Biology: Human Excretion: Urinary System Structure

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To really understand the entire urinary system, we have to kind of spiral into it and take a look at its ultrastructure, its arrangement in the organism itself. Then we'll get down into the kidney and see how the thing works, because it's a system that is intricate, and yet takes so much that you've learned in biology and makes it make sense. So this is going to be a good time. You never thought you'd say that checking out the urinary system was going to be a good time, but I know you guys are just real excited about this.
So the urinary system, let's see how it's going to work. First of all, let's look at the plumbing of the urinary system, and put some names on it. The urinary system is located pretty much centrally in your body, and the two structures called your kidneys are located on your dorsal body wall. In fact, they literally are fairly unprotected. They're one of the few organs of the body that are not somehow enclosed in some kind of rib cartilage or skull or something like that. In fact, it's one of the reasons that people who get in accidents - or in boxing even, a kidney punch is illegal, which is a punch to the back, because it can cause damage to the kidney.
So it sits on your dorsal body wall, and it's fed by a blood supply. The blood supply - anything that has to do with kidneys - you know how pulmonary meant lungs? Well, in kidneys we talk about renal. So it's fed by the renal artery, and it is cleaned out by the renal vein. And, again, we'll see what this artery does when we get down inside the kidney, but here's the thing: The kidney is going to be a filter - that's what's important here, it's a filter. So these are your kidneys. Once the kidney filters in a way I'll show you, the filtrate - otherwise known as urine - is going to end up in these two tubes, one from each kidney. And these are called the ureters. The ureters proceed down to a bag called the urinary bladder, which literally spends its day waiting to fill up, and as it fills up, nerves are stimulated, which sends a signal to you to say, "Ah, time to empty the bladder." And we empty our bladder by a tube called the "urethra." So there it is, there's the urinary system, and the plumbing. But how does it work? These are nice structures, but how do these structures have anything to do with the function of literally making and getting rid of urine? Well, let's take a look.
Let's go inside a kidney. If we take a look at a kidney - you know, you could go to the grocery store and buy a kidney, and it will look a little bit like this - and we see that the kidney is fed by blood vessels, and in comes a blood vessel. And what I really want you to see here is that there are two sections to the kidney: the outside of the kidney, which is going to be called the renal cortex - I'm going to be bringing up these words a lot as we discuss functions, so try to get them into your head - and the inside of the kidney, the inner layer of the kidney, which is called the renal medulla.
Let's go in some more. When we go deeper still, we find that the renal cortex is clearly differentiated from the renal medulla, and we see these functional units of the kidney sitting in here. Now let's just do kind of a quick glance at what's going on here, and then we'll even zoom in on this, and see it in even more detail. The functional unit of the kidney - what do I mean by that? If you think of the functional unit of the lungs, say, I'd have to say it was the alveolae - the parts that built together and were like the bricks that made up the lung. And, yes, there were tubes connected to it. Well, many of these organs are literally millions, or billions, of functional units; and this particular organ - the kidney - is literally made - besides connective tissue, there's a lot of interstitial tissue and there's a lot of interstitial fluid, and there's connective tissue - but its functional unit, the unit that functions, is called the "nephron."
A nephron is pretty much a tubule associated with blood vessels; that's what a nephron is. It is the filtration unit, it is the unit that is going to be used to take the blood, filter it, and get rid of the urine that's in the blood right now; so it's a filtration unit.
How does a nephron do that? It's a long story, but let's start the story out. Let's take a close look at a nephron, and let's get the vocabulary out of the way. Here's a nephron. What did I say? The blood vessels and the unit itself, the filtration unit itself. Let's trace the blood as it enters the nephron, then we'll trace the nephron itself. The blood comes in right here in what is called an afferent - or toward - arteriole. Afferent arteriole - remember arterioles are small arteries. Now eventually, that's going to get down to the level of a capillary, and so we have a knot of capillaries.
Now I'm going to take this and give you a big magnification of this, and then we'll come back to it. What do you see here? See this circulation portion. If I could magnify that still more, here's how it would look. You would have the afferent arteriole that would be coming in like this, and then you would have - this is going to be great artwork - a knot of capillaries. Then it would leave down here. Now we haven't gotten to the leave part yet.
But what that does is it goes inside of this portion of the nephron called Bowman's capsule, which we'll label on our diagram in a second. And Bowman's capsule looks something like this, it kind of loops around that thing, like so. So that's Bowman's capsule. I'll put Bowman's capsule over the capillary there, the capillary runs underneath it. So Bowman's capsule - so what's going to happen is blood is going to come in here, go through this knot of capillaries, and go in here. That knot of capillaries has a name - it's called the glomerulus. And as I said, this is Bowman's capsule, and we'll go back to our diagram and label those on that.
So there's my afferent arteriole - I'll just call it BC, because you guys know what that means - Bowman's capsule - and this is my glomerulus. Now remember we're tracing the blood. So then what's going to happen is we're going to leave the Bowman's capsule, and we're going to have what is called an efferent arteriole, as it moves away. Afferent and efferent are just basically anatomical terms we use - so an efferent arteriole.
Then eventually the blood is going to come down here - you see there's a few branches of it. But mostly what's going to happen, you see - look right here, it branches - but eventually we're going to go and it's going to become a vein. No, it doesn't - don't get going on the blue-red stuff; we're just showing veins in blue, because we need to color them some color. So that's a vein. But where's the filtration going to happen? Let's go over the terms here. First of all, probably the most important part of this - no, I can't say the most important part. Here I go again with the "most important part." Let's talk about this part right here. This is called the "proximal tubule," which "proximal" means "near." So that's the proximal tubule. All of this vocabulary we're going to use. This loop is called the "loop of Henle." You'll notice that there is a - if you picture that the urine is coming in here and going in this direction, there is what we call a descending loop, and an ascending loop. So we're going to refer to the descending loop of Henle, and the ascending loop of Henle.
Then we have this, which is going to be called the "distal tubule," as opposed to the proximal tubule. And then we have this - and, again, what do you think is going to happen? General overview - look, we're going to get filtrate in here, it's going to go through here, it's going to go ya-da-da-da-da, down the loop of Henle, up the loop of Henle, ya-da-da-da-da, around through the distal tubule, and what do you think it's going to be called here? Well, this is called the collecting tubule, and what do you think it collects? You got it - urine. That's where you're going to find the urine, in that collecting tubule, which then, if you go back to your plumbing, is going to end up - the urine's going to leave all the collecting tubules, go in here, go down here, end up in the bladder - and voilà, you're read to excrete. Now you're asking a tough question. You're saying, "Yes, but how do you get urine out of the blood?" I'll tell you about that one later.
Animal Systems and Homeostasis
Human Excretion
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