Osmorality: Freshwater vs. Saltwater Fish

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Osmolarity. For those of you unfamiliar with the word, it refers to the concentration of salts compared to water. The higher the Osmolarity the more salty the mixture is.

Now all organisms need to keep certain internal conditions constant, this is called homeostasis. For most organisms these need to be kept within a narrow margin: temperature, pH, glucose levels, and Osmolarity. 

So why do we need to keep Osmolarity pretty steady? Do you remember the concept of osmosis? The diffusion of water molecules across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration. Do you remember the structure of a plant cell? They have rigid cell walls allowing them to absorb water without any detrimental effects. They can also lose water and keep their structure. Do you remember the animal cell? No cell wall to be found. So what do you think happens when too much water enters it. Well to put it bluntly, it explodes. So yeah. We don't want that. On the flip side too little water will cause them to shrivel. Not sure what the negative consequences of that are... But it certainly isn't desired.

So our bodies need to be able to keep our cells in a isotonic solution. So what does that mean? An isotonic solution means that the concentration of solute (salts) is the same on the inside as the outside. This doesn't mean that there is no movement of water molecules into and out of the cell, but the net movement is 0. 

What would have in the solution was hypertonic? Well if something is hypertonic there is more solute on the outside then there is on the inside. Meaning that water will move out of the cell to the higher concentration of salt on the outside, this will cause the cells to shrink. 

If the solution it hypotonic meaning the concentration of salts is lower on the outside than in the cells, water molecules will move into the cells. Causing them to go *kaboomy*

So what does this mean for a water dwelling comrades? Freshwater fish live in hypotonic environments, meaning that water will want to go in to the fish. So what do they do? Well they only eat, they don't drink any water. And their urine is highly dilute. This allows them to control the amount of water in their bodies.

On the other side of extremes we have saltwater fish. They live in highly hypertonic environments. So what water wants to do is leave their bodies to go into the water with a high solute concentration. So what these fish do is drink a lot of water, and have highly concentrated urine. This allows them to maximize the amount of water in their bodies.

Hope this was vaguely interesting.