Now, we are going to put a bunch of the pieces of the puzzle together.
We’re going to talk through the control and regulation of blood, and urine volume.
To do this, we’re going to step one step at a time through controlling osmolality
and controlling volume at the same time.
To look at the integrated control of blood volume and osmolality,
the first thing that we need to do is to look at the osmolality component.
So what is osmolality?
This is the amount of solutes in the solvent.
The more concentrated the solutes are, the higher the osmolality.
This what occurs in the blood if osmolality increases.
Where do you sense this change?
You have osmoreceptors located in the OVLT and SFO.
These will stimulate thirst, as well as AVP, arginine vasopressin antidiuretic hormone release.
Let’s go through the AVP portion first.
There are certain neurons that are located in the hypothalamus.
These are hypothalamic nuclei – the paraventricular nuclei and the supraoptic nuclei.
These particular portions will garner the information from the osmoreceptors,
and then cause the release of arginine vasopressin.
Arginine vasopressin is released into the blood,
travels around to the kidneys,
and remember, that will allow us to reabsorb more water.
If you reabsorb more water, you excrete less.
Therefore, there’s more free water in the system
that feeds back and will decrease the osmolality
because we have more solvent that will dilute the solute in the solvent.
Besides retaining more water,
you’ll also be able to have a greater thirst so you will garner more water.
These thirst receptors are located also in the hypothalamus,
and they will allow you to increase water intake – or become thirsty.
As you drink more water, you’ll increase the free water in the system,
and that too will feedback to decrease osmolality.
So both, through sensing the osmoreceptors,
do 2 things – prevent water loss and stimulate you to want to garner more water, or drink more water.
Now that we have osmolality controlled for,
let’s talk about effective circulatory volume.
So effective circulatory volume is the amount of blood that’s going to be circulating around in the circulation.
Here, it’s going to be primarily affected by 2 things.
One is how much water you’re able to retain,
and the other is how much pressure is in the system.
So let’s talk about pressure.
Arterial pressure is sensed from the carotid and aortic bodies in the baroreceptors,
as well as the volume is also sensed through the low pressure baroreceptors in the heart and the large vessels of the lungs.
These 2 baroreceptor populations – the arterial and the cardiopulmonary baroreceptors –
will feedback information to the brain,
and that will tell us what pressure the blood has.
Now, besides looking at blood pressure,
you can also think about how much volume is circulating.
To do that, we need to talk about the kidney again.
So we have a juxtaglomerular apparatus, and that is going to sense the amount of afferent arteriole pressure.
If it is low, you’re going to stimulate the renin-angiotensin aldosterone system.
So these juxtaglomerular cells will release renin in response to low effective circulatory volume.
Renin gets converted to angiotensin I, and then angiotensin II.
And it’s angiotensin II that we’re most concerned with because that feeds back to the thirst receptors.
Besides stimulating you to want to drink more, it also stimulates you to want to eat more salt.
So this is the salt drive force to increase the amount of sodium in the body.
If you increase the amount of sodium, you will naturally increase the amount of blood pressure and circulatory volume.
That should relieve the decrease in effective circulatory volume and remove that into a normal range.
Thus, this effective circulatory volume aspect can all be regulated on its own.
The salt appetite through the sensing of baroreceptors in the aortic arch carotid bodies,
from cardiopulmonary receptors, and then in the juxtaglomerular apparatus.
All of these are coming together to sense this effective circulatory volume decrease
and then try to correct it.
So in this case, we had a situation in which you were able to have a decrease in effective circulatory volume
and increase in osmolality.
Hopefully by going through this process of increasing the sodium appetite,
by increasing the amount of free water,
you’re able to reverse these processes.
Therefore, effective circulatory volume goes up,
arterial blood pressure goes up,
and osmolality goes down.
This is the correction factor in this interrelated process of controlling both osmolality and effective circulatory volume.