Why does a dehydrated patient show a transiently raised hematocrit?

1. Hematocrit is a ratio, the volume of packed erythrocytes over the total sample volume. 2. A ratio can rise either because the numerator grows or because the denominator shrinks. 3. In dehydration, inadequate water intake or excessive water loss makes the plasma volume fall. 4. The red cell mass is unchanged, so the same cells now occupy a larger share of a smaller total. 5. The hematocrit therefore rises without any new erythropoiesis at all, which is why the rise is transient and corrects with fluids. 6. The marrow option would take weeks rather than hours, so it cannot explain a transient change. 7. The spleen stores platelets rather than the red cell reserve described here, so that option misassigns the organ. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 18 "The Cardiovascular System: Blood", section 18.3 Erythrocytes_