Antidiuretic Hormone (ADH) and Osmolality

00:01 Here, if we look at this particular graph, we have osmolality on the X-axis and the amount of antidiuretic hormone on the Y.

00:10 Normally, you go through a process in which there’s always some level of antidiuretic hormone that is secreted, and this evolves in a normal plasma osmolality, which is around 300 milliosmoles.

00:25 How does the body normally release this? Again, the hypothalamus is critically involved with the regulation of arginine vasopressin antidiuretic hormone.

00:35 Places such as the paraventricular nucleus and the supraoptic nucleus of the hypothalamus will send a signal to the posterior pituitary and release arginine vasopressin from that locale.

00:51 The process with this in terms of regulation involves both osmoreceptors and baroreceptors.

00:58 So the osmoreceptor are sensing information from what? The change in osmolality.

01:04 Baroreceptors are sending their information about changes in blood pressure.

01:08 Blood pressure is very related to the amount of blood volume that you have.

01:12 So this is both volume and osmolality interrelated.

01:18 The release of arginine vasopressin antidiuretic hormone is done from the posterior pituitary.

01:27 If we look at different volume conditions of the body, you normally look at this kind of linear relationship between osmolality and arginine vasopressin.

01:38 Normally, you’re right about 300 milliosmoles.

01:42 How about what happens if you are dehydrated? So if you’re in a volume expansion condition, that means you have taken in too much water.

01:52 You are hyperhydrated.

01:54 You’ll see that there’s a blunting of the relationship between the increase in osmolality and the increase in our arginine vasopressin.

02:03 If you’re dehydrated, or in a volume contraction state, the opposite is true.

02:08 You now accentuate this relationship.

02:11 So per change in osmolality, you get a greater change in arginine vasopressin release.

02:17 So if you think of that like this, there’s normal linear relationship and you get a volume expansion versus a volume contraction, all altering that relationship between the increase in osmolality to how much arginine vasopressin is released.

02:38 Now, where is that osmolality sensed? These come through with something called circumventricular organs.

02:45 It might have been mentioned before that what circumventricular organs do are the places in the blood-brain barrier that are a little more leaky, allowing you to sense or be able to sample portions of the blood easier.

03:00 These occur right around the ventricles of the brain, so this is the 4th ventricle and especially the 3rd ventricle.

03:08 So circumventricular organs, again, are places in the blood-brain barrier that are leakier.

03:15 This allows for sensing, so there’s sensory circumventricular organs, as well as secretory circumventricular organs.

03:23 The secretory ones allow you to move a substance across the blood-brain barrier in a more easier manner.

03:32 Now, where would you want to secrete something from? Well, maybe the posterior pituitary so you can release arginine vasopressin.

03:40 The sensory circumventricular organs – let’s review those right now.

03:45 We have the subfornical organ, which is known as the SFO, and then the organ vasculosum of the lamina terminalis, which is abbreviated as OVLT.

03:56 These are our primary places in which we’ll sense changes in osmolality.

04:02 If we review the secretory circumventricular organs, we have places like the median eminence and the posterior pituitary.

04:12 Remember, the posterior pituitary is where we release arginine vasopressin or antidiuretic hormone.

04:19 If we put these altogether, you’ll notice that they’re always in a close proximity.

04:24 So the SFO and the OVLT are pretty close to where the hypothalamus is located, and the hypothalamus is located just above the posterior pituitary.