G

Normal

Anemia Polycythemia

Figure 14-11

Hematocrits in a healthy (normal) person and in patients with anemia and polycythemia.

Figure 14-11

Hematocrits in a healthy (normal) person and in patients with anemia and polycythemia.

each of which exerts frictional drag against adjacent cells and against the wall of the blood vessel.

Hematocrit. The percentage of the blood that is cells is called the hematocrit. Thus, if a person has a hemat-ocrit of 40, this means that 40 per cent of the blood volume is cells and the remainder is plasma. The hema-tocrit of men averages about 42, while that of women averages about 38. These values vary tremendously, depending on whether the person has anemia, on the degree of bodily activity, and on the altitude at which the person resides. These changes in hematocrit are discussed in relation to the red blood cells and their oxygen transport function in Chapter 32.

Hematocrit is determined by centrifuging blood in a calibrated tube, as shown in Figure 14-11. The calibration allows direct reading of the percentage of cells.

Effect of Hematocrit on Blood Viscosity. The viscosity of blood increases drastically as the hematocrit increases, as shown in Figure 14-12. The viscosity of whole blood at normal hematocrit is about 3; this means that three times as much pressure is required to force whole blood as to force water through the same blood vessel. When the hematocrit rises to 60 or 70, which it often does in polycythemia, the blood viscosity can become as great as 10 times that of water, and its flow through blood vessels is greatly retarded.

Other factors that affect blood viscosity are the plasma protein concentration and types of proteins in the plasma, but these effects are so much less than the effect of hematocrit that they are not significant

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