Blood Volume and Composition

Blood volume varies with body size, changes in fluid and electrolyte concentrations, and the amount of adipose tissue. Blood volume is typically about 8% of body weight. An average-sized adult has a blood volume of about 5 liters.

If a blood sample stands in a tube for awhile and is prevented from clotting, the cells separate from the liquid portion of the blood and settle to the bottom. Cen-trifuging the sample quickly packs the cells into the lower part of the centrifuge tube, as figure 14.2 shows. The percentage of cells and liquid in the blood sample can then be calculated.

Plasma And Blood Solids

Figure 14.1

Blood consists of a liquid portion called plasma and a solid portion that includes red blood cells, white blood cells, and platelets. When blood components are separated, the white blood cells and platelets form a thin layer, called the "buffy coat," between the plasma and the red blood cells.

Figure 14.1

Blood consists of a liquid portion called plasma and a solid portion that includes red blood cells, white blood cells, and platelets. When blood components are separated, the white blood cells and platelets form a thin layer, called the "buffy coat," between the plasma and the red blood cells.

Capillary tube

Buffy coat

Plug-

Figure 14.2

If a blood-filled capillary tube is centrifuged, the red cells pack in the lower portion, and the percentage of red cells (hematocrit) can be determined. Values shown are within the normal range for healthy humans.

A blood sample is usually about 45% cells by volume. This percentage is called the hematocrit (HCT), or packed cell volume (PCV). The remaining 55% of a blood sample is clear, straw-colored plasma (plaz'mah). Appendix C, Laboratory Tests of Clinical Significance, (p. 1030) lists values for the hematocrit and other common blood tests in healthy individuals.

In addition to red blood cells, which comprise more than 99% of the blood cells, the formed elements of the blood include white blood cells and blood platelets. Plasma is a complex mixture that includes water, amino acids, proteins, carbohydrates, lipids, vitamins, hormones, electrolytes, and cellular wastes.

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