Portable ultra-low field MRI (ULF-MRI) systems operated in unshielded environments are susceptible to electromagnetic interference (EMI). Subject presence in the imaging region will lead to substantial noise increases, yet the dominant coupling mechanism remains insufficiently characterized. We develop a lumped-parameter circuit model of the coupled environment-body-receiver system. The model indicates that ambient time-varying electric fields induce a body common-mode potential, which is converted into differential-mode noise through capacitive imbalance between the head and the receive-coil terminals, yielding strong dependence on subject position and geometry. Circuit analysis, simulations, and controlled experiments support the model, with predicted imbalance consistent with measured noise variations. Guided by this mechanism, we implement a capacitive low-impedance bypass to clamp the body potential, achieving an approximately 3.5-fold SNR improvement on a 50 mT prototype. The proposed model offers a compact circuit-based tool for analyzing and mitigating human body-coupled electric-field interference in portable ULF-MRI.