Shen-Essence, another vital substance, drives both Primordial and Acquired Essence. Stored in the Kidney system as a fluid, it circulates throughout the body and determines growth, reproduction, development, sexual maturation, conception and pregnancy, etc. Primordial Essence and Shen-Essence are seemingly different aspects of the same object. When Shen essence and Primordial Essence are replenished by Acquired Essence, the three become one vital substance.
Essence is closely related to Qi, and one can change into the other, but there are several differences, mostly due to their different shapes or states.