Figure 1. Osmoregulation in a freshwater fish. The net osmotic gain of water and diffusional loss of salt across the gills is balanced by excretion of relatively dilute urine, active uptake of salt across the gill, and possibly some ingestion of salt in the food. Blue arrows represent passive movements of salt and water, and red arrows indicate active pathways of osmoregulation. See text for details. Outline drawing of a goldfish was redrawn from Greenwood HP, et al. (1966) Bulletin of the American Museum of Natural History 131: 339–456.

Figure 2. Osmoregulation by a marine teleost fish. The net osmotic loss of water and diffusional gain of salt across the gills is balanced by ingestion of sea water, production of small volumes of urine that contains some salt, and active extrusion of salt across the gill. Blue arrows represent passive movements of salt and water, and red arrows indicate active pathways of osmoregulation. See text for details. Outline drawing of a tuna was redrawn from Bigelow HB and Schroeder WS (1953) Fisheries Bulletin, Fish and Wildlife Service 53: 1–577.

Figure 3. Osmoregulation by a shark. The osmotic gain of water and diffusional gain of salt across the gills is balanced by production of large volumes of urine that contains some salt, active secretion of salt via the rectal gland, and possibly active extrusion of salt across the gill. Blue arrows represent passive movements of salt and water, and red arrows indicate active pathways of osmoregulation. See text for details. Outline drawing of a spiny dogfish shark was redrawn from Bigelow HB and Schroeder WS (1953) Fisheries Bulletin, Fish and Wildlife Service 53: 1–577.