The indented area of the kidney is called the hilus and attachedto the hilus is the ureter - a tube of connective tissue thattransmits the urine from the kidney to the bladder.
If we cut the kidney so that we can see it's internal structures,what we see is something sort of like where the ureter expandswithin the kidney and forms a funnel-shaped piece of sheet ofconnective tissue, that's called the renal pelvis. And then danglingdown in the middle of this renal pelvis is the papilla. If youlook at the cut surface of the tissue, it is very obvious thatthere is a distinct difference in the appearance of the dark area,which is called the cortex and the inner area is called the medulla.
The reason for that difference in appearance is that there aredifferent structures located in those areas; different parts ofa single structure. There is a very extensive blood supply tothe kidney, a large renal artery taking a large volume of bloodto it. In fact, in a resting human, something like 20% of allthe blood pumped by the heart goes to the kidney and it can bereduced during exercise. A large volume of blood goes to thekidney and in the cortex you can see the vascular structures. The medulla doesn't have any that are grossly observable butit does have some. Most of the mass of this kidney other thanthe vascular structures is composed of a very large number ofwhat are sometimes called the renal tubules or the synonym forthat is a nephron. If we want to understand the function of thekidney we basically need to understand how a nephron works andbefore we can understand how it works, we need to understand theanatomy.
There is an artery that comes into the Bowman's capsule and thena little capillary tuft (glomerulus) inside the capsule and thenanother arteriole that comes out and it breaks up into anothercapillary bed called the peritubular capillaries. So blood entersthis structure after branching off of the renal artery into what'scalled an afferent arteriole. This little tuft of capillariesis called the glomerulus which is a like a Greek word that refersto a ball of yarn. And then exiting the glomerulus, the bloodtravels through an efferent arteriole for a short distance untilit enters another capillary bed which is called the peritubularcapillaries. Tubular referring to the renal tubules, peri meaningaround. So the peritubular capillaries is the general name givento that mesh of capillaries that are found wrapped around therest of the nephron.
The blood enters the afferent arteriole, goes into the glomerulus,and that's where the process of the formation of the urine begins. The structure that surrounds the glomerulus is called Bowman'scapsule. If you can imagine taking a big soft beachball wherethe latex surface is represented by a line and your hand representsthe glomerulus, you can sort of poke it into the middle of thatball, there's a layer of the beachball that is wrapped aroundyour hand and your hand would be in the middle of that structure. So this is like a cross-section through a 3-dimensional structurewhich is spherical. In the middle of that hollow structure isa space which is continuous with the lumen of the 1st segmentof the tubule which is called the proximal convoluted tubule (PCT)and that part is highly convoluted, not a single simple shortloop. The hair-pin shaped segment collectively referred to asthe Loop of Henle (LOH) - is a structure that is found in thekidney of a bird and a mammal but not found in the kidneys ofany other vertebrates and there is a segment of it that goes down,the downward part, the 1st part is called the descending limbof LOH and then it comes back up again and that is the ascendinglimb of LOH. The Loop of Henle itself is the part that's a littlebit thinner than the rest of the tubule. We get up to the topof the ascending limb, then we get to that last segment of twistedtubule and that's the distal convoluted tubule (DCT). And thedistal convoluted tubule plugs into the final little segment ofit which is called the collecting duct and it's called that becauseit collects up a filtrate (the fluid) that's flowing through thetube from a number of different distal convoluted tubules. Soa whole series of different nephrons will all be connected upto a single collecting duct and the boundary between the cortexand the medulla is going to be right about at the top of the Loopof Henle. The collecting duct is going to collect from a wholeseries of nephrons, goes all the way down to the tip of the renalpapilla and there will be other nephrons that are connected intothat collecting duct.
The immediate proximity of the Loop of Henle and the collectingduct making up the tissue of the renal papilla is a very importantfeature of the design of this system. When we understand howthis thing works, what the collecting duct does, what occurs inthe collecting duct and what the Loop of Henle does, it's goingto be very important to understand that those 2 are immediatelyadjacent to one another here in the renal papilla. Now some ofthe nephrons (there's about a million in each human kidney) don'thave Loops of Henle, it's really just the ones that are locatedright near the boundary of the cortex and the medulla that haveLoops of Henle, the ones that are further out don't have a Loopof Henle, they have just a proximal distal convoluted tubules.
Where's the point where the cortex and medulla separate? It'sfairly obvious that the tissue looks different and the reasonfor that is that the medulla is composed of Loops of Henle andcollecting ducts and they are all running sort of parallel toone another. The cortex looks much rougher because it has allthese convoluted tubules that are all wrapped around each other,proximal and distal convoluted tubules, it has glomeruli, it hasLoops of Henle. The real reason for the difference in the appearanceof the cortex and the medulla is that they are composed of differentstructures. When you look at the cortex under the microscopeyou can't see the distinct tubules. These things are microscopicstructures. In fact the walls of this nephron are composed ofsimple cuboidal epithelium.
Now what I want to talk about is the process of the productionof the urine. There are really 4 processes (steps) in the productionof urine. This is in fact the study of how the urine is formedand it is on-going. There are still details of this process thatare not completely understood and there are opposing theoriesand so forth. The explanation that I'm going to provide you withis a simple version of it, believe it or not.
4 processes (steps) in the production of urine:
1) Filtration - This occurs in the glomerulus and Bowman's capsule. Basically what's happening here is that some of the plasma (lipidportion of the blood) is physically squeezed/filtered throughextremely small holes in the wall of the capillary and in thewall of Bowman's capsule. Technically, we should refer to thisprocess as ultrafiltration because the holes that the fluid issqueezed through are so small that large protein molecules cannotget through. Some kind of physical force pushes the fluid throughand separates the fluid from the stuff that's left behind andthe stuff that's left behind in the case of this system are thered blood cells, the white blood cells and the large proteins. The stuff that comes through is referred to as filtrate and thefiltrate is a protein-free extract of the plasma. So the largemolecules of protein don't come through but other than that, thechemical composition of the filtrate is identical to the chemicalcomposition of the plasma. That is a very important point. It'sa protein-free extract of the plasma, it has exactly the samechemical composition, it has the same composition of salts, sodium,chloride, phosphate, amino acids, sugars, metabolic waste products,hormones and everything that is in the plasma, it shows up inthe filtrate. With the exception of the large protein molecules.
Now in the 2 kidneys of the average human being, the total volumeof filtrate is 200 liters per day (200L/day). This is the firststep in the production of the urine. The driving for this filtrationis the blood pressure produced by the heart. That's what causedthis filtration to occur.
2) Reabsorption - This is what happens next. At least 90%(sometimes as much as 98% or 99%) of the filtrate is going tobe reabsorbed. The water itself is very important for the waterbalance of the animal but remember that the filtrate has exactlythe same chemical composition as the plasma so it has lots ofsugars, amino acids, and other things in it that the body reallycannot afford to just lose. So reabsorption involves retrievingsolutes that the body wants to keep as well as retrieving waterthat the body desperately needs to keep.
Reabsorption has 2 components and one of these components happensall the time in a normal healthy human and that is called obligatoryreabsorption. That's the part that happens all the time. Obligatoryamounts to a total of 175 liters per day (175 L/day), of those175 liters a day, 160 occur in the proximal convoluted tubuleand 15 occur in the descending limb of the LOH. The remainingreabsorption called the facultative reabsorption is a volume offluid with solutes dissolved in it that the animal can eitherreabsorb or not reabsorb, it can vary it according to it's needsto remain in water balance. If the animal is short of water,it's dehydrated or it's in some kind of dehydrating environment,then the animal wants to save more of that water, it will reabsorbmore of it and it will produce a fairly small volume of urine. If the animal is hyperhydrated, that is it has an excess amountof water in it's body, then the animal has got to get rid of theexcess amount of water and so it will reabsorb less. This isfacultative reabsorption and facultative is a term that we usein a variety of different instances where we're talking aboutsomething that the animal has the option to do all or part ofaccording to it's own particular needs. And facultative reabsorptiontherefore we don't give a single value for, we can only set limitson.
Facultative reabsorption is anything from 0 to 24.6 liters perday (0-24.6 L/day). Any volume of fluid that is not reabsorbedis going to appear as urine, at least in a mammal. So the differencebetween filtration and reabsorption is going to give us urineproduction. If we look at filtration, it's 200 liters per day,reabsorption is going to be 175 + 0 to 175 + 24.6 which is 199.6= so reabsorption is anywhere from 175 to 199.6 and that leavesthe remainder for urine production as being anything from 25 to0.4 liters per day. If the body is reabsorbing nothing, thatis the facultative reabsorption if 0, obligatory reabsorptionis 175 then you're going to have 25 liters a day of urine production. Facultative happens in the distal convoluted tubules and in thecollecting duct. It happens at the same time as concentration.
3) Secretion - Refers to the active transport of solutes intothe filtrate. And this occurs primarily in the distal convolutedtubule, a couple of solutes that are transported are potassiumions and hydrogen ions [DCT - K+, H+].
One of the two hypothesis for how the mammalian kidney works,is that urine is produced by secretion, secretory hypothesis. That is the body has this tube and it just puts into the tubethe stuff it doesn't want. Then the stuff goes flowing down thetube and it comes out in urine; that seems logical. The othertheory was referred to as the filtration hypothesis and that iswhere the blood was filtered producing a filtrate and then thebody reabsorbed what it wanted. The reason that this filtrationmechanism exists and the reason that this inefficient system continuesto exist is that this is a fail-safe system. So this system filterseverything out and keeps the stuff that it knows it wants. Thesecretory kidney has to know what you want to get rid of and itdoesn't work the way the filtration system does. And if you accidentallyput into the something that you need to get rid of but you didn'tknow it, you won't be able to get rid of it because you won'thave the active transport mechanism to secrete it.
So the filtration kidney is a fail-safe system - it will automaticallyget rid of the things that it doesn't know that it wants to keepand that's why this business of having a facilitated diffusionmechanism for reabsorbing the things that you know you want tokeep is a good idea rather than having the opposite kind of system.
4) Concentration - How is it possible for this kidney to producea urine that has a higher solute concentration than the body fluids,because notice up to this point the total solute concentrationof the filtrate has always been exactly the same as that of theblood flowing in the capillaries.
Obligatory Reabsorption:
No one can demonstrate the Active Transport of water. You mightimagine that one way to reabsorb water would be to have in thesecuboidal epithelial cells, some kind of a membrane-bound protein,an active transport mechanism that just can grab a hold of watermolecules and pull them from the lumen and stick them into thecapillaries which are on the other side, and if that were possibleto just move water molecules across, that would be an easy explanation. But nobody has ever been able to demonstrate that. Apparentlyvertebrate animals and no animals are capable of actively transportingwater molecules.
What they can transport are the solute molecules and rememberthis filtrate has lots of sugars, lots of amino acids, and lotsof various kinds of ionic species dissolved in it and so whathappens in obligatory reabsorption is as follows:
The active transport of solutes and renal physiologists studyingthe function of the nephron have been able to demonstrate severaldifferent transport mechanisms.
1) One of these is for the active transport of sodium ions andthat's probably the single most important one. Remember thatactive transport means that this is moving against the concentrationgradient and it's an energy utilizing step. ATP in these cellsis used to grab a hold of sodium ions and transport them acrossfrom the lumen in the capillary. So the first step in obligatoryreabsorption then is the active transport of sodium ions.
2) Then we have the facilitated diffusion of other solutes. Those other solutes include: chloride, phosphate, amino acids,and sugar molecules (monosaccharides & disaccharides haveseparate transport mechanisms that are located in the wall inthese cells that comprise the walls of the proximal convolutedtubule). There are actually something like 3 separate transportmechanisms for the different classes of amino acids. What actuallyhappens is that as the solutes are transported, a volume of watermolecule moves with them simultaneously. In some ways, it's almostas though each one of these solutes has an enclosing envelopeof water molecules that has to go with it.
Facilitated diffusion - this is where in the cells of the tubules,there are membrane-bound proteins that are very specific for particularsolutes. There are membrane-bound proteins that are in thereand they recognize the amino acids and the sugar molecules thatthe body wants to keep. They recognize the phosphate and chlorideions that the body wants to keep also. All of those things goback. There are other solutes in this filtrate such as metabolicwaste products, hormones and any kind of unknown toxin that happento get into the animal's body but those solutes cannot get acrossthe membrane. They are large molecules and they can't dissolvein lipid bilayer so they are trapped. What we have done now sofar is we filtered all of the water in the body in a quarter of a day, we removed the water and we removed the things thatwe know we wanted to keep and we left all the bad stuff behindat an elevated concentration than it was in the plasma. All thisbecause the body was just reabsorbing the things that it wantedto keep.
In summary, remember there are 4 major processes/steps involvedin the production of urine by the mammalian kidney. Filtrationbeing the 1st one - 200 liters a day of ultra-filtrate produced. Reabsorption is the 2nd one which has 2 components - obligatorythat happens all the time and amounts to 175. 160 of that happensin the proximal convoluted tubule - active transport of sodium,facilitated diffusion of a bunch of other solutes and the watergets reabsorbed.
There are another 15 L that occurs in the descending limb of theLoop of Henle (LOH) and we need to understand what's happeningthere. So that's obligatory reabsorption.
The second component of reabsorption which varies between 24.6and 0 is the facultative reabsorption. This is the part thatthe animal varies in order to keep itself in water balance. Inorder to have it's total water gain equal to it's total waterloss. It varies the amount of water that it loses in the urineso that if it's gaining lots of extra water, it has to lose lotsof extra water and it's going to have a large urine production. It will have a large urine production by having a small facultativereabsorption of water. So facultative reabsorption is anywherefrom 0 to 24.6 leading to a urine production of anywhere from25 to 0.4.
The third major process is secretion. That's the active transportof things like hydrogen and potassium into the distal convolutedtubule.
The 4th step is the process of concentration. The body fluidsof a typical mammal have a total fluid osmolality of around 300milliosmoles per liter (300 m Osm/L). That's the units in whichwe measure total solute concentration in animal physiologicalsystems. An osmol is a mole of osmotically active particles. It could mean a mole of sodium ions and/or it could mean a moleof protein and in physiological systems it's a very complex mixof all those things.
The typical body fluid concentration is around 300 in mammals. Human beings can produce a urine with a total solute concentrationof 1,200 m Osm/L. In other words, for humans the maximum concentrationof solutes in urine can be 4 times as high as the body fluid concentration. What is happening that results in the reabsorption of 15 litersof day in the descending limb in the LOH? How does facultativereabsorption occur? How does concentration occur? The answerto these 3 questions all involve the interaction between the LOHand the collecting duct.
The overall answer to the question of how these things happenis the following: The LOH creates a concentration gradient inthe interstitial fluid of the renal papilla and the collectingduct uses that concentration gradient to perform facultative reabsorptionto concentration. Concentration gradient = A gradual increasein the total solute concentration of the fluid that is surroundingthe cells that make up the nephron. The total of solute concentrationin most of the body and also most of the cortex of the kidneyis around 300.
If you remember the position of the nephron in the kidney, theLOH starts out in the cortex, dips down into the medulla and goesall the way down to the tip of the renal papilla. So the totalsolute concentration in the fluid surrounding these cells at thislevel is around 300 and it gradually increases by the time youget to the tip of the renal papilla. At that point the concentrationis 1,200. The numbers I am giving you are for humans by the way.Remember that the number 1200 is the maximum solute concentrationin the urine of the human being and it is not coincidence thatthis value of 1200 is the maximum concentration in the interstitialfluid.
This gradually rises just a couple of points to indicate thatgradual rise. It doesn't really jump up in increments of 300,it's a more or less linear increase as you go down the LOH. That'swhat I mean by concentration gradient. Remember gradient meansa difference. So we have a gradual increase in the concentrationof this interstitial fluid (the fluid that surrounds the cellsand that concentration gradient is created by the LOH) and thecollecting duct is going to use it.
Now how does the LOH make that gradient? Well, this part is alittle bit complex and I'm not really going to try to explainin sufficient detail to have you understand it. But there area couple of things that are going on that I think you need toknow about. And we can start by following the changes in thetotal solute concentration of the filtrate of the fluid that isflowing down this nephron. The filtrate itself starts off witha concentration of 300 and as it goes down the descending limbthe filtrate equilibrates with the surrounding interstitial fluid. That is it's moving slowly enough that cuboidal epithelium ispermeable both to water and to salt and so the concentration inthe descending limb remains the same as the concentration in thesurrounding interstitial fluid. So where it's 300 on the outsideit's 300 inside, where it's 600 on the outside it's 600 on theinside.
What causes that equilibration? 2 things. One of them is theosmotic removal of water. In other words, if there was a dropof water that came in with a concentration of 300, as that littledrop of fluid moves down the nephron, it's going to be seeinga progressively higher solute concentration on the outside. Sothere's going to be an osmotic gradient and water is simply goingto move by osmosis out of the nephron because this filtrate ismoving down. That's where the 15 liters a day of obligatory reabsorptionoccurs, just by osmosis in the descending limb of the LOH. That'sone of the 2 processes that contributes to this increasing concentration.
The second process that contributes to that is the movement ofsodium chloride into the nephron. It diffuses in and the reasonfor that is the NaCl concentration in the interstitial fluid ishigher than the NaCl concentration of the filtrate so there'sa concentration gradient for NaCl and it just diffuses. The wallsof the descending limb of the LOH are permeable to NaCl and theyare able to diffuse in. That must mean that those cells havemembrane-bound proteins that facilitate the diffusion of NaClinto the nephron because we know that ions are not going to beable to get across cells without some help from the cells.
So those 2 factors, the movement of solute in and the removalof water caused this increasing concentration in the nephron.
We still haven't explained why there's a concentration gradientin the interstitial fluid. We've just sort of begun to outlinethe story here. The explanation for what's causing that increasereally comes when we look at what's happening in the ascendinglimb. On the side of the LOH where the fluid is going back up. And let's start by tracking what happens to the concentrationof solutes in the filtrate.
The main thing is that it becomes lower, it decreases and thereason that the concentration of solutes in the filtrate decreasesis because there is an active transport mechanism in these cellsthat is removing NaCl. All of these cells that form the descendinglimb have in them an active transport mechanism. That is a membrane-boundprotein that grabs a hold of NaCl and expels it from the filtrate. So clearly the cause of the decreased concentration of the filtrateis the removal of solutes. If you take a volume of fluid andpull solutes out of it, you make it more dilute. It's that removalof NaCl that explains why the concentration of NaCl in this interstitialfluid is high. In other words, this NaCl that's diffusing intothe descending limb was pumped out of the ascending limb. NaClis removed from the ascending limb and it diffuses into the descendinglimb. That combined with the fact that the fluid is moving inopposite directions in the ascending and descending limb explainswhere this concentration gradient comes from.
The LOH is in fact one of several what are called counter currentmultipliers. It's not a counter current exchanger. The countercurrents refer to the fact that we have fluid going in oppositedirections in the ascending and descending limbs. That's wherethe counter current part of it comes from. The multiplier partof it comes from the fact that the difference in solute concentrationacross the wall of the nephron at any given point is about 200m Osm/L. Down here where it's 900, the concentration on the insideis 700. Up here where it's 600 on the outside, it's 400 on theinside. So the difference in total solute concentration in theactive transport mechanism can produce a difference of about 200m Osm/L. This active transport mechanism is like a pump and anypump has a limited capacity to pump. You go out and buy a waterpump, it may be able to pump water 5 feet high but it can't pumpwater 75 feet high. Any kind of a pumping mechanism has a limiteddifference that it can produce. You buy a better pump, you canpump more. That's the difference in total size concentrationthat can be created across the wall of the ascending limb of LOH.
So why are we calling it a multiplier? Because the total differencein solute concentration between the top and the bottom is 900m Osm/L and it's produced by a pump that can only produce a differenceof 200. Now how do you take a pump that can only produce a differenceof 200 and produce a total difference of 900? Well you multiply200 by 4 1/2 and that multiplication results from the countercurrent flow. In other words, the effect that each one of theseindividual pumps gets is added onto the pumps that have gone beforeit because of the counter current flow of the fluid.
Another way of thinking about this is let's pretend that we'rea Na ion and we're floating down and all of a sudden we turn aroundand we come up the ascending side and a pump squirts us out. We're sitting out there in the interstitial fluid. Now it's sodiumcrowded and there's a lot less sodium inside the tube so we wantto diffuse back in. So we diffuse back inside the tube, go aroundthe corner, start back up on the ascending side, another pumpgrabs us, and kicks us out again. Then we diffuse back in andstart the same cycle over. In other words, there's that tendencyfor NaCl to get trapped in the system, to get kicked out beforeit can escape out of the top of the LOH. There's a tendency forNaCl to be sequestered towards the tip of the renal papilla andthat tendency is what really creates this concentration gradient.
It takes about 40 minutes to sit down and go through a step bystep development of how you could produce this gradient by justlittle simple jerking movements which create differences of 200and it's not worth the effort. You need to have some kind ofa general intuitive appreciation of the fact that this mechanisminvolves an active transport mechanism that can produce a differenceof 200 combined with the counter current flow manages to multiplythat difference of 200 into a total difference of 900, that'ssufficient. You also need to recognize that that is producedby the counter current multiplier which is in the LOH. So that'sreally what I want you to remember about this. The LOH createsthe concentration gradient in the interstitial fluid because it'sa counter current multiplier.
Now what's going to happen next? We've explained really so farjust this first question, how did the 15 liters get obligatorilyreabsorbed. We still need to explain facultative reabsorptionand concentration.
There are a couple of other things that are involved in this process. When the animal has had too much water to drink, or for somereason the body of the animal has had more water intake than waterloss and it's becoming hyperhydrated, that causes a dilution ofthe plasma in the blood. There is an organ called the posteriorpituitary which is a structure which senses that dilution of thebody fluid. It measures the total solute concentration of theplasma and it actually releases the hormone. The hormone is made somewhere else. But it releases a hormone which goes by a coupleof different names and one of these names is antidiuretic hormone(ADH). In a mammal, the ADH is a specific peptide hormone calledvasopressin. In other vertebrate animals the ADH can be slightlydifferent, and there's something like 7 amino acids in this peptide. In some vertebrates, there's a difference in 1 or 2 of thoseamino acids and they go by a slightly different name. When theposterior pituitary determines that the body needs to conservewater, the posterior pituitary releases vasopressin and it tellsthe kidney that you've got to facultatively reabsorb more waterin order to save water. That's why it's called ADH.
Diuresis is the production of a large volume of urine. So a diureticis something that causes you to produce a large volume of urine. Antidiuretic is a hormone that prevents you from producing alarge volume of urine. So what we need to do is we need to seewhat happens to this filtrate under 2 extreme conditions. Ofcourse in the living animal, it lives most of it's life in someplace in between these 2. If we understand the 2 extreme conditions,then we can understand how things happen most of the time.
The simplest situation is where the animal has too much waterin it's body. If it has too much water in it's body, the posteriorpituitary is going to be releasing no ADH. And under those conditionswhat happens is that this filtrate simply flows through the distalconvoluted tubule and through the collecting duct and nothinghappens. So what that means is that by the time we got down hereto the tip of the LOH, we had accounted for a total of 175 litersa day of obligatory reabsorption. If nothing more happens, thenfacultative reabsorption is 0 and urine production is 25 litersa day. Notice that under those circumstances the concentrationof the urine is going to be 100 m Osm per liters just as it waswhen we came out of the top of this.
Now notice that in order for that to happen, this fluid that hada concentration of 100 flowed down through this collecting ductand did not osmotically equilibrate. In fact, it also flowedthrough the distal convoluted tubule and it went past a lot ofinterstitial fluid that had a much higher solute concentrationbut there was no removal of water. It turns out that the distalconvoluted tubule and the collecting duct have a variable permeabilityto water. It could either be fully permeable to water or it canbe impermeable to water. Under these conditions where there isno antidiuretic hormone, the duct is impermeable to water. Thewater cannot get out. This is not your classical semi-permeablemembrane that would allow the water to move, osmosis to take placeand equilibration to occur. This is a membrane that is a water-proofmembrane.
That's only true if there's no antidiuretic hormone in the systembut let's go to the other extreme. Let's say that this is a dehydratedanimal and it's losing water faster than it's gaining it. It'sbody fluids are becoming concentrated due to the loss of water. What happens then is that the posterior pituitary releases ADHand it makes the distal convoluted tubule and the collecting ductpermeable to water. And then all that happens is osmosis. Thisfluid that has a concentration of 100 when it gets to the topof the ascending limb sees an interstitial fluid concentrationof 300 and water diffuses out. Then as it comes down the collectingduct, that process of diffusion of water or osmosis continuesto occur because the ADH made the collecting duct in the distalconvoluted tubule permeable to water. This water that's diffusingout is the water that is being facultatively reabsorbed. That'show facultative reabsorption of water occurs. It occurs in thedistal convoluted tubule, and in the collecting duct. It is madepossible by vasopressin and the driving force is the concentrationgradient that was created by the LOH. What's going to be the concentration of urine after equilibration is completed? The concentration of the urine is going to be 1200 mOsm per liter. In other words, the urine/filtrate is going to equilibrate withthat interstitial fluid concentration and the urine is going tohave a maximum concentration of 1200 m Osm per liter.
So facultative reabsorption and concentration occur simultaneouslyin the distal convoluted tubule and in the collecting duct underthe influence of vasopressin or ADH produced by the posteriorpituitary.