Re: ps: passed lots of stones

different cells have different renewal times (skin cells = 30 days for instance) but every cell has to deal with fluids, both within the cell and between the cells, and those fluids can carry a lot of dreck. You can attach a lot of dreck to fat cells, even contaminate hard bone.

The intestines are designed to wall things off so they don't recirculate and kill us. It's a beautiful system really. :)