Current has only one path where it can travel through the components one after the other. There is no junction between components. In our body pulmonary and systemic circles are connected in series.
$ V = V_1 + V_2 + V_3 $
$ I = I_1 = I_2 = I_3 $
$ R = R_1 + R_2 + R_3 $
Current has multiple paths to go from one junction to another passing through multiple branches and components at the same time in a circuit. In our body the internal organs are connected in parallel.
$ V = V_1 = V_2 = V_3 $
$ I = I_1 + I_2 + I_3 $
$ \dfrac{1}{R} = \dfrac{1}{R_1} + \dfrac{1}{R_2} + \dfrac{1}{R_3} $
In the case of a parallel connection, the reciprocal of the resultant is equal to the sum of the reciprocals of the resistances, which is more simply expressed in the following form for three parallel resistors:
$ R_{total} = R_1 \times R_2 \times R_3 $
If only two resistors are connected in parallel, the simplification of reciprocals gives an easy-to-remember formula. It is important to know that the resultant of parallel resistors is always less than each resistor, and that the resultant of two resistors of the same value is exactly half their value.
$ R_1 \times R_2 = \dfrac{R_1 ⋅ R_2}{R_1 + R_2} $
The capacitance of capacitors is calculated opposite to resistors. In series connection, the replus operation is applied to the capacitances, and in parallel connection, they are simply summed.
The resultant of the inductances of coils is summed like the resultant of resistors in series connection, and the use of the replus operation is neccessary in parallel connection.