 That's it! Hey everybody, Dr. O here. This video is going to be about the renin angiotensin aldosterone system. Some people just call it the renin angiotensin system, but I do think you'll see that aldosterone plays a big role. This is actually a special request. I've talked to a lot of former students and asked them, what are some of the things that you wish I would have taught you a little bit more about? And the renin angiotensin system is one that continuously pops up. Part of that reason is because we usually only cover it when we get to the kidneys and you'll see why the kidneys are involved. But I think it's important that we talk about it now with the endocrine system and then review it again with the kidneys. And I think you'll thank me when you move on to your program courses later. Alright, so it's called the renin angiotensin aldosterone system. So let's watch the steps here, but first let's talk about what it does. It's ultimately, its job is to bring blood volume and blood pressure back up and you'll see that's why it starts with the kidneys. So let's start on the left hand side here. The macula densa specialized cells in the kidneys sense low fluid flow, low blood flow or low sodium concentration. The reason this is a big deal is because your kidneys are a filter and they need hydrostatic pressure to work. They need the pressure of fluid flowing through them. So you could say like if you see a 20% drop in blood flow through the kidneys, the glomular filtration rate or GFR would basically become zero. The kidneys would quit working. So the kidneys, you could make an argument they are more sensitive to changes in blood pressure than any other structure in your body. So if these cells start to sense that the blood pressure is going down, they're going to freak out. And if these macula densa cells sense that, then the juxtaglomerular cells are going to secrete a compound called renin. So renin is going to be dumped in the bloodstream where it meets angiotensinogen as you can see there, which is a protein produced by the liver. So the liver produces angiotensinogen and then when it comes into contact with renin, it's going to become angiotensin 1. And now we're heading down this pathway. So angiotensin 1 is going to go to the lungs. And in the pulmonary blood, in the blood of the lungs, angiotensin 1 is going to be converted to angiotensin 2, which is probably the most important compound here, by an enzyme called angiotensin converting enzyme or ACE. And that's going to be very significant because that's where most of our blood pressure medications come from. They're called ACE inhibitors. So we'll end by talking about that. So we've gone from renin, which creates angiotensin 1, which has now been converted to angiotensin 2. So let's talk about all the key things that happen. This is really where most of the action occurs. Number one, as you can see here, angiotensin 2 leads to widespread basal constriction. So your blood vessels are going to constrict. I think for two reasons. Number one, this will prop the blood pressure up. You're trying to move the blood that you do have through smaller tubes. Number two, if less blood is flowing in other parts of the body, there'll be more blood available to flow to the kidneys. So that's why you see widespread basal constriction to decrease blood flow to other parts of the area. Next thing that's going to happen is angiotensin 2 is going to stimulate the kidneys, as you can see here, to reabsorb sodium chloride, salt, especially to reabsorb sodium, and to reabsorb water. The third major thing that angiotensin 2 will do is to stimulate the adrenal cortex to release aldosterone, and aldosterone will reabsorb that sodium, having the water follow it in via osmosis. You will also see an increase in ADH, antidiuretic hormone, which is going to further reabsorb water by opening more water channels. So that's what's happening here. So we'll kind of, let's review them one more time because this is very, very important. So I'll just go through the picture here now. So angiotensin 2 is going to stimulate the adrenal cortex to produce aldosterone. And aldosterone's job is to reabsorb sodium, having water follow it in. And as you can see there, aldosterone stimulates sodium uptake in the distal convoluted tubules of the kidneys and in the collecting ducts. Then you're also going to see antidiuretic hormone open those aquaporn water channels, which is going to lead to more water reabsorption. So in the end, what has happened? We've seen basal constriction to bring the blood pressure up and to make sure more blood is flowing to the kidneys. We've seen more water reabsorbed at the kidneys, more sodium reabsorbed at the kidneys, both of those should increase plasma volume and bring the blood pressure back up. So in the end, the kidneys saw a drop in blood flow and they were able to fix it. This is the problem though. We now live in a time where low blood pressure is rarely an issue compared to high blood pressure. You can make an argument that our ancestors would have had lower blood pressure, maybe something like 100 over 60. So it was more likely that they would reach periods of time where they had really low blood pressure. But now we weigh a lot more than them, we're less physically active. For lots of reasons, high blood pressure is a much bigger concern. And we don't really have major systems that can bring our blood pressure down. So we've decided to pharmacologically interfere with this system. And that's where I want to end here, right there in the middle. You see that angiotensin converting enzyme is what turns angiotensin one into angiotensin two. So we can actually stop that from happening with your ACE inhibitors. So that would be where many of your antihypertensive medications or your blood pressure medications would interfere with this process and keep this from happening so that you wouldn't reabsorb all this fluid. Your plasma volume, blood volume would go down and then hopefully your blood pressure would go down as well. This is also why your ACE inhibitors, this is not a pharmacology class, but your ACE inhibitors are usually not always, usually they will add a mild diuretic to them. So you're actually going to take a diuretic to cause you to get rid of excess fluid while you're taking the ACE inhibitors to stop this process from happening. Okay, kind of long-winded but very, very important and like I said, students have begged me to talk about this more. So this is the renin angiotensin aldosterone system, clearly important because so many of us Americans have high blood pressure and this is one of the ways that we counteract it. I hope this helps. Have a wonderful day. Be blessed.