In this section, we start off with a working osmolality system, i.e. a feedback system that keeps osmolality tightly in range, specifically by changing water. Let's now think about how to keep volume within the same range.
We also start off with a 'dehydrated body' (the current water level is lower than the newly displayed 'normal volume' level) but the osmolality is normal.
How might you go about increasing volume back up to normal? You may try just increasing the water level. This would be similar to a person drinking lots of pure water, or their kidneys conserving lots of pure water. Try this by using the slider below the beaker and see what happens.
This may have been surprising, or it may have been obvious from the previous section.
If you fill the body with lots of pure water, then the water simply gets lost, because we have a working osmolality feedback system. Increasing pure water content decreases osmolality, and so the system works hard to bring osmolality back to normal by getting rid of any extra pure water that goes into it. Osmolality is very tightly controlled, and this is because it equalises across the whole body and affects a huge amount of cellular processes. The system is set up in such a way that this is prioritised over volume.
So if all the body did to get keep volume controlled is change water levels, then it would always only be very temporary and our kidneys would constantly be working very hard.
Here is the key: now try increasing the amount of sodium instead.
Note that you can freely change the water level to whatever you want it to be, simply by changing the sodium amount. In fact, this is exactly what the body does in response to changes in volume.
Under normal circumstances, and as long as the body has a working osmolality regulation system, the body cannot directly influence volume by changing water itself, because as you've seen above, this disrupts osmolality in such a way that any change in water is counteracted relatively quickly. However, when sodium is changed, it disturbs the osmolality feedback system in such a way that water follows sodium. We have seen how across a semi-permeable membrane, sodium amount is closely tied to water amount, but I hope this shows how this is also true at the level of the whole body and its regulation of osmolality and volume. The key point is that:
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.
How exactly does it do this? Various baroreceptors and sensors of perfusion across the body control the activity of the renin-angiotensin-aldosterone system. This then tells the kidneys to conserve sodium when the body volume is perceived to be low, or get rid of more sodium when the body volume is perceived to be high. Once again though, the mechanism isn't the focus of this tutorial.
Click next to see both the osmolality and volume feedback systems working together.
