Transcription
I'm about to draw this diagram of a nephron. It's the functional unit of filtration in the kidney. But before we jump to the whiteboard and do that, let's take a look at the kidney model and see where these things are located.
So here on my model is one of the nephrons. You'll see this loop that comes down, and you notice that loop is also in the diagram. There's going to be this capsule that surrounds some blood vessels. That's going to be right in there. You can kind of see those yellow dots. And there's going to be a collecting duct, which we see on the right side of our diagram that I drew. And that collecting duct is going to take the urine that gets filtered out of the blood. It's going to take it down to the bottom. It's going to enter into the calyx, then to the major calyx, and it'll travel down here through the ureters to get down to the bladder.
These nephrons, which include the capsule, the tube that loops down, the collecting duct, there's millions of those throughout the kidney. They're located in the renal pyramids, including the renal medulla, and then part of them is in the renal cortex. Those tubes of the nephron are also surrounded with blood vessels that come into the kidneys. So we see those blood vessels right there. We also see those in the diagram that I've drawn. So now that we know where those nephrons are located within the kidney, let's jump to the whiteboard and get started.
So there's two main regions in our diagram here. Here at this top half, above the dotted line, is the renal cortex, and everything below the dotted line is the renal medulla. Also being drawn right now, we've got something that's called the Bowman's capsule. This Bowman's capsule is going to be the beginning of the nephron where filtration first happens. But before we get into that, we have to deliver some blood to the nephron. And so we're going to have a renal artery coming in, or really a branch of the renal artery that's going to be bringing blood in. And if you notice here, once that renal artery gets to the Bowman's capsule, it's going to kind of split into a couple sections and sort of form this thin coiled section of artery here that kind of bunches up in a ball like this inside of the Bowman's capsule. Now, that bunch of artery is called the glomerulus. So blood will come in through this branch of the renal artery into the glomerulus.
Now, if you notice, the branches of the glomerulus are a lot thinner than the artery coming in. And so imagine if you try to take a bunch of fluid that's in a vessel like this and then suddenly force it into a thinner or smaller vessel. Well, that's going to greatly increase the pressure. That increased pressure from trying to force a lot of fluid into tiny little vessels is going to cause a lot of that fluid to leak out of the glomerulus into the Bowman's capsule. And that's the whole point of this is to get fluid from the glomerulus into the Bowman's capsule. We call that process filtration. And basically, that's going to mean that the blood plasma, and it's going to be about 20% of the blood plasma that comes through here, is going to exit the glomerulus and go into the Bowman's capsule.
Now, once that blood plasma, or that fluid, is in the Bowman's capsule, we call it the filtrate. So if you hear me refer to the filtrate throughout the rest of the video, I'm talking about fluid from the blood that was blood plasma that once it's in the Bowman's capsule and then the rest of the nephron, we stop calling it blood plasma, we start calling it filtrate. So in a period of 24 hours, we're going to filter about 180 liters of our blood plasma through the Bowman's capsule to become filtrate. Now, if we urinated out 180 liters per day, we would get dehydrated super quick, and we would not have enough fluid for our bodies to survive. So luckily, filtration is not the only thing that's going to occur. We also have the process of reabsorption. So we're going to take a lot of fluid out of our blood, but we're going to put most of it back. And we're just going to keep out stuff that we want to get rid of. That could be water if we have too much water in the body. It's going to be other wastes that we're trying to get rid of. It's going to be salt. So if we have too much salt in the body. And it's really kind of a weird, inefficient system because why not just take out the stuff we want to get rid of? But that's not how our kidneys work. We're going to filter out lots of things, even stuff that we want to keep, and then we'll selectively determine what do we put back into the bloodstream to keep in the body. And that's how we'll regulate what we urinate out and what we keep inside. Filtration is going to take place in the Bowman's capsule and glomerulus, and then reabsorption is going to take place through the whole rest of the nephron that we're about to draw on our diagram.
Okay, so where does that filtrate go next? Well, first, it's going to go through something called the proximal convoluted tubule. Proximal because it's right after the Bowman's capsule, it's going to be proximal to it. We'll have a distal convoluted tubule that's farther away, but proximal because it's close to the beginning. Convoluted just means that it sort of meanders, like it doesn't just go straight to where it's going. It's going to kind of take some turns and stuff like that. Once it reaches the end of the proximal convoluted tubule, it's going to descend down into the medulla in what we call the nephron loop, or sometimes called the loop of Henle. So far, everything we've done has been in the renal cortex, or the outer layer of the kidney. But now this loop is going to dip down into the medulla, and we'll talk about why that's important in a minute. But just know for now that it dips down into the medulla and then it comes back out of the medulla into the renal cortex. There, the nephron loop is going to connect to the distal convoluted tubule. Notice it's kind of windy and convoluted, just like the proximal one was. Once it gets to the end of the distal convoluted tubule, it's going to connect with something called the collecting duct. If you notice in the collecting duct, there's lots of branches coming out of it. That's because there's lots of nephrons that all connect to the same collecting duct. But the big idea here is that all of the urine that we're filtering out is collecting together into collecting ducts, which then joined together to form the calyx, which is going to connect to the ureter so we can bring that down to the bladder. So our end of the collecting duct there will connect down to the ureter.
But we haven't talked about that reabsorption component yet. Let's go ahead and add in the other blood vessels here. So we said that we have blood coming in. About 20% of our blood plasma is going to filter out into the Bowman's capsule from the glomerulus. But 80% of the plasma is going to stay in these blood vessels, as well as all the red blood cells and white blood cells. Those are going to stay in the blood vessel, they're too big to really go through filtration here and end up there. Also, a lot of our bigger proteins in our blood plasma are too big to actually get filtered out. So most of those will stay in the blood vessel here. That blood vessel is going to take that blood over here to where the nephron loop is, so that reabsorption can occur.
Let's start in the proximal convoluted tubule and see what happens here. So we filtered out a bunch of the fluid, and then H2O and nutrients are going to get reabsorbed back into the bloodstream. Like I said, it's kind of inefficient. We filtered a bunch out, but we're going to put most of that back into the bloodstream. About two-thirds of the water that we filtered out is going to reabsorb back into the bloodstream from the proximal convoluted tubule. And a lot of the important nutrients that we need, such as sugars and vitamins and stuff, are going to diffuse out of the proximal convoluted tubule into the blood vessels again, so that we don't urinate out all of that good stuff that we need in our bloodstream.
Okay, so about two-thirds of the water and most of the nutrients have been reabsorbed back into our artery here. And then as this nephron loop descends down into the medulla, it's gonna get increasingly salty. In other words, it's gonna have a lot more sodium, it's gonna have a lot more chloride and other ions in that area. We're going to talk about how the medulla got so salty in just a minute, but for now, know that this is very salty. It's a lot more salty as we descend down into the medulla. And what salt does is it essentially attracts water. So as the nephron loop descends, water is going to be diffusing out of that nephron loop back into our blood vessels. And that process of water diffusion because of a highly salty area is osmosis. So the big idea there, water is leaving the descending nephron loop and it's being attracted essentially to the saltiness of the medulla. And that water is going to enter back into our bloodstream here.
From there, the filtrate is going to wrap around the nephron loop and enter the ascending nephron loop. Now, in the ascending nephron loop, we suddenly have an area that's impermeable to water. Water can't leave or enter in this ascending part of the loop. So it's different physiologically than the descending part. In the descending part, water was leaving due to osmosis. In the ascending part, water can't diffuse back and forth into or out of the nephron loop. What is happening though, is sodium and chloride ions are being actively transported out. They're being pumped out. And that's what's actually making the medulla salty in the first place is that we have pumps for sodium and chloride to pump out those ions, which is going to make this medulla salty. And just because it's happening here on the diagram, it's really happening all throughout this whole medulla area right here. This active transport of the salt ions is really what makes this osmosis part earlier on in the nephron loop possible. But of course, it's active transport, so it takes a lot of energy. So we need a lot of energy for our kidneys to function. Like I said, it's a little bit inefficient.
Okay, great. From there, the filtrate is going to travel through the distal convoluted tubule. And at the distal convoluted tubule, water can diffuse out. Note that I said it can diffuse out, I didn't say that it will. That depends on something. The water can also diffuse out from the collecting duct, but again, that's going to depend. This is where the true regulation happens, where our body is going to decide, do we want to try to conserve as much water as possible, or do we want to try to urinate out a lot of water? So basically, if you're dehydrated, we're going to reabsorb as much water as we can. The distal convoluted tubule will become very leaky to water, so that water is going to be leaving. And so will the collecting duct. The collecting duct will become very permeable to water, so the water can leak out into the salty medulla area. And again, that's if we're dehydrated. We're trying to conserve water. We don't want any water leaving through the collecting duct to the ureter, because then we're going to lose that water whenever we urinate. Instead, if we're dehydrated, we want to get any water that we can that's still left over in our filtrate. We want to get that water back into the bloodstream. So we're going to let it leak out of here and diffuse back into our bloodstream so we can keep it in circulation and keep all the water in our body.
But how does our body regulate that? Well, there's a special hormone that's involved. And so this process of reabsorption of water in the distal convoluted tubule and collecting duct is only going to happen if ADH is present. ADH stands for anti-diuretic hormone. It's a hormone released by the pituitary gland, that little gland hanging off the front of your brain. It's also known as vasopressin, but I like the term ADH because it describes what it does. A diuretic is anything that makes you pee more, and an antidiuretic is something that would make you pee less. So anti-diuretic hormone makes you pee less. How does it make you urinate less? Well, it's going to cause the distal convoluted tubule and the collecting duct to leak more water. So water leaves there and enters back into the bloodstream. The plus in my diagram just means that it stimulates water getting reabsorbed back into the bloodstream.
But what if we're not dehydrated? What if we've been drinking plenty of water and our body actually has more water than it needs and it wants to get rid of some of that water? Well, then our brain, our pituitary, is going to stop releasing ADH, anti-diuretic hormone. If anti-diuretic hormone is not present, well then the distal convoluted tubule and the collecting duct will not be permeable to water. So in other words, all of the filtrate that enters in the distal convoluted tubule is going to stay there as it travels through and down the collecting duct. All that water will stay in the collecting duct and then into the ureter so we can urinate out a lot of water. That would make our urine a lot lighter because it has so much water present. In other words, it's very dilute. To contrast that, if we had a lot of ADH, well then we're going to be reabsorbing most of that water. And so that's going to make the urine a lot darker because it has less water compared to the amount of solute or other waste that we're getting rid of.
So to summarize one more time, if we're dehydrated and our brain is releasing a lot of ADH, that's going to cause the distal convoluted tubule and the collecting duct to become permeable to water or leaky to water. Water is going to get reabsorbed back into the bloodstream at a greater amount, so that we're not producing a lot of urine because we're trying to keep water in the body. However, if we've got plenty of water in the body, our brain won't be releasing ADH. And so if there's no ADH, then the distal convoluted tubule and the collecting duct will not be permeable to water. That water is going to stay in the urine as it goes down the collecting duct and will produce a lot more urine.
Filtration and reabsorption in the kidneys is a lot more complex than what I've presented in this diagram. There's lots of other ions involved, there's other hormones involved like angiotensin one and two and aldosterone and a bunch of other things. But this is a pretty good overview of how the system works.
All right, that was a lot of information. Let's do a quick recap here. Blood is going to enter into the glomerulus from a renal artery. About 20% of that blood plasma is going to get filtered out into the Bowman's capsule. That filtrate will travel through the proximal convoluted tubule. About two-thirds of the water that got filtered out will get reabsorbed back into the bloodstream just here in the proximal convoluted tubule. Nutrients also get reabsorbed so we make sure we keep all those important nutrients in the bloodstream. The filtrate will travel down the descending nephron loop where more water is going to diffuse out in the process of osmosis because this medulla is very salty. So water is going to diffuse out back into the bloodstream here. As the filtrate goes up the ascending nephron loop, sodium and chloride ions are going to get actively transported or pumped out of the nephron loop, which is what caused this medulla to be salty in the first place, which is driving the osmosis which is happening in the descending loop. As that filtrate passes up into the distal convoluted tubule, more water can be reabsorbed into the bloodstream in the distal convoluted tubule as well as the collecting duct. But that's only going to happen in the presence of ADH. Our pituitary gland is going to release more or less ADH in order to regulate how much of this fluid gets reabsorbed. And that's how we'll regulate whether we produce a lot of urine or just a little bit of urine. Any fluid or waste that doesn't get reabsorbed will travel down the collecting duct to the ureter and then to the bladder. We can expel it in the process of urination.
Speaking of which, I need to go use the bathroom. But before I do that, take a moment, pause the video, and see if you can explain all of the stuff that I just explained. Talk about the process of filtration, what happens in the different tubules and loops, and then how this is regulated by ADH at the end. And if you can use the diagram here to explain that whole process, then you know this process pretty well. Hey, uh Mortimer, can you believe this is the last anatomy and physiology video of the year? What are we gonna do all summer?