There are many processes working to maintain homeostasis in the body. Fluid volume and temperature are examples of two conditions regulated by homeostatic mechanisms. Osmoregulation is the homeostatic process of maintaining water and salt balance in the body by controlling the movement of salt and water across cellular membranes. Cells are surrounded by and contain substances dissolved in fluid. The balance of fluids and solutes (dissolved substances) affects the integrity and function of the cell. Osmoregulation is an active form of regulation controlling the pressure of bodily fluids. Osmotic pressure is a type of pressure generated as water moves across a membrane due to osmosis. Osmosis is the movement of water from areas of low solute to high solute concentration.Salmon use osmoregulation to maintain fluid homeostasis. They live in both fresh and salt waters and moving between these types of water affects water and salt balance in their bodies. When an imbalance occurs, the body relies on osmoregulation to restore the balance through negative feedback. The negative feedback system detects a change in fluid content and responds by restoring the appropriate water concentration in the body. Thermoregulation is the regulation of a body's internal temperature. There are two groups of animals based on the way they maintain thermal homeostasis, ectotherms and endotherms. An ectotherm is an animal that does not have internal control over its body temperature. Ectotherms usually maintain an internal body temperature similar to the temperature of the environment. They are typically called cold-blooded. Examples include certain fishes, reptiles, and amphibians. Ectotherms must depend on external sources such as sunlight to regulate their internal body temperature. An endotherm is a warm-blooded animal that maintains a constant internal body temperature independent of temperature changes in the environment. Examples of endotherms include mammals and birds. An endotherm may increase muscle contraction to generate heat and raise its body temperature when the external environment causes a loss of heat. Endotherms will also perspire or pant to increase heat loss in environments where the temperature is high.