Excretion and Elimination of Toxicants and their Metabolites The first topic that was covered by this chapter was the excretion of wastes by the Renal system. The first step that occurs in the kidney deals with the nephron, which is the functional unit of the kidney. In the glomerulus the formation of urine begins with the passive filtration of plasma through the pores that are found in the glomerulus. The plasma is forced through these pores by hydrostatic pressure. The only things that determines if a molecule will pass through the pores of the glomerulus is it's molecular weight. The lower the molecular weight, the easier it will pass thro