ZOO 138, Monday, February 24, 1997, 12:00 p.m.
The next topic is the function of the vertebrate kidney.
First, we need to understand the anatomy of the kidney. Andif you take the kidney out of your rat and you section it, whenyou pull it out a little smaller than this, what you'll see issomething that's shaped like that, a kidney-bean shaped. It hasa connective tissue tube attached to it, which is called the "ureter."
If you cut it in the plane of the board, what you will see insideis something that's shaped sort of like that. The connective tissuearea here expands within the interior of the kidney.
And this expanded area here includes all of this area that Ihave crosshatched, and that is called the pelvis, the "renalpelvis." "Renal" is a word that means kidney. Soit's a name that occurs over and over again in these terms thatwe're going to be learning for this.
And when you look at the tissue that you have cut, you'll seethat there is very a distinct difference. Even to the naked eyethere is a distinct difference to the 2 parts. The outer partis very rough. And if your rat was injected with red and bluelatex, that's where you'll see some small arteries and veins.That outer area is called the "cortex."
And "cortex" is a word that anatomist use many timesin different locations just for the outer layer of anything. Youradrenal glands have a cortex. Your brain has a cortex. And theinner part is also in those cases called the "medulla."It's smooth. There are no blood vessels, although there are somecapillaries in there, obviously. And there is this a nipple-shapedstructure in the medulla which is called the "renal papilla."
And "papilla" just means a nipple-shaped structure.And it is surrounded by the renal pelvis. The pelvis is like afunnel. This secreted tissue expansion is like a funnel. And thepapilla is in the middle of it. What happens is that the urineis formed by a series of processes that I'll be talking abouthere. And it comes out of the end of the renal papilla. And it'scollected in that funnel of the renal pelvis and it trickles downthe ureter to the bladder where it is stored.
Now, in order to understand those processes that lead to theproduction of the urine, we need to understand the structure ofthe something called the "nephron." Which is also sometimescalled a "renal tubule."
Those are synonyms, the nephron and renal tubule. It's a tubularstructure and there is a million or so of them in each of yourkidneys.
So we're not going to draw all of them, we're just going todraw one of them. But this is a drawing that I do want you tobe able to reproduce. If one of the possible essay questions onthe final midterm or the third midterm is to discuss this businessof structure and function of the kidney, even if it isn't that,I have some people diagram it to produce this diagram and labelit.
So let's just look at the first part the diagram. This is across-section through a 3-dimensional structure. So this wouldbe a spherical structure here with this indented area. And theinterior of this spherical structure is continuous with this tubularpart that is, in fact, very highly twisted and contorted in 3dimensions. It folds back on itself.
You know, if you took a piece of spaghetti and dropped it ona plate, it would be folded and bent. That's what this twistedpart is, it gets part way, about half way through this nephronand it bends in. We'll see what that "in" means. It'srelevant to the structure over there. And after it bends in itbecomes very thin. It's not the cross section, the width of thistube is suddenly reduced. And that's an important part of theanatomy of the thing.
Be sure to include that. And this thing is U-shaped. And afterit comes -- gets through making a U, it expands back up again.
All right. I didn't draw that very well.
So the diameter of this tube suddenly reduces and the thinggoes into this U. And then when it gets back up towards the topit, broadens back out to its original diameter.
And then there is another sort of twisted end segment whichis just as twisted as the first part, much more twisted than I'mdrawing it here. And this connects into another tube that hasother nephrons connected to it.
So, again, there are many other nephrons connecting into thesame last -- this last little terminal segment which is calleda "collecting duct." And a large number of nephronswill be collected up to this collecting duct.
So I do want you to be a able to produce the diagram. You don'thave to put a lot of fancy contortions and convolutions here,but I do expect to see this thing folded over.
In fact, it is important, and I'm not going to explain to youthe importance of this, but when you get into a more advancedphysiology class, you'll find out that this is important, thatthis bent part right here that is close to the top of the U partthere has some real important interesting control stuff thatgoes on because of the proximity of those 2 components. So that'san important part of the diagram.
The fact that this is U-shaped and thinner is also importantin terms of its function. You don't have to lot have a lot ofcontortions in it or make it fancy. This is a diagrammatic representation.I don't expect you to be an artist. I want you to be able to labelthe parts of this structure. And we need to start with some vascularstructures, some blood vessels.
So inside of this little indented area here is a little tuftof capillaries. It's called the glomerulus. And "glomerulus"is Greek for a ball of yarn. So this looks like a tuft of bloodvessels in the capillaries. Blood is brought to the glomerulusby the afferent arterioles. And blood is carried away from theglomerulus by the efferent arterioles.
Afferent and efferent are 2 words that you will encounter overand over again in anatomy which differ by only a single letter.Anything called afferent is used with respect to nerves. It'sused in respect to the branchial arteries in the fish that youhave looked at recently.
Anything that is called "afferent" is carrying somethingtowards some point of reference. In this case the point of referenceis the glomerulus. And anything called "efferent" iscarrying the same thing away from that point of reference. Youhave efferent branchial arteries in fish and afferent branchialarteries.
How can you keep track of afferent and efferent? It's not goodEnglish, but afferent carries something at something. And efferentis carrying something exciting. If you remember "exit"means efferent, you will be ale to remember these. It's importantto remember those arterioles, they are not branchial arteries.They're arterioles. They are very small. An arteriole is not thesame thing as artery.
Now, there are some other structures here. The ball-shaped thing,which is the glomerulus in the middle is called "Bowman'scapsule."
And the tube that connects the Bowman's capsule to the U-shapedstructure is called the proximal convoluted tubule. That convolutedtubule, the one that carries the fluid away from the U-shapedstructure, is called the "distal convoluted tubule."
Again, "proximal" and "distal" are 2 anatomicalterms that you will encounter over and over again. They just meancloser to and further away from. The "proximal" segmentof your arm is the upper arm closer to the body. That's the proximalsegment.
Your forearm is the "distal" segment, it is furtheraway. So proximal and distal. What's the point of reference? Itis the U-shaped structure.
If the point of reference wasn't the U-shaped structure, youwould use the words proximal and distal. So what is the pointof reference? It's Bowman's capsule. What would be the words youwould use if your point of reference was the U-shaped structure?What would you call this over here? You would call it afferent.This is bringing fluid to this, and that's going to be carryingit away. Never mind that. What they are really called is "descending"and "ascending."
This is the descending limb. And the U-shaped structure is calledthe "Loop of Henle." So the first part which is attachedto the proximal convoluted tubule is the descending limb of theLoop of Henle -- I'm abbreviating Loop of Henle "LOH."
The Loop of Henle, if you are asked to structure it, if youwrote LOH, I wouldn't give you full credit. If you wrote Loopof Henle once and in parentheses you put "LOH," youcan use it the rest of the time. You must first define this descendinglimb. And this is the ascending limb.
Then this last part here is the "collecting duct."
And when you come back on Wednesday, we will figure out wherethe nephron is oriented and located with respect to the kidney,and we'll be talking about the production of the urine.
ZOO 138, Wednesday, February 26, 1997, 12:00 p.m.
Today I want to finish talking about the function of the kidney,and remember I had just diagrammed the anatomy of a nephron. Soyou should have a diagram that looks something more or less likethis. So that's the nephron itself. There is an afferent arterioleglomerulus and an efferent glomerulus. I left out one item ofthis circulation that you want to add into this diagram. And thatis that this efferent arteriole gives rise to a capillary bedwhich is called the "peritubular capillaries."
"Peritubular" means around the tubule. Around therenal tubule. And I'm not going to draw it in because it's wrappedaround all the rest of the nephron. There is even a looped-shapedsegment that goes down with the Loop of Henle. And if I were todraw that in it would make it difficult to do the rest the diagram.That's the anatomy of the system.
And what I want to talk about today is the function of the system,how does this system operate to produce the urine?
We can identify 4 steps or 4 processes that are involved inthe production of the urine. And so what I'm planning to do hereis put all 4 of them up here so you get the big picture andgo back and review them.
The first step, that is chronologically the first thing thathappens is the "filtration." And one of the things thatI'm going to be doing in this lecture is to present some numbersabout the volume of fluid that's involved in these various processes.You might want to leave a little room to the right, I'm goingto run a tally sheet here.
Second process is "reabsorption."
And reabsorption has 2 components. So skip a couple of linesthere.
The third major step or process is called "secretion."Secretion.
And the forth and final one is "concentration."
And in talking about these different processes you want to keeptrack of where they are occurring and what causes them to happenwith the mechanism of the processes as well as the volume of fluidthat's involved, if that's a relative issue. Filtration occursin a Bowman's capsule. Actually what happens is blood passingthrough the glomerulus has a fraction -- a fairly good fractionof its fluid pushed out of it.
About 20 percent of the plasma that is passing through the glomerulusis physically squeezed out of the capillary and across the wallof the Bowman's capsule into the interior of the Bowman's capsule.
Technically, we should call this ultra filtration, because inthe process of doing this filtration the pores through which thefluid is being pushed are so small that the large protein moleculesare retained inside the wall of the capillary and the glomerulus.And so when a scientist is able to do a filtration using a filterwith that small of a pore they call it ultra filtration.
And so what is produced as a result of this process we referto as the filtrate. It appears here. So it's being pushed outof the vasculature and into the lumen of the Bowman's capsule.And it is a protein-free extract of the plasma. It has everythingelse that that plasma has in it, except for large proteins moleculesthat can't get through the very tiny pores in the capillaries.
And that means that it has a lot of good stuff in it. A lotof important things that the body wants to keep. It has whatevernutrients are being transported into the blood, like sugars andamino acids. It has a lot of sodium chloride which sometimes thebody has a hard time keeping ahold of.
It also has all of the waste products that the animal is goingto want to get rid of. Metabolic breakdown of products and hormonesand urea or uric acid, whatever the nitrogenous waste productsare, whatever toxins the animal may have accumulated in its systemas a result of eating whatever it ingested.
So this filtrate is going to have be fairly heavily modifiedbefore it becomes urine. And the volume of filtrate is reallysubstantial. There are 200 liters a day of filtrate produced inyour kidneys. And the average human being's combined filtratein a couple million nephrons found in the 2 separate kidneys addsup to a total of 200 liters a day.
Now, many Americans are not used thinking in the metric system,so let me give you some frame of reference. If you way a 150,160 pounds, you weigh 70 kilo grams. 70 percent of your body weightis water. You have about 50 liters of water in your body. Andso the volume of filtrate produced is 4 times the total volumeof water in your body.
And, obviously, you're going to have to retrieve or reabsorb,as the process is called. You are going to reabsorb the overwhelmingmajority of that fluid, or you would become seriously dehydratedin a very short period of time. So 200 liters of filtrate areproduced. It has a lot of nutrients and waste products. It's producedin Bowman's capsule and the glomerulus. The driving force, themechanism of this filtration, is the blood pressure. The hydrostaticpressure of the blood physically squeezed the fluid across thewall just like -- have you ever had a chemistry class where hadsome kind of a percipitation in a solution, and put it in intoa funnel with a piece of filter paper, the driving force thenis just gravity. The filter paper is performing the same functionas the walls of the capillary.
It's exactly the same kind of thing, only it's occurring witha filter that has a lot smaller pores in it? That's why it's ultrafiltration.
Now, the next process then is reabsorbing. This filtrate isbeing continuously produced, and it is being pushed down alongthe nephron. And the next process is a component of reabsorptionwhich is called "obligatory reabsorption." The majorprocess is reabsorption. There are 2 components. One part of thathappens all the time, and one part is a variable. The "obligatory"as the name applies, obligatory reabsorbing is the part that happensall the time. It's the part that the animal is obligated to do.It always occurs.
And it occurs in several places by a couple of different mechanisms.The first part of obligatory reabsorption is going to occur inthe proximal convoluted tubule. Remember, this first twisted segmentis the proximal convoluted tubule. And out of that tubule, the body is going to pump 160 liters a day out of the 200 that aregoing to be reabsorbed right away.
That's one of the components of obligatory reabsorption.
Another component is down here in the descending limb of theLoop of Henle where another 15 liters a day of filtrate will bereabsorbed. So the total reabsorption by obligatory mechanismis a 175 liters a day.
That's seven eights of all the filtrate produced and reabsorbedright away. Now, a reasonable question might be why bother tofilter that if you are going to immediately reabsorb it? And theexact mechanism for the answer to that question is complicated.I don't want to try to get into it.
But, basically, what is going to happen, particularly in theproximal convoluted tubule is that the body is going to reabsorbthe things that it knows it wants to keep. It has in the cells,which are simple cuboidal epithelial cells that make up the wallof the proximal convoluted tubule, in the membrane of those cellsare membrane-bound proteins that perform active transport andfacilitated diffusion, and are specific for the things that thebody wants to keep -- amino acids.
There are like 3 separate -- 3 transport mechanisms or aminoacids that the body knows that it is are going to be there inthat filtrate there, because they are present in the plasma inthe animal.
There are several different transport mechanisms for sugar,for monosaccharides and disaccharides, there are specific transportmechanisms for particular ions that the body wants to keep, phosphatesand so forth. Everything else that is in that filtrate is trappedinside the nephron. All of those other molecules, because thereis no transport mechanism, they are trapped inside the nephron,and, therefore, they are going to be excreted.
So this is a mechanism where the body filters it 4 times a totalof the amount of water in the body every day. And it just takesback the things that for 500 million years of vertebrate evolutionit has been wanting to keep, even though the kidney I'm describingfor you, a mammalian kidney, the only thing that is peculiar aboutit is the Loop of Henle. The rest of it, I'll be telling you aboutis basically true of all vertebrate kidneys.
So 175 liters a day of obligatory reabsorption, the balance,the difference between the 200, and that is filtered. And the175, that is reabsorbed. The difference between those 2 is 25liters a day. And those 25, if the body of the animal is hyperhydrated, if the mammal has more water in its body than it wantsto have, let's say it's been consuming lots of water, maybe it's been spending a lot of time down at the pool hall drinking beer,or some other mechanism has caused the organism to consume anexcessive volume of fluid. Then the body is going to be wantingto get rid of water.
And it will -- it can, in fact, under extreme conditions, theanimal can actually -- a human being can actually produce a dailyurine volume of 25 liters a day.
And this is a result of an experiment that they did during --early in World War II, the University of Nevada, there was a renalphysiologist who was hired by the army. And he was given somevolunteers from the army because the army needed to know how muchwater they needed to send with the troops that were going to beinvading North Africa. And so they wanted the physiologist tofind out what these guys could tolerate in terms of a minimumwater consumption.
Since he was a good physiologists, he was going to try to findsomething of interest in general. He got them to drink massiveamounts of water. And he found out that the typical human beingcould drink about 25 liters a day. That's going to be a reallyboring day because your bladder holds about a quarter of a litter.So you do the arithmetic. These guys are making a head call every15 minutes going over to the urinal to empty their bladders. Therewere about to burst, so this is not a fun way to spend a day.And it's something that you might tolerate for a very short periodof time.
But that's the maximum amount. That's the difference betweenthe 200 and 175 that will be reabsorbed in the obligatory fashion.
Now, the body rarely obviously does that. And the minimum urinevolume that can be produced by a human being in our healthy conditionis about a 400 mls.
So these numbers here are urine production. Urine productionin a human being can range from 25 liters a day to 0.4 litersa day. And the way in which that occurs -- in other words, theway that is adjusted is by the animal performing the second componentof reabsorption. Which that part is called "facultative reabsorption."
Now, facultative reabsorption is the part that can be adjusted.That is, if the animal needs to get rid of water, facultativereabsorption is zero and urine production is 25.
If the animal is dehydrated it's needing to conserve as muchwater as it can. Facultative reabsorption is 24.6 liters a day.The balance being the .4 liters that are excreted.
So if you start off with 200 filtered, obligatory reabsorptionof 175. That's leaves 25 that you can play around with. And whenyou facultatively reabsorb zero, the only other thing that canhappen to the stuff is that it drips out of the end of the renalpapilla and trickles down the ureter and into bladder and youurinate it. That means your urine production is 25.
On the other hand, the extreme, you can take those 25, and youcan reabsorb 24.6 which means that your urine production is thebalance. And that's the 0.4.
So once the stuff is filtered, there is only two things thatcan happen; either it gets reabsorbed or it gets urinated.
Excreted from the body.
Obligatory reabsorption, the 160 or the 15, occurs in mostlythe proximal convoluted tubule, some in the descending limb ofthe Loop of Henle. The facultative reabsorption occurs primarilyin the collecting duct, but a little bit of it occurs in the distalconvoluted tubule.
So a little bit of it here in the distal convoluted tubule.Most of the collecting ducts, that's where we find the facultativereabsorption.
Now, that's variable. It's under the control of the endocrinesystem, and I'll be telling you about that in just a minute, amechanism of control -- the actual mechanism by which this facultativereabsorption occurs. But let's go down this list of processesa little bit further.
Secretion is a process. That's important to know about becausethere are a couple of things that are secreted in the kidney.And secretion refers to the active transport of something, thenephron. In other words, all this reabsorption dealt with stuffcoming out. But secretion is achieved by the active transportby membrane-bound proteins located in the cell membranes of thecells that make up the distal convoluted tubule. And what we findbeing actively transported normally are hydrogen ions and potassiumions.
Those are the two things that can be secreted by active transportmechanisms is in the distal convoluted tubule.
Now, those are both very very important ions. That's why theyhave this special mechanism for their excretion. Once they getput into the urine they are going to be lost from the body. Hydrogenions -- the Ph of the blood and kidneys perform a very importantroll in regulating the Ph of the blood by controlling the excretionof hydrogen ions.
And potassium ions are also extremely important. If you willremember, when I talked about the membrane potential which isfound in all cells and which changes during the action potential-- and nerve and muscle, that membrane potential results fromthe fact that there is a very substantial difference between thepotassium ion concentration across the membrane of the cell.
Potassium is at high concentrations inside the cell and at alow concentration, like maybe 4 millimoles in the plasma. Andone of the quickest ways to kill an animal is to increase thepotassium ion concentration, in a human being, it's only about6 or 8 millimoles. And what happens is all of the excitable tissuesfire off. All the muscles go into spasms. All nerves fire off.The heart stops functioning, and you have a dead animal in a bighurry.
So potassium ion concentration is possibly the most preciselyregulated of all physiological parameters that have been studied.This mechanism is here to allow for careful regulation of theconcentration of the potassium in the blood. And so secretiondoes not significantly affect the volume of the urine. It doesn'tsignificantly affect anything that has to do with the concentrationof solutes in the urine. But it is very important, and we needto mention it for that reason.
Now, all of these mechanisms occur in the kidneys of all vertebratesexcept for last one, which is concentration. The things that distinguishthe mammalian kidney from the kidneys of other vertebrates arethe presence of the Loop of Henle, which we find both in mammalsand in birds, and the ability to produce a concentrated urine.
And that's the last mechanism. Only mammals and a couple ofspecies of weird birds -- only mammals can produce a urine whichthe total solute concentration is greater than the total soluteconcentration of the blood.
And in the process of producing that concentrated urine, that'swhen the mammal performs this facultative reabsorption. That'swhat I want to talk about.
Now, the mechanism of this production of a concentrated urineand the mechanism of facultative reabsorption, in order to understandthat, we need to understand some things about the concentrationsof solutes in the filtrate and in the body fluids.
Your body fluid has a total solute concentration of about 300milliosmoles per liter. That is 0.3 moles or 300 millimoles ofosmotically active solutes. That's the sum of all the ions, andsodium and chloride being the primary ones, proteins, amino acidssugars, all of the stuff that is dissolved in your plasma. Thetotal a number of moles of those osmotically active species isabout 3 tenths of a mole or 300 millimoles.
And in terms of understanding water balance and the functionof the kidney, it doesn't really matter the exact compositionof that, whether it's amino acids or sugars or proteins or ions.So that's why physiologists will use this term "osmo"it is just osmotically active. It can measure it directly withoutknowing the exact concentrations of the sodium and chloride.
You can measure the total milliosmoles, the total concentrationsof solutes or body fluid of concentration of about 300. Most vertebrateshave a total body fluid concentration of about around 300 milliosmoles.And so the filtrate, when it's produced has a total concentrationof about 300 milliosmoles. And that concentration remains constantas the filtrate travels along the proximal convoluted tubule evenown though there is a vast amount of water being provided.
The mechanism is really there to reabsorb all those importancesolutes like amino acids and sugars, and so the total solute concentrationremains constant at about 300.
However, when the filtrates start to go down the Loop of Henle,things start to change and it gradually increases from 300 upat the top of the Loop of Henle to about -- in a human being about1200 at the tip of the renal papilla, but at the tip of the hair-pinbend, at the bend of the Loop of Henle it reaches about 1200.
It gradually decreases so that by the time it comes out ofthe top of the Loop of Henle it's down to 100. So it increasesfrom 300, it goes up to 1200, and then decreases back down to100.
And the mechanism of that is complicated. And I'm not goingto try to explain it to you. But what's important to understandis that that change in concentration parallels the change in concentrationof the solutes in the tissues surrounding the Loop of Henle. Sothat the concentration outside the Loop of Henle also undergoesa similar change.
It starts out at about 300 up at the top and then graduallyincreases. I'm going to just write in here some big steps of about300, but it's actually more or less a linear and gradual increaseup to 1200.
So down here at the tip of the renal papilla, it's about 1200.And I keep saying that. I better go back and explain to you whythe hair-pin bend at the end of the Loop of Henle correspondsto the tip of the renal papilla.
Remember, you have a diagram of the cross section of a kidneywith the ureter attached here. And we did this section. We get-- that's the renal papilla right there. Remember, there is 2layers the cortex, that rough outer layer, and the smooth innerlayer, the medulla.
The reason for the difference in the cortex and the medullaof the kidney is that medulla is composed of the Loop of Henleand collecting ducts. So if we were to diagrammatically representour nephron here, here is our Bowman's capsule. Here is our convolutedtubule. Out there is the cortex, then the Loop of Henle comesdown here. The tip of the renal papilla, it goes back up again.Then there is the distal convoluted tubule and there is the collectingduct.
It goes back down the tip of the renal papilla, so the diagramhere is a little bit more accurate in terms of the relative sizeof these things because the size of the white board. This is very,very sort of condensed here, but right about here is the boundarybetween the cortex and the medulla. And these structures thatare below that line are then going down to the tip of the renalpapilla and coming back up again.
And so this gradual increase in solute concentration from 300right at the boundary between the cortex and medulla down to 1200,that really exists within the renal papilla itself.
So its solute concentration in the tissue surrounding the renalpapilla is around 300 here and it goes up to gradually 1200 atthe tip of the renal papilla. This is a gradient -- a concentrationgradient in this interstitial fluid that surrounds the Loop ofHenle. It is produced by the Loop of Henle. So you need to rememberthat the function of the Loop of Henle is to produce this concentrationgradient.
Write that down, underline it. Make sure that you never getfooled into believing that the Loop of Henle has any other function.The function of the Loop of Henle is to produce this concentrationgradient in the interstitial fluid.
Now, that concentration gradient produced by the Loop of Henleis going to be used by the collecting duct to concentrate theurine. And the mechanism of that I'll explain to you in a minute.
So the concentration gradient produce by the Loop of Henle isused to perform both facultative reabsorption and concentration.Those two things occur simultaneously. When the fluid is beingfacultatively reabsorbed the urine is becoming concentrated.
That occurs in the collecting duct primarily. And it is thisconcentration gradient produced by the Loop of Henle that is responsiblefor that ability of a mammalian kidney to do these two things.Remember, I said there were two things. It has a loop of Henleand it concentrates urine. And those two things aren't necessarilyconnected to one another, but they are occurring in the same place.
So the Loop of Henle produces this concentration gradient,and the filtrate comes out of the top of the Loop of Henle witha concentration of 100 milliosmoles per litter.
Now, we have talked about facultative reabsorption as beinganywhere from 25 to 0.4 liters a day causing urine productionto vary anywhere from -- facultative reabsorption is any wherefrom 25, 20, 0.4 causing urine production to -- something is notadding up here. 24.6. It varies from 0 to 24.6 causing urine productionto go from 25 to 0.4.
What it is that causes this change, normally you don't do eitherone -- and the control of this process starts with the endocrinegland which is called the posterior pituitary.
The posterior pituitary is immediately adjacent the anteriorpituitary which is where gonadotropins are produced. It's importantto remember this is the posterior pituitary, it operates in adifferent mechanism. It does not have releasing hormones producedby neurons. The posterior pituitary produces its own hormonesdirectly, and one of those hormones is a hormone that controlsthe absorption of water, not only in the kidney, but in otherplaces in the body.
And just as there are hormones that go by the general name ofgonadotropins found in many different kinds of animals, thatcontrol -- the function of the gonads, there are also hormonesfound in the many different types of animals that control waterbalance. And those hormones that control water balance usuallyfunction by preventing an animal from losing too much water.
And they are called antidiuretic hormones.
Antidiuretic hormones diuresis is a production of a large volumeof urine. So antidiuresis is the process of causing an animalto have a small volume of urine. And so ADH is a genetic namefor a hormone that causes the animal to produce a small volumeof urine.
Different classes of vertebrate animals have different antidiuretichormones. They are all like 7 amino acids long as small peptidehormones. Well, these are peptide hormones that are usually like7 amino acids long.
There are small differences between them in terms of exactlywhich amino acids they have. That is interesting from an evolutionarypoint of view, the antidiuretic hormone in mammals is called vasopressin.
Amphibians have different ones. We don't need to worry aboutthe name of them. And other reptiles have different ones. Butin a mammal the anti diuretic hormone is vasopressin. It's producedby the posterior pituitary
The posterior pituitary has blood flowing through it, and theposterior pituitary monitors the total solute concentration ofthe blood. And when the animal is hyper hydrated, the animal hasbeen drinking too much beer or water, whatever, so that it hastoo much water in its body, that causes a very slight dilutionof the blood plasma.
The solute concentration of the blood plasma is a little bitlower than the animal wants it to be, the posterior pituitarydetects that and it stops producing ADH.
And so in order to understand the mechanism of facultative reabsorptionand concentration I'm going to describe what happens under these2 conditions when there is -- when the blood is diluted, the posteriorpituitary does not produce ADH or vasopressin.
And what happens is that as this volume of filtrate with a concentrationof 100, the volume here is 25 liters a day, concentration is 100,what basically happens is nothing happens, and that filtrate travelsthrough the proximal convoluted -- distal convoluted tubule andout the collecting duct.
And so you have a production of a volume of urine of 0 facultativereabsorption. You have 25 liters of urine produced a day. Andit has a concentration of a 100 milliosmoles per litter.
There is no vasopressins circulating in the blood of the animal,and that would be if animal was really seriously hyper hydrated.
So if there is too much water in the body, then the animal'sposterior pituitary detects that, it shuts down the productionof ADH, and the 25 liters a day of diluted urine, because thisis now diluted compared to the body fluid. The body fluid of theanimal will be 300 milliosmoles. This urine is diluted.
The lower solute concentration, it has 100 milliosmoles perliter, and 25 liters of urine are produced in this human beingunder these circumstances.
The opposite --
STUDENT: The concentration, what is the concentration on theoutside of the Loop of Henle that you drew over there, that 300,600, 900, and 1200. What is the concentration of--
INSTRUCTOR: That's the concentration of the interstitial fluidthat surrounds the Loop of Henle and collecting ducts. And that'sbeen produced by the Loop of Henle, it's in the renal papillahere.
STUDENT: And that's the channel that you drew?
INSTRUCTOR: That's the symbol that physiologists uses for activetransport. Active transport of hydrogen ions and potassium ions.That's secretion. So that's where this thing right here representssecretion, and that's the third mechanism here.
STUDENT: You didn't get to that, yet?
INSTRUCTOR: I already did. It's secretion.
STUDENT: What controls causes the secretion?
INSTRUCTOR: I'm not going to tell you about that. Let's talkabout concentration and facultative reabsorption.
When the body is dehydrated it has lost more water than it hasgained. All right. It's the animal that is wandering around inthe desert not drinking enough water. It's dehydrated. That causesthe plasma of the blood to be become slightly more concentrated,be slightly higher than 300 because it's lost water. It hasn'tlost solute but it's lost water. So is the animal is concentratedabove what the regulated level is, 300, above what the body wantsit to be.
And the posterior pituitary detects that concentration of theplasma. And in response to that concentration of the plasma, theposterior pituitary releases vasopressin in the blood. Vasopressintravels through the blood to the kidney. And the target cellsin the kidney are in the distal convoluted tubule and in the collectingducts.
And the response to this peptide hormone in those target cellsis to increase their permeability to water. They have what arecalled water channels. They're like sodium and potassium channelsonly they are for water.
And this is a very interesting situation because we normallythink about cell membranes being completely permeable to waterand varying their permeability to ions and things. But in thistissue the distal convoluted tubule and collecting duct normallyhave very low permeability to water; they are like water proof.
But they have membrane-bound proteins that can open in responseto the presence of vasopressin and allow water to diffuse outof the collecting duct and the distal convoluted tubule.
And so what happens when the plasma is concentrated by dehydration,the posterior pituitary releases vasopressin. And these membranesof the distal convoluted tubule and collecting duct become permeableto water and so osmosis takes place. And when the filtrate comesout of the top of the ascending limb of the Loop of Henle it hasa solute concentration of 100. The interstitial fluid surroundingit has a solute concentration of 300.
And so water diffuses out of the filtrate, that's osmosis, thediffusion of water from an area of low solute concentrationto an area of high solute concentration, and that is the facultativereabsorption of water that occurs in the distal convoluted tubule.
The concentration goes up to 300, it becomes equilibrated withthe interstitial fluid surrounding this. And there is little facultativereabsorption of water. The majority of the facultative reabsorptionof water occurs as the filtrate is traveling down the collectingduct. It's going past or through this interstitial fluid thathas this concentration gradient that was produced by the Loopof Henle.
And so the equilibration continues to take place in the membranesof collecting duct, cells are made permeable by the presence ofthe peptide hormone vasopressin. The water channels open up andwater diffuses out of the filtrate as it travels down the collectingduct. And that water represents the balance of the facultativereabsorption.
In the process, the solutes are left behind. The only thingthat is coming -- you have this collecting duct, the solutes havebeen left behind -- the only thing that is water. So the solutesare left behind and they become more concentrated. So by the timethe urine, the little drop of urine gets down to the tip of therenal papilla it has equilibrated and it has a concentration of 1200 milliosmoles per litter.
And the volume, that has been reduced dramatically. In fact,under maximal secretion of vasopressin, facultative reabsorptioncan equal 24.6 leaving only 400 mls of urine produced, and theconcentration is up to 1200 milliosmoles per litter.
And so facultative reabsorption is maximally 24.6 liters leadsto the concentration of the urine, the forth step in the productionof the urine. And the mammal is able to conserve water and minimizeits urinary water loss. Remember the reduced availability of waterleads to the necessity for increasing water conservation. Anda decrease in water loss, one of the mechanisms of that is decreasingurinary water loss.
And this is how mammals do this. They have a Loop of Henle thatproduces a concentration gradient, vasopressin allows the collectingduct to use that concentration gradient to perform facultativereabsorption of water. And in the process, the urine becomes concentratedto a value that is 4 times the concentration of the body fluids.The body fluid of plasma has a concentration of 300.
Here the water the concentration of 1200, it's 4 times as highin terms of it's concentration. And so the waste products arepackaged into a minimal volume of water and the animal conserveswater. Now, some mammals have longer Loops of Henle. The concentrationof the tip of Loop of Henle is proportional to the length of theLoop of Henle.
This rodent that lives in Australia has a long Loop of Henle.That is, the renal papilla actually dangles out in the ureterand it has a really long Loop of Henle. And concentration at thetip of that animals Loop of Henle is 9,000 milliosmoles per liter.
The urine concentration, therefore, is 9,000 milliosmoles perliter. And that's the world champion mammalian kidney in termsof the ability to produce concentrated urine. These guys are nearlypeeing dust. They still have a body concentration of 300, buttheir urine is 9 molar. And the only adaptation that does it is thelonger Loop of Henle that produces a much bigger concentrationgradient.