Electrochemical impedance spectroscopy has great potential for laboratory blood tests. The overall aim of this study is to develop a microfluidic sensor for determining the physical properties and hematological parameters of blood based on its dielectric spectra. Impedance was measured in flowing blood to prevent aggregation and sedimentation at frequencies between 40 Hz and 110 MHz. Two major factors make accurate analysis of impedance spectra difficult: forced orientation of erythrocytes in a microchannel and hemoglobin hydration. A theoretical approach based on the effective medium theory was applied to find the preferred erythrocyte orientation and dielectric properties of blood components. The cytoplasm of erythrocytes was considered a colloidal suspension of hemoglobin molecules surrounded by a double hydration shell. The proposed preferred orientation factor demonstrates that approximately 66% of the erythrocytes in the microfluidic channel have a random distribution and approximately 34% of them occupy random positions and are oriented along the blood flow. The experiments did not reveal any significant changes in the preferred orientation factor when the blood flow rate changed from 2 to 20 mL/h. Finally, several hematological parameters of blood samples were determined (erythrocyte count, hemoglobin level, hematocrit, mean corpuscular volume, mean corpuscular hemoglobin, and mean corpuscular hemoglobin concentration). A comparison of routine hematological studies and the developed technique proves its effectiveness.