EPO and anemia (phenotype): The resulting vasodilation, natriuresis, and anemia caused by the SVRR decrease vascular resistance. The decrease in red cell mass must be proportional to the decrease in plasma volume to maintain a constant hematocrit. A decrease in plasma volume without a decrease in red cell mass would cause hemoconcentration and increased blood viscosity. Kilbridge, et al. have demonstrated this arm of the response by showing that hyperviscosity blunts the erythropoietin response to hypoxia in mice [2].