The graph is 米 — grain — plus 青. The base sense is grain that has been picked over and refined, and the medical sense follows directly: 精 is the refined part of what the body takes in, set aside as a reserve.
The chapter treats it as finite. People of the present, it says, exhaust their essence in desire and scatter their true endowment in waste (以欲竭其精,以耗散其真). Whatever else is being claimed, the grammar is one of spending a store.