One of the fundamental structures with which fishes regulate their internal ions and maintain osmotic homeostasis are ionocytes (also known as chloride cells and mitonchondrion-rich cells), located in the gills. These cells are critical for the uptake of ions in freshwater and the excretion of ions in seawater. To help explain the complicated function of these cells, Dr. Arne Christensen developed this video model contrasting chloride cell function in freshwater (FW, first) and seawater (SW, second).