The excess conductivity of the Bi2Sr2CaCu 2O8+y system is measured as a function of carrier concentration. The analysis shows that the system is highly anisotropic and the interlayer coupling strength decreases very rapidly with the decrease of holes. In the low-hole-concentration region Tc depends strongly on the interlayer coupling as compared to that in the optimally and heavily-doped regions. From the value of the interlayer coupling, the out-of-plane coherence length and dimensional crossover temperature are estimated.