Starting in the left cardiac ventricle the reader is taken on a guided tour on a fetal erythrocyte as measured by Doppler ultrasound. Up in the ascending aorta we move through the aortic isthmus to the descending aorta and the internal umbilical arteries, which fuse around the umbilical cord. With fresh oxygen from the placenta our erythrocyte moves in an accelerating continuous flow along the umbilical vein to the ductus venosus. After having reached the left ventricle again it now passes through a coronary artery to the right atrium and eventually the ductus arteriosus or the pulmonary circulation. Concepts of pulsatility, impedance and resistance are presente in the context of their clinical applicability in Doppler waveforms of various fetal vessels.