All of the electron carrier in the electron transport chain interact according to their redox potentials.  Every time whereas an electron transfers occurs, the accepting carrier has a higher affinity for electrons than the giving carrier. Thus there is a net flow of electrons from NADH (most negative redox potential, least affinity for electrons) to oxygen (most positive redox potential, highest affinity for electrons). These make sure a unidirectional flow of electrons.  Furthermore, note that each cytochrome, every FeS middle and every copper atom can carry only one electron but each NADH donates two electrons.  Additionally, every molecule of oxygen (O₂) requires to accept four electrons to be reduced to a molecule of water, H₂O. The  various  components   are  arranged  in  such  a  manner  as  to  permit  their different electron-handling properties to work in harmony.