Fluid Balance: Videos & Practice Problems

Osmoregulation and Nitrogenous Waste

Hi. In this lesson, we'll be talking about the excretory system and osmoregulation. Osmoregulation is the homeostatic mechanism that allows organisms to balance their solute concentration and body, which is why the excretory system is heavily involved with osmoregulation. But the excretory system has another important job, and that's getting rid of nitrogenous waste, which we'll talk about in just a moment. Now, the excretory system is made up of a few components.

The main part is the kidney, that's like the business end of the excretory system. That's this bean-shaped organ; you actually have two, one on either side of your body, and it's going to filter blood plasma and form urine. But its job is so much more sophisticated than that; we'll really get into the details in just a moment. Now, the kidney is going to give off urine that will be transported to the bladder by the ureters. These are going to be tubes basically that lead from the kidneys to the bladder, which is the storage organ for urine.

And urine will be stored there until it is ready to be eliminated through the urethra, which is the opening to the environment. Now, here you can see an example of a fish trying to maintain its osmotic balance by drinking seawater, and passing water and solutes in and out of its body to excrete solutes so that it can maintain an osmotic balance in its body. Now, nitrogenous waste is bad because ammonia is a super toxic substance, and it's only safe in the animal body if it's heavily diluted. It's going to form from the breakdown of proteins and nucleic acids. Right?

They both have nitrogens in their structures, and those nitrogens are going to be given off as ammonia. This is, for some organisms, okay because they can just heavily dilute the ammonia and eliminate it that way. Here you can actually see what ammonia looks like. It's just a nitrogen with some hydrogens attached to it; and, you know organisms that have plenty of water around them, for example, like tadpoles, they'll often eliminate their nitrogenous waste as ammonia because water is very plentiful for them. So it's okay for them to waste a lot of water diluting the ammonia because there's plenty more available.

For organisms that have less water available, urea tends to be a better choice in terms of eliminating nitrogenous waste. Now it consumes energy to make urea. Those organisms have to take their ammonia and convert it into urea. As you can see, ammonia has one nitrogen, urea has two nitrogens, and a carbon and an oxygen. It's actually basically formed by combining ammonia and CO₂.

That's a, you know, sort of oversimplified version of how urea is made. The difference is it's way less toxic than ammonia, and it doesn't need to be as heavily diluted. It can be excreted with minimal water loss, which is super important for, for example, terrestrial animals like us. We excrete urea because we don't have that endless supply of water, you know, like all around us like a tadpole for example. So we don't want to waste as much water getting rid of our nitrogenous waste.

Some organisms, like organisms that live in really dry climates, like reptiles in a desert for example, will actually go even further and choose to excrete their nitrogenous waste as uric acid. You can see the uric acid right here; jump out of the way so my head's not blocking it, and as you can see, ammonia had one nitrogen, urea had two nitrogens, and uric acid has four nitrogens. So it is a bigger molecule but it gets rid of more nitrogen. It costs more energy to produce than urea. It's more energy-intensive.

However, it's basically insoluble, so it can be excreted with almost no water loss, which is why it would be the appropriate choice for a desert-dwelling organism, like a lizard for example, where water is extremely scarce. Now, the basic point I'm trying to make is that the type of waste excreted by an organism is tied to things like its evolutionary history, and its habitat, and its level of osmotic stress. So, you know, for example, some birds excrete most of their nitrogenous waste as uric acid, but ducks, for example, excrete some, like almost half really is urea, and the rest is uric acid because, you know, they live in water, they're waterfowl, they have more water available to them. So it's not, you know, species necessarily species-specific. It really depends on a variety of factors, including habitat.

Now, the other point I'm trying to make is that there's a fitness trade-off between how much energy it costs to produce the particular molecule that will get rid of this nitrogenous waste, and conserving water. Right? You know, you might be able to save a lot of water by making uric acid, but it might cost you more energy than you can afford to produce it. So perhaps urea, for example, would be the better choice for you. You know, these are just the trade-offs of using either strategy.

Point is, nothing's perfect in biology. It's just about doing the best you can given your conditions. So with that, let's flip the page.

Kidney and Nephron

Kidneys are the major organs of the excretory system, and they're heavily involved in osmoregulation. They actually have two layers: an outer layer called the cortex and an inner layer known as the medulla. The inner layer is the saltiest layer. The kidney is mostly comprised of functional units called nephrons.

These are tube structures that transport filtrate, which we'll discuss more shortly. These tube structures are surrounded by blood vessels. Nephrons use the active transport of solutes to create salty environments that help the nephrons and the kidneys reabsorb lots of water from the filtrate. There are two layers to the kidney: the cortex, the outer layer, and the medulla, the inner layer. There are also two types of nephrons in the kidneys.

You'll find cortical nephrons, which are the most common type of nephron. The tubules of these nephrons do not extend deeply into the medulla. Here's the boundary between the cortex and the medulla; you can see a nephron that only extends a little bit into the medulla. Another type is a juxtamedullary nephron, mostly responsible for maintaining the osmotic gradient for reabsorption. Cortical nephrons filter the filtrate, create urine, and reabsorb many substances, whereas juxtamedullary nephrons, which are less common, play more of a support role.

They ensure that this area remains extremely salty. That's why they extend deeply into the medulla, unlike the shallow extension of the cortical nephrons. This diagram shows a slice of this area; it's as if you took a chunk out and zoomed in. You can also see just how riddled with blood vessels the kidney is, which is crucial for its function.

So with that, let's actually go ahead and flip the page.

Renin-Angiotensin-Aldosterone-System

Okay, everyone. In this lesson, we are going to be talking about the renin angiotensin aldosterone system that is utilized to regulate your blood pressure and your blood volume. So the Renin Angiotensin Aldosterone System has an incredibly long name, so we usually don't call it by that name. We usually call it the RAAS system or simply RAAS.

And like I said, it's going to control your blood volume, and it's going to do this by increasing your salt and water reabsorption inside of your kidneys, but it's not that simple. Based on its name, you can tell it has many components, and it's going to have many different hormones that work together to raise that blood pressure and that blood volume. Now, the loss of blood pressure or blood volume could be caused by many things. It could be caused by severe dehydration where you don't have enough water in your blood. It could be caused by blood loss, something like that.

You need to make more blood. You need to have more water in your blood. This system is going to help you do that. So, the very first cells we're going to talk about are going to be these juxtaglomerular cells in the Juxtaglomerular Apparatus. That word is always hard for me to say.

So these Juxtaglomerular cells are found in the Juxtaglomerular Apparatus. And from now on, I'm simply going to call them JG cells because that is a common shortened version of their name since it is kind of difficult to say. And these JG cells are going to be found in the kidney, but specifically they're going to be found in the blood vessels of the kidney. And what these cells are going to recognize is when the blood pressure or the blood volume drops.

Renin is going to be the beginning of this entire process. Renin is going to go on and start signaling all of these other hormones to begin. It says here that renin leads to the cleavage of Angiotensin to Angiotensin two. And actually, it's not that simple. There's Angiotensin, but it can also be called Angiotensinogen, it's just a longer name for the same thing.

And what's going to happen, I'll draw it down here, angiotensinogen is going to interact with renin. So these two are going to interact. Now, renin is made by the kidney cells, those JG cells, and angiotensinogen is going to be made by your liver cells. But angiotensinogen can't do anything in the form that it's in. But once it reacts with renin, it's going to be transformed into angiotensin one.

Angiotensin I. Angiotensin I is going to be an active form of this Angiotensin Hormone. Angiotensinogen isn't active until it interacts with renin. And what renin is going to do is it's actually going to cut part of the Angiotensinogen molecule off and create Angiotensin I. And then Angiotensin I flows through the blood vessels, and it's going to be converted to angiotensin two.

This is going to be the one I want you guys to remember because it's going to do a whole bunch of really interesting things. So how is angiotensin two actually made? Angiotensin two is going to be made by these very specific enzymes called angiotensin converting enzymes or ACE. So ACE is going to do this to Angiotensin one and it's going to change it to Angiotensin two. That is going to be the magical form.

Angiotensin two is going to do a lot of things. Angiotensin two raises blood pressure by vasoconstriction, and it's also going to stimulate the adrenal cortex to release aldosterone. Now, Angiotensin two can also directly affect the kidneys and tell the kidneys to reabsorb more water, and it can also tell the pituitary gland to secrete ADH, the antidiuretic hormone, which is also going to make you reabsorb more water inside of your kidneys. So angiotensin two is going to tell the adrenal cortex to create aldosterone.

Aldosterone is going to be the other hormone that is in this system. Aldosterone does some interesting things. Aldosterone is going to stimulate the distal tubules in the collecting duct of the kidney to reabsorb more salt. And you may be thinking, well, to increase blood volume, we don't want salt, we want water. But remember, the process of osmosis, water is going to flow towards the area of higher solute concentration.

So if inside of your body has a higher solute concentration, you're going to reabsorb more of that water from your urine, and you're not going to release as much water. So this is actually a good thing. So the ADH, antidiuretic hormone, is going to make you reabsorb more water. The aldosterone hormone is going to make you reabsorb more salts, which will inevitably also make you reabsorb more water. So as you can see here, it says water follows the reabsorption of salt, leading to the increase of blood volume and blood pressure.

That is great. Now, I also want you guys to know that this whole system can be triggered by the sympathetic nervous system as well in case there's an emergency, if you go into fight or flight syndrome or response, you will also stimulate this particular pathway, but you'll stimulate it through a different means, and it's going to be this pituitary hormone, ACTH, and it's going to stimulate the production of aldosterone, which is going to increase your blood pressure.

And you may be thinking, why do you want high blood pressure whenever you're in a fight or flight system? Well, you want this because it allows all of your organs to get all of the blood that they need because it's increasing blood volume. It's ensuring that all of your organs, your muscles, your lungs that help you get away from the threat have the blood supply that they need. So you need to reabsorb as much water as possible to create that perfect high blood volume and blood pressure.

But the majority of the time, we are going to use the renin angiotensin aldosterone system or RAAS whenever we have low blood volume for whatever reason and generally not in the sympathetic nervous system. So now I have this really neat diagram that I think is really helpful. This is basically just diagramming all of the stuff I just told you.

So it doesn't fit all perfectly. I will get out of the way in a second. The only thing that's really being cut off are going to be the lungs and the top word up there, but I will explain that. So let me scroll up a bit so you guys can see everything.

So where does this whole process start? Remember, the whole process is going to start whenever those kidneys realize that the blood volume is not high enough, so that's going to be right here. The decrease in renal perfusion or there's not enough blood in the kidneys is going to cause those JG cells to realize that there's low blood volume, and they're going to create renin. So this is going to be the very first step. So they're going to create renin, and then it's going to go this way.

So what does renin do after that? Remember, after this renin is going to interact with the Angiotensinogen hormone that is made by the liver. And once it interacts with the Angiotensinogen hormone, it's going to create Angiotensin one. And this is the second step in the pathway.

And then the ACE enzyme is going to turn Angiotensin one into Angiotensin two. And here's the ACE enzyme up here. So now we have Angiotensin two, and this is going to be the third step in the process. And remember I told you that Angiotensin two is a really important molecule? Because it does all of these really crazy cool things.

So let me go out of the picture so you guys can see the rest of it. So you guys can see that Angiotensin two is going to do a lot of things. Remember, I told you that it's going to be important because it's going to constrict the blood vessels, which are going to physically increase the blood pressure of that individual because those blood vessels are smaller, they're constricted, the blood pressure is going to go up, and remember, I also told you that it's going to cause the secretion of the ADH hormone from the pituitary gland, and this is the antidiuretic hormone, and this is going to cause the increased absorption of water in the kidneys. Okay, everyone? Now, it's also going to do even more.

It's going to cause aldosterone to be secreted from the kidneys as well, and what aldosterone is going to do is it's going to cause those salts to be reabsorbed, which you guys can see is going on right here. These salts are being reabsorbed into the kidneys, which means the water will follow and be reabsorbed as well. And, guys, remember, I also told you that Angiotensin II is important for your sympathetic nervous system. It's going to play an important role in keeping your blood pressure high during emergency situations to ensure that you have enough blood volume to stimulate your running away from whatever it might be. Okay, guys?

Now, once the blood volume has gone back to normal, what is going to happen? Once it's gone back to normal, you guys can actually see down here this red line. This red line down here is going to be a negative feedback, and so once the kidneys realize that there is enough blood volume that it is back to normal and enough water has been reabsorbed, they're going to send this negative signal back to the JG cells to tell them to stop producing renin, which will stop the process, which means that angiotensinogen will no longer be turned into angiotensin one and eventually into two, and aldosterone will not be secreted. So this is only going to be used whenever you don't have enough blood volume or your blood pressure is too low or you are in an emergency situation. But when you're not and your blood volume is perfectly fine, your kidney is being told to not make the renin hormone.

Okay, everyone. I hope that was helpful. I know that these particular pathways can be rough, but it is really, really interesting how all of these different hormones actually work together to control your blood volume. Okay, everyone, let's go on to our next lesson.