Paul Andersen explains how organisms regulate their internal osmolarity or not. He starts with a brief description of osmosis and why it is important for animal cell to be surrounded by an isotonic solution. He then explains how freshwater and saltwater fish osmoregulate. He finally discussed the nephron within the kidney as an osmoregulatory organ. He explains how the loop of Henle sets up a gradient which is used to reclaim or remove water with the use of the antidiuretic hormone (or ADH).