9.4 Water Balance
The body’s internal environment of extracellular fluid must maintain a constant volume, solute content, and often temperature. Humans and other organisms require a stable aqueous environment to survive. They carry this aqueous environment inside their body, and they must continuously replenish and maintain it. Aquatic organisms must also maintain their internal environment, since the conditions of the external environment change in terms of solutes and temperature. Atlantic salmon spends part of its life cycle in both freshwater and saltwater environments. For the first few years of its life, the salmon lives in freshwater rivers and lakes, where the solute concentration is less than the concentration within its body. This concentration gradient results in the uptake of water into the salmon’s internal environment, and therefore homeostatic mechanisms must expel the excess water from the body. As the salmon matures, it abandons the freshwater environment for the salt water of the Atlantic Ocean, where the concentration of solutes is higher. In this saltwater environment, the salmon experiences a continuous loss of water from its body, which it must replenish. Because water is not as abundant on land as it is in aquatic environments, terrestrial organisms require mechanisms to conserve water and maintain a homeostatic balance of solutes within their body.

OSMOSIS – define the following: During osmosis, water molecules move from a region where they are highly concentrated to a region where their concentration is lower. This movement occurs across a selectively permeable membrane, which allows water but very few solute molecules to flow through. The different water concentrations on the two sides of the membrane are produced by different numbers of solute molecules. The side with a lower concentration of solutes has a higher concentration of water