It’s now time to wrap up the renal system.
We’re going to do that by looking at body fluid compartments.
And some of the alterations that happen clinically within those body fluid compartments.
But what really is the body?
The body is really nothing more than a bag of water.
So it's going to be represented here by a water balloon.
So let’s just put some arms and legs on our person here.
We are about 60% water,
and that can vary a little bit between person and dependent upon someone’s body composition.
But let’s just look at a person’s average body water.
You have about 14 liters of water,
and what we’re going to call the extracellular fluid compartment.
That is made up from about 3 liters of plasma volume,
and about 11 liters of volume in the interstitial fluid.
The extracellular fluid, however, only encompasses about 1/3 of total body water.
The other component is what is located within cells,
which is known as the intercellular fluid compartment.
You have normal body water intake,
which is what you drink and what you eat during individual day.
And then, of course, you have to subtract out the body water that you lose.
So this example is a nice way to think about body fluid compartments
and we’re going to utilize this particular 14 liters of extracellular fluid,
28 liters of intercellular fluid as kind of the typical
maybe the 70-kg adult male.
So in terms of the intakes, how can we break that in?
So let’s look at fluid ingestion.
You might ingest in a normal day about 2.1 liters.
You have a little bit of water that is made from metabolism.
So this is the breakdown of glycogen. There’s going to be some release of water.
So your total intake maybe around 2,300 milliliters.
In terms of your output per day, it’s going to be very dependent upon your metabolism
and how much energy you’re producing.
So in a normal resting environment, you lose about 350 ml through the skin and through the lungs.
This just happens by just evaporation.
There’s a little bit that's lost by sweat, a little bit lost in the feces.
The urine, however, is the biggest component of water loss,
and that’s why we talk about body fluid balance so much with the kidney.
If you contrast that in someone who’s undergoing heavy exercise,
which increases their basal metabolic rate and increases their temperature,
you still have some insensible water loss in the skin through evaporation.
The lungs level of evaporation increases because your respiratory rate has gone up.
You’re now sweating, so you can lose a lot of water through sweat.
And then the feces and urine will decrease a little bit according to how much water you’re trying to save.
So in the normal environment, you might lose 2.3 liters.
During heavy exercise, you might lose 6-6.5 liters.
If we now rotate this body fluid diagram on its side,
I think it’ll better have the ability to integrate what’s happening between the various components.
So let’s start off with the intercellular fluid, which is about 28 liters.
There are cell membranes that contain that intercellular fluid.
So the reason why it’s not just floating around the body, it's within a cell.
The reason why that is important is because we have to think about the cell membrane
if there’s going to be changes in intercellular fluid volumes.
The interstitial fluid exchanges directly with the intercellular fluid.
The interstitial fluid also directly exchanges across the capillary membrane to the blood.
And so, these are how the various movements of fluid can occur.
So if you have a change in plasma volume, it first will affect the interstitial fluid,
and then second it will affect the intercellular fluid.
If you have a change in intercellular fluid, then it will change interstitial fluid, and then plasma –
same with our 14 liters being about 1/3 total body water being our extracellular fluid.
It’s much easier to change extracellular fluid than it is intercellular fluid.
Body fluid compartments don’t just have water in them.
They have ions.
So let’s go through what the different ions are in each of the body fluid compartments.
Let’s start in the extracellular fluid compartment, and let’s look at the positively charged cations.
There’s quite a bit of sodium in the plasma and interstitial fluid.
There’s a little bit of potassium, a little bit of calcium.
If we contrast that to intercellular fluid,
you see that the majority of the cations are potassium, and there’s only a small amount of sodium.
If we look at the anions, or negatively charged molecules,
you can see in the intercellular fluid compartments – it doesn’t have a lot of ions.
It’s mainly phosphate, a little bit of protein.
Most of the ions that are anions are located in the extracellular fluid compartment –
things like chloride and bicarbonate.
So where or how do we analyze both the body fluid compartments and the ions?
For this, we have to do some clinical testing.