This work provides a unique view into the physics behind the heat pipe operation which was considered a thermal network of various components. Transient heat pipe behavior was described by first-order, linear, ordinary differential equations. The working fluid undergoes a thermodynamic cycle which was analyzed by T-s diagrams. The heat pipe dimensions must be "thermally compatible" with the heat pipe materials to establish the thermodynamic cycle. This was illustrated by a dimensionless number proposed here for the first time. Validated by comparisons with previous experimental and numerical studies, the present thermodynamic theory may lead to simplified heat pipe design schemes.