Animal life on Earth can exist in a wide range of environments by adopting different means of osmoregulation, which is the regulation of osmotic pressure. Osmoregulation can be thought of as the regulation of water content, keeping internal fluids from becoming too concentrated (high osmotic pressure) or too dilute (low osmotic pressure). Osmoregulation, like energy balance (Chapter 40), is a form of homeostasis. High osmotic pressures inside a cell can damage the cell, sometimes even causing it to burst and thus disrupt some of the animal’s functions. Low osmotic pressure can lead to dehydration. Although cells and tissues can tolerate dehydration, often as high as 50% or more, excessive dehydration impairs a cell’s metabolic function because many chemical reactions, such as hydrolysis reactions that break down proteins and nucleic acids into individual subunits, depend on the presence of water.
How do cells control their internal osmotic pressure? We have seen that water follows solutes across selectively permeable cell membranes. To regulate water and solute levels, and hence the osmotic pressure inside cells, the cell controls the solute concentration of the inside of the cell relative to the solute concentration outside of the cell. At the level of the cell, then, osmoregulation is achieved by the movement of solutes, particularly electrolytes.
At the level of the organism, osmoregulation is achieved by balancing input and output of water and electrolytes. Let’s first consider how animals gain and lose water. Animals gain water by drinking water that is less concentrated in solutes (hypotonic) than their body fluids. Humans and many other vertebrates, both freshwater and terrestrial, gain most of their water this way. Animals also gain water through the food they eat. Finally, animals produce water as a product of cellular respiration (Chapter 7). This is the primary source of water for some animals adapted to deserts and oceans, where drinkable water is not readily available. Animals lose water in their urine and feces. Terrestrial animals lose water through evaporation from their lungs, and, in the case of humans, by sweating. Freshwater fish gain water through their gills, whereas most marine animals lose water through their gills.
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Now let’s consider how animals gain and lose electrolytes. Animals gain electrolytes in the food they eat. Marine animals can gain electrolytes by drinking salt water, which has a higher solute concentration than their body fluids. Marine aquatic animals also gain electrolytes as hypertonic water moves across their gills. In contrast, freshwater aquatic animals lose electrolytes through diffusion across their gills into the hypotonic watery environment. Humans lose electrolytes as well as water when they sweat. Drinking beverages high in electrolytes is helpful before demanding physical activities—