General overview of the RAAS system: Cells and hormones Learn the important cells and hormones that are working together to control your blood pressure! Rishi is a pediatric infectious disease physician and works at Khan Academy.
General overview of the RAAS system: Cells and hormones
- The body has a really really cool way of controlling blood pressure.
- You'll hear about this RAAS-system.
- RAAS stands for Renin Angiotensin Aldosterone System.
- Let's go through this RAAS-system, kind of like an overview.
- Just looking at where things start from and where things go in terms of cells and hormones.
- Those are the two things I want to distinguish between.
- This RAAS-system begins with a set of cells.
- I'm gonna draw all of my cells as little blue houses, like that.
- The hormones they release are gonna be orange messengers.
- I'm gonna draw a little messenger, this will be a little person.
- The person is the hormone and the blue house is the cell.
- The key cell in the RAAS-system is the juxtaglomerular cell. The JG cell.
- These JG cells are actually in the kidney, but in a specific location, in the blood vessels.
- If you look closely, these JG cells are nothing more than very special smooth muscle cells.
- If you look in the blood vessels, they're actually just like smooth muscle cells.
- I'm gonna write smooth muscle, just to remind you where they are.
- Of course these are in the kidney.
- It may not look like a kidney, but that's what it's supposed to be.
- The JG cells are releasing a hormone called renin.
- When would they do that?
- Renin is gonna help us raise blood pressure.
- If the juxtaglomerular cells, these little guys, notice the blood pressure is low,
- that would be a trigger for releasing renin.
- That's the first trigger, low blood pressure.
- That's not the only trigger, there are actually 3 triggers.
- Let me write down 2 and 3.
- Let's go through what they are.
- The second trigger is a neighboring cell is actually a sympathetic nerve cell.
- We know that sympathetic nerve cells, they fire whenever something big is going on.
- For example you're running away from a bear, or let's say you're trying to win a fight,
- or let's say you're in a car accident and you start bleeding.
- Any sort of major stressor is gonna cause these nerve cells to start firing.
- When they fire, that JG cell starts releasing renin.
- So the second trigger would be the sympathetics.
- The sympathetics, or maybe sympathetic nerves.
- If these are your neighboring cells, these little sympathetic nerve cells, because they end right on the JG cells.
- Then a little ways away, still in the kidney of course, not touching the JG cells, would be the macula densa cells.
- Stay with me for this, these macula densa cells are also in the kidney
- and actually they're specifically in the distal tubule of the nephron, so remember the distal convoluted tubule.
- They're there, their interesting ability is the ability to sense sodium.
- When you have low blood pressure, not a lot of blood is moving through that glomerulus.
- Not a lot of blood is moving through the nephron as a result.
- A lot of the salt is being reabsorbed.
- So by the time it gets to the distal convoluted tubule,
- the macula densa cells, they're kind of tasting or sensing the fluid that goes by
- and they say 'There's not a lot of salt here.'
- They put 2 and 2 together and realize that the reason there's not a lot of salt, is that the blood pressure is low.
- So when they don't sense much salt, they say 'Hey, JG cells, wake up!'
- 'Do something about this, raise blood pressure for us.'
- They send a message over in the form of prostaglandins.
- Prostaglandins are local messengers.
- Unlike renin, which is more of a long distance messenger, prostaglandins act locally.
- Actually, lots and lots of cells in our body use prostaglandins to send local messages.
- The third trigger, just to summarize it, is low salt in the distal convoluted tubule.
- You know specifically that it's the macula densa cells of the convoluted tubule.
- So these are the three major triggers for renin release.
- This is all happening in the kidney.
- That's were all this action is occurring.
- But you know there are other organs involved in blood pressure control as well.
- The one that is next on our list, is the liver cells.
- The liver cells, here we go, a little house for cells, are also making a hormone of their own.
- It's gonna meet up with renin in a second and it's called angiotensinogen.
- Angiotensinogen is kind of a sleepwalker, if we would zoom in on its face, it would be asleep.
- So I'm gonna draw it that way.
- It's there and it's moving around the body, but it's not active and that's the key thing, it's not active.
- But it needs renin and renin literally chops of a big chunk of angiotensinogen.
- And if that doesn't wake you up, I don't know what would.
- Angiotensinogen becomes angiotensin I after meeting renin.
- So renin is an enzyme that cuts a big chunk of this angiotensinogen protein away.
- Angiotensin I is the result. If you'd zoom in on this guys face, it would be awake.
- Maybe even a little smile.
- Angiotensin I now floats through blood vessels and of course blood vessels have cells lining them.
- So let's draw a little house.
- These are the endothelial cells. These are the cells that are lining the blood vessel inside.
- Classically we used to think that this is almost always happening in the lungs,
- but more and more we're realizing that it definitely does happen in the lungs, but it's in other vessels as well.
- Endothelial cells in a number of parts of our body, including the lungs,
- are able to convert angiotensin I into angiotensin II.
- So angiotensin II is formed.
- This is also a hormone, so I'll draw this little person.
- Angiotensin II is happy as a clown, because angiotensin II has lots of activity.
- It's a very very active hormone, it does all sorts of things.
- I'm gonna draw that activity now.
- Angiotensin II will go out to a number of different places.
- I'm gonna draw four arrows, 1, 2 and two little ones, 3 and 4.
- It goes to four places and four different cell types are affected by angiotensin II.
- Keep in mind, at the beginning of all this, we're trying to raise blood pressure.
- Four different cell types are affected and here is the fourth.
- The first one over here is smooth muscle cells in the blood vessels.
- This is smooth muscle cells all over the body, not just in the kidney.
- These smooth muscle cells all over the body are going to contract, they're gonna constrict down.
- They're gonna cause increased resistance.
- Remember that as the blood vessels constrict, vasoconstrict, that will increase resistance.
- That's one effect.
- In a different cell type all together, in the kidney cells, you actually have the ability to,
- angiotensin II makes these kidney cells hold on to more water.
- So you have more volume.
- It helps the kidney to hold on to more water and more volume.
- Think about it in terms of stroke volume. It's gonna increase stroke volume.
- So you have increased resistance and now increased stroke volume.
- Those are two cell types for angiotensin II to act on.
- It also acts on a couple of glands.
- I'm gonna try to draw for you the pituitary gland.
- This pituitary gland is sitting at the base of the brain.
- This gland is called that because it secretes hormones as well, actually it's sending out messengers as well.
- Here's a little hormone, again in orange, remember all our hormones are in orange.
- This one is called ADH. AntiDiuretic Hormone.
- That ADH does some of the same stuff, at the end of the day, that angiotensin II will do.
- In that it will increase resistance of blood vessels
- and it will also increase volume by making the kidney hold on to more water.
- The fourth cell type, is gonna be the adrenal gland.
- The adrenal gland is here.
- It's called ad-renal because it's sitting on top of the kidney, which is the renal.
- This adrenal gland is also making a hormone, because it's a gland.
- That hormone is going to act right there.
- This is the little messenger and this is called aldosterone.
- So you've got aldosterone and ADH that are also acting on some of the same cells.
- I should rephrase that, not on the same cells, but on the same organs as the angiotensin II.
- So here aldosterone is gonna act on kidney cells to increase volume.
- ADH is gonna act on, like I said before, the kidney and smooth muscle.
- So let's scroll up and show you from the top some interesting things I want to point out.
- We've got at the very top, all the action, you remember, started in the kidneys.
- It started in the kidney with the macula densa cell and our JG cell and even our nerve endings were in the kidneys.
- One of the key target organs down here is of course the kidney.
- Things are starting in the kidney, but they're also ending in the kidney.
- Then you'll say 'Well what about the smooth muscle cells all over the body?'
- You're absolutely right it does also affects the smooth muscle in other parts of the body.
- I just want to point out the fact that the kidney is a major player in this game.
- That's one point.
- The other point is that when people talk about the RAAS-system, they're talking about certain pathways.
- They're specifically talking about, for example, this arrow right here, this hormone obviously.
- They're talking about this angiotensinogen and this angiotensin I.
- They're also referring to angiotensin II and all of its targets.
- They're gonna talk about angiotensin II affecting the smooth muscle and the two glands,
- the pituitary as well as the adrenal gland and its effect on the kidney.
- So they're really referring to all of those things.
- I want to make sure you remember that it's affecting four, at least four target cell types.
- Finally that aldosterone right here has a huge effect on the kidney as well.
- These are the important points to take away from this overview,
- that there are many different hormones involved, and I've tried to keep them color-coded all in orange,
- to make sure we keep track of them.
- And the fact that the kidney is a major player in blood pressure control.
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