Question

The reduction of blood volume occurs following severe dehydration and hemorrhage; however, although dehydration results in an increase in hematocrit, immediately following hemorrhage hematocrit is unchanged. Explain this difference.

Answer 1

During bleeding, both formed elements (platelets, white blood cells, red blood cells) and plasma are lost from the circulatory system. They are lost proportionally, so initially there is no change in hematocrit.

Hematocrit is the percentage of the blood volume made up of elements (Hct = cell volume/blood volume). During dehydration, only water and electrolytes are lost, and the number of cells remains constant - the same number of cells in a smaller volume leads to an increase in hematocrit. When the body tries to restore blood volume, the first thing to recirculate is water from the ECF and this increases the amount of water without increasing the amount of red blood cells, so the compensatory mechanism causes the hematocrit to fall.

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