Osmolality and volume are both crucial variables that a body needs to maintain in balance. But out of the two, you may be surprised to know that it is osmolality which is kept under tighter control on a day-to-day basis, so let's consider the system that controls osmolality first.
From the previous page, we know that osmolality is easily kept the same everywhere within the body. For example, a large change in osmolality in the extracellular compartment will be buffered by the intracellular compartment and so it won't have as large an effect as is immediately obvious. But after a certain point, the osmolality in the body may be too high/low and it will need to adjust osmolality using the external environment.
From now on we can consider the whole body as just one beaker. Any changes in its levels of sodium or water are due to interactions with the external environment (e.g. an increase in water may be from drinking water, or a decrease in water may be from excreting it in urine).
Osmolality
Note the starting osmolality level.
Increase the amount of sodium in the body with the slider on the top, and let's pretend this is a sudden increase in sodium from the external environment (e.g. a pizza with plenty of cheese).
Note the new osmolality level after you have done this.
Notice that the system has been turned off, as nothing happens in reaction to your change. If I now give you the task of bringing the osmolality back to normal, how might you go about doing this?
Well, you could certainly just decrease the sodium back to what it was before. However, the body actually does something different. Instead, it changes the amount of water in the system until osmolality reaches its desired value.
You can simulate this by increasing the water with the slider at the bottom until osmolality reaches what it was before. (If you've forgotten the initial levels, then just press the reset button and start over)
Where does the body get this extra water from? Well, it secretes ADH to increase your thirst and make you drink more fluid, and also tells the kidneys to hold on to more pure water. Similarly, when the body's osmolality is too low, it decreases ADH, which gets rid of more pure water via the kidneys. So the body gets any extra water it needs from oral intake, and it gets rid of any water it needs to lose via the urine. However, the specifics of the mechanism by which this happens really aren't that important for now. The key point is that:
When the body senses a change in osmolality, it reacts by changing the amount of water (NOT amount of sodium)
Great! What you did on this page was to increase osmolality, and then manually bring osmolality down in the way that the body normally would respond.
Now that you understand what is happening with the osmolality feedback system:
Press the reset button on the left of the beaker.
Increase osmolality again.
Click the button below to turn the osmolality feedback system of the simulation on. Once you've done this, the simulation will be responding automatically just as the body would.
Experiment and get used to seeing what happens as you increase or decrease sodium. Note how osmolality always eventually always reaches the same value, no matter what you do.
