You had me until thermodynamics. We're obviously not getting every substance in our bloodstream like we are with plastics so I don't think thermodynamics is the issue here.
Quoting Wikipedia: In statistical mechanics, entropy is an extensive property of a thermodynamic system. It is closely related to the number Ω of microscopic configurations (known as microstates) that are consistent with the macroscopic quantities that characterize the system.
and
The second law of thermodynamics states that the entropy of an isolated system never decreases over time.
Applied here, when plastic breaks into pieces, the number of microstates -- ie, the entropy -- increases.
It's not the same as the ideal gas law, but similar.