Blood is classified as a connective tissue: It consists of cells suspended in an extracellular matrix of complex, yet specific, composition. The unusual feature of blood is that the extracellular matrix is a liquid, so blood is a fluid tissue.
The cells of the blood can be separated from the fluid matrix, called plasma, by centrifugation (Figure 49.15). If a sample of blood is spun in a centrifuge, all the cells move to the bottom of the tube, leaving the clear, straw-colored plasma on top. The packed-cell volume, or hematocrit, is the percentage of the blood volume made up by cells. Normal hematocrit is about 38 percent for women and 46 percent for men, but these values can vary considerably. They are usually higher, for example, in people who live and work at high altitudes because the low oxygen concentrations at high altitudes stimulate the production of more red blood cells.
In this section, we will consider two classes of cellular elements in blood: the red blood cells and the platelets, which are pinched-off fragments of cells. We discussed the other important class of blood cells—white blood cells, or leukocytes—in Chapter 18.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.