In a series circuit, the current remains constant because there is only one path for the flow of electricity. The total resistance in the circuit is the sum of all individual resistances, which affects the overall current according to Ohm's law (I = V/R). This concept can be likened to water flowing through a hose; if water is poured in at a certain rate, the same amount must exit, regardless of any restrictions along the way. Any increase in resistance requires more voltage to maintain the same current flow, but the current itself does not change. Thus, the fundamental principle is that in a series circuit, the current is uniform throughout due to the single pathway for flow.