 Dear students, in this topic we shall discuss osmoregulation in freshwater animals. The body fluids of freshwater invertebrates and vertebrates are generally hyper osmotic to the surrounding freshwater aquatic environment. The blood osmolarities of freshwater vertebrates range from 200 to 300 milliosmoles per liter. In comparison to this, the osmolarity of freshwater is about 50 milliosmoles per liter. In this hypotonic environment, the freshwater animals face two types of osmoregulatory problems. That is, the osmotic flooding of their body fluids which results in swelling and continuous loss of salts into the surrounding dilute medium. In order to maintain the osmolarity of their body fluids, they need to get rid of excess water and also to prevent net gain of water. They also need to prevent loss of salts from their body and also to gain salts to replace the net salt loss. Dear students, now we shall discuss the osmoregulatory strategies of animals living in freshwater. All freshwater animals are osmoregulators. Even the protozoa, unicellular organisms, such as amoeba and peramecium, who live in freshwater, they have contactile vacuoles to remove excess water. Freshwater vertebrates adopt different strategies for osmoregulation. The first strategy is that they do not drink water. They do not drink water to reduce excess water gain from the environment. Freshwater fishes or amphibians produce dilute urine in a large amount so that excess water is removed. Freshwater fishes have large glomerulus of nephrons and their tubular system is small. They have more filtration than the agglomerulus and their water reabsorption is less. Useful salts can be reabsorbed to the tubular system as much as possible, so there are many salts that need to be reabsorbed because salt loss causes potential problems for them. So vital salts such as sodium chloride, calcium chloride and potassium chloride, they can be obtained through their food. Other salts are also actively absorbed from the surrounding dilute medium. Freshwater is a dilute medium but it has salts that can be reabsorbed through the surface of the gills in fishes and through the skin in amphibians. Reptile birds or mammals reduce water uptake or salt loss through their skin and minimize them by having a less permeable integument.