Osmoregulation: Definition, Challenges & Energetics

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  • 0:00 Keeping a Healthy Balance
  • 1:22 Osmoconformers
  • 3:51 Land Osmoregulators
  • 5:05 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Cells need a certain balance of water and solutes to survive, but this balance is different for different animals. Just as different is how animals regulate this balance, which you'll learn about in this video lesson.

Keeping a Healthy Balance

Did you know that your body is almost 2/3 water? What's more amazing is that this water is in constant flux. You drink new water in and excrete old water and waste products out through means like urination and sweat. the collective processes that maintain water balance in an organism are referred to as osmoregulation. We could also think of osmoregulation as regulation of the concentration of the fluids in the body of an organism, and that concentration is called osmolarity.

Do the terms 'osmoregulation' and 'osmolarity' sound slightly familiar? If you said yes, you might be thinking of osmosis, which is the diffusion of water from a low solute concentration to high solute concentration. This occurs so that both sides are balanced in terms of the ratio of solute molecules to water molecules. Too little water and cells shrivel and die, too much water and they burst. So really, osmoregulation is much more than water balance - it's a balance of everything else in that water, too!

Osmoregulation occurs differently in different organisms. It all depends on the surrounding environment and how it influences the water balance in their bodies. Let's learn about how animals keep their internal fluids in check with this unique and important process.


One way that some animals regulate their body fluids is to not regulate them at all! Called osmoconformers because they conform to their environment, these animals have body fluids with solute concentrations the same as their surrounding environments. We see this in many marine invertebrates that live in saltwater environments such as our friend Sammy the Squid. And for the most part, he has it easy because he doesn't have to regulate most of the solutes in his body. However, there are some solutes he does have to actively pump, like sodium ions, because his neurons need a lower concentration of sodium ions inside the cell than outside.

Sea and Freshwater Osmoregulators

But say that one day Sammy turns into a salmon. He is no longer an osmoconformer and instead is an osmoregulator who has to actively regulate water movement in his body. This is true of other marine vertebrates (such as sharks), freshwater fish (like Sammy's friend Tony the Trout), as well as land-dwelling animals.

Now, instead of just swimming along and being one with the ambient environment, Sammy has to actively work to maintain that important fluid/solute balance. He's used to being a squid, so he had to turn to Tony for some help. Turns out that as a trout, Tony's body has a much higher solute concentration than the freshwater around him. This means that his body is constantly taking on water (by osmosis through his gills and skin); it also loses solutes to the outside environment. To counter this, he has to actively take in salt through his gills and get certain ions through his food. And just like us, when he has too much water he can get rid of large amounts of it (while holding on to solutes) through urine.

But Sammy is a saltwater fish, so his fluid regulation is a bit different. In contrast to Tony, Sammy's body has a lower solute concentration than the surrounding seawater, and therefore, loses water to the environment instead of taking it on. His body also takes on solutes (like salt) through the same diffusion as well as from his food. Another key difference between these two friends is that Tony doesn't drink water, and why would he when he already has a hard enough time keeping it out of his body? But Sammy does drink seawater, and lots of it! This helps to bring in water that was lost to the surrounding environment through osmosis, and instead of taking in salt with his gills this is where he expels excess solutes. He also produces very little urine, which is pretty concentrated in order to help save on water loss as well.

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