Now let's analyse what's happening. Here are some important characteristics of such a system:

• There are two compartments, left and right intracellular and extracellular, separated by a membrane phospholipid bilayer.
• This membrane is completely permeable to water, but NOT permeable at all to solute sodium
• Due to the laws of physics, osmolality between neighbouring compartments will always equalise if it can.

Because of these characteristics:

• Whenever there is a difference in osmolality between the two compartments, osmolality cannot equalise via moving of solute sodium molecules. Instead, it equalises by water rushing in from the side with relatively less solute sodium (more water) to the side with relatively more solute sodium (less water) until osmolality is the same in both compartments. This is what is happening in response to you changing the solute sodium level in either compartment.
• A result of this behaviour is that the total volume of each side of the beaker fluid compartment is mainly determined by the amount of solute sodium that side compartment has. The side fluid compartment with more solute sodium always has more water. This is because the system body will always be adjusting to make the osmolality of the two compartments the same. You can confirm this by seeing that the amount of space any molecule has to move around is always roughly the same on both sides in both compartments no matter what you do.
• More specifically, the volume on each side in each compartment is determined by:
  1. the total amount of water in the beaker body
  2. the ratio of solute sodium molecules on one side in one compartment to the other
  The total water will always be shared between the compartments in the same ratio as the solute sodium molecules.
• If plain water is added to one compartment, then it will have some effect in increasing the water level of that compartment, but only because water is being added to the whole system body. The system body will once again adjust so that the total water is shared between compartments in a ratio corresponding to the amount of solute sodium in each compartment. The overall effect is that any water added is always shared in this ratio.
• So it is not possible to increase the volume of any specific compartment simply by adding water. If water is added to the side compartment with more solute sodium molecules already, then more of it will stay there, but if water is added to the side with less solute sodium already, most of it will move to the other side compartment.
• This is why when 5% glucose is added to the blood, 2/3rds of it ends up in the intracellular compartment and 1/3rd ends up in the extracellular compartment, and only a small amount (around 8% of the total) stays in the vascular compartment (i.e. the blood plasma) as this only has around 8% of the total solute sodium in the body.

Try this out with the following:

1. Press the reset button to the left of the beaker
2. Add lots of solute sodium to the left side and remove lots of solute sodium from the right side, using the sliders at the top.
3. Try adding water to either side using the sliders at the bottom. What do you notice?

You should have noticed that no matter which side you add water to, it will end up increasing the volume of the left side. The reason is that this is the side with more solute sodium.

The body is very similar:

• As the solute of the extracellular component is mainly made up of sodium, changes in sodium have a large effect on the total amount of extracellular solute.
• The body is made up of an intracellular fluid compartment and an extracellular fluid compartment which are separated by a semi-permeable membrane.

Click the button below to make the necessary replacements and see the similarities directly:

Next steps

I hope this helped you understand, as a general concept, how volume is so closely tied to amount of sodium, and also how the relative difference in osmolality across the whole body will always be minimised. With these points in mind, we're ready to consider the whole body and how it regulates both osmolality and volume.