Human Story. I talked about how genes and that can be analysed for stuff so that scientists can make evolutionary relationships, by comparing with the genes of other stuff and then blah blah blah.
And for diffusion, in the case where you require high pressure within the glomerulus and Bowman's capsule or the active transport of urea into the renal tubules through secretion (which is against a con/c gradient), diffusion is simply insufficient.