In this section, we're going to finally see both osmolality and volume feedback systems working together. You can now see toggle buttons to the right of the beaker to turn both systems on or off independently, but the volume system is switched off for now.
Gently decrease sodium using the slider at the very top and observe the volume decrease to normalise osmolality.
Turn on the volume system with the toggle button to the right of the beaker.
Pay close attention to the sodium level at the top and the sodium particles in the water.
You should have seen both sodium and water very gradually rise until volume eventually reaches its normal level.
As you may have noticed, a key difference is that the volume regulation system is much slower than the osmolality system. This is because sodium conservation via the RAAS is a slower process than water conservation via ADH.
Take some more time to explore how both systems interact before moving on.
Here is a recap of the key points:
When the body senses a change in osmolality, it reacts by changing the amount of water (NOT amount of sodium)
When the body senses a change in volume, it reacts by changing the amount of sodium (NOT amount of water). This then automatically triggers a corresponding change in volume by the osmolality feedback system.
Well done for sticking through and getting this far! If you understand these two points, then you understand the fundamentals of how the body regulates both osmolality and volume, which will be very useful when trying to truly understand hyponatraemia.
This is a good time to mention some caveats:
The numbers used so far are completely arbitrary. They do not have any units because they are not meant to be reflective of real physiological values
The extent to which derangements of volume and osmolality are possible in this simulation is highly exaggerated in order to make the changes and responses easier to see, but in real life derangements in osmolality or volume don't occur to the degree or magnitude seen here.
This model is of course an oversimplification designed to emphasise the two points above.
There is one more crucial caveat which changes these principles in an important way. We'll consider this in the next section.
