasma. Plasma that lacks coagulation factors is called serum. For many biochemical laboratory tests, plasma and blood serum can be used interchangeably. Serum is preferred for several specific tests because the anticoagulants in plasma can interfere with the results. However, tests of clotting ability require that all coagulation factors be preserved; therefore, serum is inappropriate for these tests.
The interstitial fluid of connective tissues is derived from blood plasma.
The fluid that surrounds tissue cells, called 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 nonconnective 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. 10.1a). 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 preparation may then be examined with a high-power oil-immersion lens, with or without a coverslip (Fig. 10.1b and Plate 17, page 302).
The modified Romanovsky-type stain commonly used for blood smears consists of a mixture of methylene blue (a basic dye), related azures (also basic dyes), and eosin (an acidic dye). On the basis of their appearance after staining, leukocytes are traditionally divided into granulocytes (neutrophils, eosinophils, and basophils) and agranulocytes (lymphocytes and monocytes). Although both cell types may contain granules, the granulocytes possess obvious, specifically stained granules in their cytoplasm. In general, the basic dyes stain nuclei, granules of basophils, and the RNA of the cytoplasm, whereas the acidic dye stains the erythrocytes and the granules of eosinophils. Scientists originally thought that the fine neutrophil granules were stained by a “neutral dye” that formed when methylene blue and its related azures were combined with eosin. The mechanism by which the specific neutrophil granules are stained is still not clearly understood. Some of the basic dyes (the azures) are metachromatic and may impart a violet to red color to the material they stain.
Erythrocytes are anucleate, biconcave discs.
Erythrocytes or red blood cells (RBCs), are anucleate cells devoid of typical organelles. They function only within the bloodstream to bind oxygen for delivery to the