Blood Volume

Blood Volume

Blood contains both extracellular fluid (the fluid in plasma) and intracellular fluid (the fluid in the red blood cells). However, blood is considered to be a separate fluid compartment because it is contained in a chamber of its own, the circulatory system. The blood volume is especially important in the control of car-diovascular dynamics.

The average blood volume of adults is about 7 per cent of body weight, or about 5 liters. About 60 per cent of the blood is plasma and 40 per cent is red blood cells, but these percentages can vary considerably in different people, depending on gender, weight, and other factors.

Hematocrit (Packed Red Cell Volume). The hematocrit isthe fraction of the blood composed of red blood cells, as determined by centrifuging blood in a “hematocrit tube” until the cells become tightly packed in the bottom of the tube. It is impossible to completely pack the red cells together; therefore, about 3 to 4 per cent of the plasma remains entrapped among the cells, and the true hematocrit is only about 96 per cent of the measured hematocrit.

In men, the measured hematocrit is normally about 0.40, and in women, it is about 0.36. In severe anemia, the hematocrit may fall as low as 0.10, a value that is barely sufficient to sustain life. Conversely, there are some conditions in which there is excessive production of red blood cells, resulting in polycythemia. In these conditions, the hematocrit can rise to 0.65.