Now we are going to take a look at how you measure the ions and fluid compartments.
In the US, we oftentimes use something like a basic metabolic panel or what we term a Chem-7.
This involves 7 different substances: sodium, potassium, chloride, bicarb, glucose, blood, urea, nitrogen, and creatinine.
If you want to get a little bit more expanded panel,
you can get other things like calcium,
the total protein concentration albumin.
These are all very important aspects to get a good feel for the ions,
the osmolality potential, as well as the osmotic potential.
If you want to measure osmolality directly, you have to put it into a machine.
We do that with either freeze point depression or vapor depression.
If you don't have those available and only have the Chem-7, you can calculate it.
And this calculation is just done by this formula:
you take 2 times the sodium concentration, glucose divided by 18
and the blood, urea, nitrogen divided by 2.8.
So I provided you just an example here at the bottom.
These are very typical sodium values of 140,
glucose value of 80 mgs/dL, and a BUN of 8 mg/dL.
If you go through the formula, it comes out to be 287 milliosmoles.
And this is a very typical blood value.
So the interstitium and the blood will be right around this 285-287.
The intercellular component is just a teeny bit higher, maybe around 300 milliosmoles.
So this is what is going to be circulating around in the body in comparison now to what is within a cell.
We don't really know what cell osmolality is on a minute-to-minute basis
because we don't really puncture the cell to measure its fluid concentration.
We only measure what's in the blood.
And that is why, measuring blood levels of various substances is very important for us clinically.
Now what else is important besides the osmolality?
And that is the oncotic pressure.
So oncotic pressures help us to determine if we are going to move fluid into a blood vessel
or it's going to be moved out of the blood vessel.
And this is based upon proteins.
So we have the total protein concentration,
the albumin, which is the major component of a serum protein analysis.
We have a few globulins that are present, but these are more minor in nature.
So we also look at total protein and albumin as our primary factors to determine the oncotic pressure.
Now that we have our ions being measured,
we are able to measure our osmolality and measure our oncotic pressures.
How do you determine how much is in each fluid compartment?
Some of these fluid compartents we can directly measure, and other ones we have to calculate.
So let's go through the first one, which is probably the most important which is plasma volume.
For plasma volume, we can directly measure this by radioactively tagging albumin
or we could tag it with a dye such as Evans blue dye.
Then we can look at it's concentration and determine the plasma volume.
We can also measure extracellular fluid volume by looking at another radioactive dye with sodium
or we could give a substance like inulin or mannitol and we look at it's concentration.
We can't directly measure interstitial fluid volumes.
But we what we can use is back calculate it from the extracellular fluid compartment minus the plasma volume.
Finally, we can measure total body water
and this is done also through nuclear medicine where you radioactively tag water
and this can be measured overtime to see someone's total water clearance.
Once you have this particular measurement and the extracellular fluid compartment,
you can use that to calculate the intercellular fluid
by taking the total body water, minus the extracellular fluid volume.
And this gives us your intracellular fluid.
Because again, you cannot directly measure intracellular fluid.