This letter presents a molecular communication receiver model grounded in Langmuir adsorption kinetics, offering a physically consistent alternative to passive and fully absorbing models. The receiver detects information molecules through reversible binding to a finite number of surface-anchored receptors (probes), thereby capturing the saturation and competition effects in realistic biosensing environments. We derive closed-form solutions for finite-duration pulse inputs under reaction-limited conditions and propose simplified asymptotic approximations for short- and long-pulse regimes, which accurately characterize the binding dynamics under limited receptor availability. An equivalent resistor-capacitor circuit analogy is introduced, mapping molecular binding and unbinding to time-varying and fixed resistances. Particle-based Monte Carlo simulations verify that the proposed model accurately captures the channel behavior and temporal memory of realistic biochemical receivers with finite receptor capacity.