Actually, Mace didn't crush Grievous' actual organs (using the Force on a living being like that, consciously, is Dark Side material "Apologies accepted, captain Needa!").
Mace thought (as all the Jedi did) that Grievous was nothing but a very powerful droid. So he proceeded to crushing Grievous' torso (silly General, thought he could take on Shaft, Jedi Knight). The very humbled Grievous started coughing then and there, and Mace would've crushed him in mere seconds if the shuttle hadn't gotten away.