AP Biology The Excretory System

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In this section, we are going to continue our discussion of animal physiology with the focus on the excretory system.0002

The excretory system has two major functions.0010

One is the excretion of waste. The other is a role in homeostasis particularly osmoregulation.0013

Beginning first with its function as an excretory system, nitrogenous waste are produced by the breakdown of nitrogen-containing compounds,0024

for example, proteins and nucleic acids.0035

One product of this breakdown is ammonia, which is NH₃. The thing about ammonia is it is very toxic in high concentrations.0040

So, there are some animals that directly excrete ammonia. However, many animals convert ammonia to less toxic substances and then, excrete those.0053

The less toxic substances that I am talking about are urea and uric acid. Those are the two main ones.0065

First, talking about animals that actually directly excrete ammonia, these are animals that live in the water,0070

so, animals that live in the water, that live in an aqueous environment directly0080

excreted by some animals that live in an aqueous environment- aqueous-dwelling animals.0090

This is because if an animal lives in the water, it is constantly exposed to very large amounts of water.0103

So, all this water coming in and bathing the animal dilutes out the ammonia during transport and during the excretion of ammonia.0109

So, the ammonia cannot concentrate and then, damage the animal cells.0119

Fish, for example, excrete ammonia through their gills, and a lot of water is passing through the gills; and it dilutes out the ammonia.0123

Simpler aquatic animals such as a hydra just excrete the ammonia through diffusion directly from their cells, directly into the water.0141

There are, however, some aquatic animals like sharks that do not secrete ammonia directly.0152

Sharks convert their ammonia to urea like many land-dwelling animals do.0158

So, let's now talk about the second common waste product, which is urea.0164

Many animals including mammals as well as annelids like earthworms excrete urea as their nitrogenous waste, and urea is much less toxic than ammonia.0171

Without this constant exposure to water, urea is a safer material to transport and excrete.0187

In mammals, urea is formed in the liver. In the liver, ammonia is converted to urea, which, then, leaves the body via the excretory system.0196

Another option for nitrogenous waste for animals is uric acid.0213

Reptiles and birds secrete or excrete nitrogenous waste in the form of uric acid. This is also less toxic than ammonia.0219

A major difference between urea and uric acid is that uric acid is poorly soluble in water.0231

In fact, uric acid has a consistency that is somewhat like paste.0243

And this is an advantage to animals like reptiles and birds that may have to go without water for a while. They live in drier environments.0247

So, instead of secreting urea in the urine and having to have this solute in a lot of water and lose a lot of water that way,0257

they can secrete uric acid from the body with very little water loss, so uric acid is an advantage for an animal that lives in a drier environment.0265

Now, the conversion of ammonia to either urea or uric acid has a cost, and that cost is energy.0275

This is an energy-requiring process, but it is a necessary one because the toxicity of ammonia would be a problem for animals,0283

especially land animals that do not have all this water diluting out the ammonia.0291

Excretion of nitrogenous waste is one major function of the excretory system, but another one is osmoregulation.0297

In many species, the second major function of the excretory system is a role in homeostasis.0306

Osmoregulation is a maintenance of a constant solute concentration in an organism.0313

So, to understand this, we are just going to go back to some basics of chemistry and talk about solutes and movement of water and osmosis.0319

Let's say I have a compartment separated by a semipermeable membrane meaning that water can cross this membrane but solutes, particles, cannot.0331

On one side, let's say I have a higher concentration of solute. The other side is going to be more dilute, a lower concentration of solutes.0344

This side is the hyperosmolar side. This side is hypoosmolar.0354

Water is going to move from the area of more water, or you could look at it either way.0369

You could look at it as the area of lower solute concentration, the hypoosmolar side to the hyperosmolar side to dilute this out.0379

Or you could look at it as water via osmosis is going down its concentration gradient0387

to an area of greater water concentration to an area of lower water concentration.0392

This is very similar to what we talked about when we talked about cells and we said hypotonic, hypertonic and talked about0399

water entering the cells if they were hypertonic, water leaving the cells if they were hypotonic relative to their environment.0407

So, here I am talking about just two different liquids, but it is the same idea with what we are talking about with a cell in a solution.0413

Osmotic pressure is a term that you should understand, and the solute concentration here is drawing water in.0423

The pressure that would needed to be applied to prevent this water from rushing in, to counteract,0434

there is a certain pull that these particles are essentially pulling water in.0441

Well, the pressure that you would need to counteract that is the osmotic pressure of the solution.0446

So, the osmotic pressure on this side is much lower because there are fewer solutes. You would need much lower pressure.0451

Well, actually, there would not be in that movement to that side.0457

But, the osmotic pressure exerted by these solutes, since they are less concentrated, is lower than the osmotic pressure on this side.0460

When we talk about osmolarity, we are really discussing solute concentration, and the unit of measurement of osmolarity is the osmole.0471

And a lot of times, we will talk about milliosmoles. We might say "oh, a solution is 500 milliosmoles per liter".0481

Therefore, when we talk about osmoregulation, we are talking about the maintenance of a constant solute concentration in the blood.0492

Solutes and fluid are obviously very closely-related, so by regulating osmolarity, the kidney is also helping to regulate fluid volume.0504

In mammals, the kidney has a major role in osmoregulation.0514

Before we get on to talking about mammals though, I want to talk about osmoregulation in some very challenging situations.0520

Animals that live in freshwater or in saltwater both face very challenging situations0526

because they are immersed in either a hyperosmolar or hypoosmolar environment.0532

So, let's talk about fish that are living in saltwater. The ocean water is hyperosmolar, their environment.0539

This fish wants to keep its body fluids at a lower osmolarity than the surrounding ocean.0557

So, it is constantly having to fight against the fact that the water is going to want to diffuse out of the fish.0567

So, you have this fish, and he is swimming around in the ocean; and the ocean has a higher solute concentration than in the fish's body.0575

Water wants to leave the fish, his body, so he is fighting dehydration.0592

If he does not fight against it, water will just leave until there is an iso-osmolarity and the solute concentration inside and outside the fish is the same.0599

In order to compensate for loss of water in a saltwater environment, the fish has some mechanisms.0612

One: they drink a lot. Now, when they are drinking, they are drinking seawater, so they are getting the water; but they are getting salt too.0620

So, he is losing water, so he is compensating by drinking water; but as he drinks water, salt is coming into his body.0630

So, what he does is he keeps the water and gets rid of the salt, and in fact, fish in saltwater environments excrete salts via their gills.0636

And this requires active transport because the salt is being moved against its concentration gradient, so this is an energy-requiring process.0647

In addition, they excrete salts via their kidneys, so the kidneys excrete salts into the urine to leave the body.0656

And they only urinate small amounts, so they get rid of the salt essentially and keep the water, so just urinate small volumes.0670

They do not lose a lot of the water via urine.0680

The opposite challenge is presented to a freshwater fish. This fish is living in a hypoosmolar environment.0687

Here is a fish, and there is not a lot of solutes out here. It is a low osmolarity environment.0700

There is more solutes inside the fish.0709

As a result, water wants to keeps rushing in and diluting out his body fluids and diluting out, then, the salt concentration.0711

What this fish needs to do is maintain his salt concentration, and he needs to get rid of excess water and hold on to salts.0721

So, this fish needs to get rid of salt. This fish needs to get rid of excess water.0729

This is accomplished by one: not drinking water. This fish is not going to drink water all the time like this one.0741

This fish is going to take salts in via his gills. In a saltwater environment, a fish excretes salt through the gills.0753

In a freshwater environment, the fish is going to take up salt through its gills, and this fish is going to urinate large amounts of dilute urine,0765

so getting rid of the water by just urinating out the excess water, holding on to the salt, getting rid of the water.0782

I do want to note that some invertebrates in, for example, saltwater environments, have a different method of adaptation.0791

Instead of fighting against the osmolarity of the saltwater and trying to get rid of all the salt,0798

some certain animals actually have a high osmolarity that is the same as seawater.0806

So, their body fluids are as concentrated as the water around them, and there is not this net loss of water to the external environment.0810

Let's start out with an overview of types of excretory systems before focusing on the human excretory system.0823

Starting out with some simpler animals and talking about a system called protonephridia, now, some flatworms just remove nitrogenous waste via diffusion.0831

The nitrogenous waste just diffuse across body surfaces. There are others, though, some types of flatworms use protonephridia.0847

Rotifers use protonephridia, and what protonephridia are, are a network of small tubes; and these small tubes lead to ducts.0858

And the nitrogenous waste are, then, excreted through the ducts, so the ducts connect to the outside of the body,0875

so, a network a tubes leading to ducts leading to the outside.0885

These ducts, though...so, let's say we have this worm - I will draw it a little bit bigger - and he has got this protonephridia.0888

And it leads to a duct to the outside where waste or nitrogenous waste are excreted.0906

Essentially he is urinating. Let's look at it that way, but it is a much simpler system, and these ducts are blind-ending.0913

So, they are open on this end. On this end, it is blind-ending, and at this thermal end, it is what is called a flame bulb.0922

If you look at the flame bulb, there are cilia that project inside in the lumen of the tubule.0935

And what happens is, then, fluid goes in, and these cilia help to move the fluid along the tubule and then, on out.0946

In this way, nitrogenous waste can leave the body, exit their tubules and then, exit out of the surface of the body via ducts.0959

Earthworms and other annelids have a different system. It is called metanephridia, and recall that earthworms have segments.0974

They also have a true coelom, and each segment in an earthworm has a pair of nephridia, so there is one pair per segment.0986

And what these consist of, what a metanephridia consist of, is a tubule surrounded by a network of capillaries, and there is also a bladder.0995

There are two openings here in this system, so here, we are going to have an earthworm, and there are actually two openings:1014

An external opening, and then, we have a system, and then, we have an internal opening.1026

So, the external opening is called the nephridiopore, and then, the internal opening is called the nephrostome.1033

Body fluid within the worm is going to enter this internal opening, the nephrostome.1054

It is going to pass into this tubule, go through the tubule and be processed and then, reach the bladder.1062

It will be urine, and then, exit the body as urine. As the fluid is going through the tubule, it is being processed.1071

And a lot of what we are going to talk about today, we talked about the human kidney is this processing.1081

By processing, I mean that at first, the fluid that goes in here might have some things that the animal does not want to lose.1086

It might need to retain salt. There might be nutrients in there, so things that we do not want to lose as waste are reabsorbed.1093

In addition, there may be waste products that are not initially in the fluid that is leaving the body.1103

So, those waste products can be secreted into the tubule to leave the body.1110

Just very simply, processing takes place in which certain substances are picked up and retained in the body.1115

Others are secreted to leave the body, and then, the urine with the waste products exits the body.1121

So, the constitution of fluid in an excretory system changes as it goes to the body.1129

Here, it is filtered. Processing takes place, and what starts out is what we call filtrate.1135

It ends up being urine.1143

Malpighian tubules are found in many arthropod excretory systems.1146

And this is a different system because these are tubules that project out from the GI tract.1162

So, if you had some insect, and he has this GI tract here, there are tubules the project out from the GI tract.1169

Food is being processed in the GI tract, and then, waste is leaving, exiting the GI tract.1187

Meanwhile, these Malpighian tubules are absorbing or picking up fluid from the body.1193

The hemolymph, which is the circulatory fluid, would be filtered, and this is processed; and then, it actually enters the GI tract.1203

The urine is going to be eliminated along with feces in this type of system because it dumps out into the GI tract.1213

Most of the water that is initially in this urine is reabsorbed along the way in the GI tract.1223

So, this is a good system for insects that live in dry environments and that need to conserve water because very little water is lost.1229

Most of what is exiting the body through the excretory system is actually nitrogenous waste.1238

We are now going to focus on the human excretory system, and a lot of this will apply to other mammals, as well, but again, this is the human system.1245

Humans have a pair of kidneys that filter the blood, and function in both osmoregulation, as I mentioned, and in the excretion of waste.1254

After the urine is formed in the kidneys, it exits via the ureters. There is a ureter leading from each kidney to the bladder.1267

Urine is stored in the bladder and then, leaves through the urethra.1279

So, this is just the basic structure, and we are going to go into a lot more detail particularly about the kidney.1288

The kidney has two sections. This outer portion here is the cortex, and this portion is the medulla.1295

The functional unit of the kidney is the nephron, so the kidney contains nephrons; and that is where the urine is made.1306

There are about 1 million nephrons per kidney, and a huge volume of blood passes through the kidneys every day.1319

In fact, one quarter of the blood that is leaving the heart goes through the kidneys.1327

Blood supply: the renal artery delivers blood to the kidneys, so we have the renal artery.1333

And then, we have the renal vein. The blood leaves the kidneys through the renal vein.1351

Before we talk about how the kidney works, you need to make sure that you understand certain terminologies.1368

The first term you need to be familiar with is filtration. Filtration is the process of the blood passing through a semipermeable membrane.1375

During filtration, certain substances will enter the kidney, those that can pass through the membrane.1397

Other substances in the blood just stay in the blood vessel and stay in the circulation.1405

Those items that pass through...so here, we have the blood vessel, which I will do in detail later about the nephron and the tubule.1411

And there is filtration right here. Those things that can pass through the capillary membrane enter the renal tubule.1424

And this liquid in solutes in this solution is the filtrate.1433

The filtrate consists of everything that passed through the membrane, that made it through the membrane into the kidney.1439

As the filtrate goes through the nephron, the filtrate enters the nephron, it enters the tubule, it is processed.1446

And processing consists of reabsorption and secretion.1456

So, it is important that you understand the difference between these two. These terms get commonly turned around and mixed up.1468

Reabsorption is the process of returning substances to the blood, so substances go from the nephron, from the tubule to the blood.1475

These are things that the body wants to keep, for example, glucose. The body uses glucose.1494

We do not want it lost in the urine. That would be reabsorbed.1500

In certain cases, some substances can be reabsorbed at certain times, secreted at others.1504

But, just keeping it simple, reabsorption are substances that we are putting back into the body, either temporarily or permanently you want to keep.1510

Secretion, that is the opposite process. Secretion is removing substances from the blood.1522

So, in reabsorption, substances are going to go back into the bloodstream.1540

In secretion, substances are going to go into the tubule to leave the body, so removing substances from the blood.1546

In other words, a substance or something that we want to get rid of. It could be excess hydrogen ions.1555

In a certain part of the kidney, it is urea. A substance goes from the blood into the tubule.1563

So, these are things we are putting into the filtrate or into the urine. Reabsorption is removing things from the filtrate or from the urine.1573

This sketch shows a nephron that has been, sort of, flattened out and stretched out.1585

In the kidney, they are actually a lot more coiled up, but to look at this for clarity, this is flattened out.1590

And remember that the kidney has two parts. It has the outer cortex, and it has an inner medulla.1596

So, if I cut the kidney in half, and I sectioned it, I am looking at this section of an opened-up kidney, and I looked at nephrons,1608

and there is a million of them here, what I would see is that these nephrons have portions.1617

If I relate their order into a certain way so that this portion - actually, I will draw that a little bit lower - all of this here is in the cortex.1624

And then, all these, this part, all these down here is in the medulla.1644

So, if I laid this, if I took this and I laid it out here, I would see this glomerulus up here in the cortex, the first part of the tubule here and then,1651

the loop reaching into the medulla and then, this latter part in the cortex, this long collecting duct going down into the medulla, so this is how it is oriented.1660

I am going to go through just an overview of what the different sections are and then,1674

what happens in each section because there is quite a lot to understand, so we are going to spend a lot of time on the nephron.1680

There are two major structures that comprise a nephron. The first is the glomerulus, and the second is this big, all this whole tubule, this renal tubule.1688

The glomerulus is a cluster of capillaries, so it is a cluster of capillaries; and it is located inside this cups-shaped structure called...1705

So, this is the glomerulus. This is Bowman’s capsule.1716

This is the section where filtration occurs, and I am going to go into that in detail after I just finish naming all the structures.1726

So, after filtration, the filtrate enters Bowman’s capsule and then, the renal tubule.1734

This first part of the renal tubule here is called the proximal convoluted tubule or just the proximal tubule.1743

This entire structure here, this loop, is called the loop of Henle.1759

And it is divided into two regions: the descending loop that goes down, and then, we have this turn; and the ascending loop.1765

So, there is a descending loop of Henle and an ascending loop of Henle, and they each have different structure and different functions.1774

As the filtrate enters the renal tubule, it is being processed. There are things being secreted into it.1782

There are things being reabsorbed. Water is moving around.1788

All this is happening throughout, and then, we get through the loop of Henle and enter the distal convoluted tubule or just the distal tubule.1793

Finally, we get to this terminal part of the nephron, which is the collecting duct.1812

And you will notice this is open at the top, and the reason is that urine from other nephrons can drain into a single collecting duct.1819

Urine from this nephron and then, there could be another nephron that drains into here.1829

When this fluid starts out, it is called filtrate. By the time it gets to the collecting duct, it is urine.1834

And then, the collecting ducts are all going to drain into the renal pelvis.1841

The urine will exit the kidney, go through the ureter, be stored in the bladder and go out the urethra.1846

This is just the big picture, and now, what we are going to do is focus first on the glomerulus and filtration.1854

Blood enters the glomerulus through this afferent arterial. It is called an afferent arterial with an A.1865

And then, the glomerulus is this network of capillaries. Blood exits the glomerulus through the efferent arterial.1882

Well, where is this blood coming from?1895

The blood enters the kidney through the renal artery, branches from the renal artery, branch off into afferent arterials.1899

These branch further into capillaries, then, form the efferent arterial, and the blood leaves the kidney.1908

Filtration is driven by the blood pressure within the glomerular vessels.1917

The fluid within these vessels is creating pressure, and that pressure is pushing this filtrate through.1923

And the blood is filtered across the glomerular membrane.1931

Certain substances can pass through the capillary membrane of the glomerulus.1937

These substances will enter this little space here called Bowman’s space and then, enter Bowman’s capsule and now be in the renal tubule as filtrate.1943

So, here is where the filtrate is created through filtration. What you should know is what substances enter the filtrate, are part of the filtrate.1957

So, what substances are small enough to get through the glomerular membrane and enter the filtrate?1968

Well, a major component of the filtrate is water. Other substances that can enter the filtrate are salts.1975

You will find sodium and chloride in the filtrate. Glucose can also enter the filtrate.1989

Amino acids are small enough to enter the filtrate, so things need to be small to get through this capillary membrane.1996

Vitamins can enter the filtrate, nitrogenous wastes such as urea.2003

Now, what cannot enter? What is too big to be filtered?2013

So, cannot enter the filtrate, cells are an example and proteins, red blood cells, white blood cells, proteins.2022

Amino acids can get in, but cells and proteins cannot; and, of course, there is plenty of fluids still left in the vessel here.2030

If somehow red blood cells and white blood cells are passing through this glomerular membrane, there is a problem with the glomerular membrane.2038

There is some kind of disease process going on, and so, in fact, if somebody has blood cells or protein or things in their urine that can be a clue,2046

there are many possibilities that different levels of the urinary tract system, but one possibility is something is going on with the glomerulus.2056

So, cells and protein should not be in the filtrate. These items are in the filtrate.2064

By the time we get to the urine, things like glucose will no longer be in there. They will have been taken back up by the body.2069

Other things will be secreted and added to the filtrate, so the composition of the filtrate changes a lot as it goes through each step until it becomes urine.2076

Urine is very different than filtrate. Filtrate reflects some of the composition of the plasma, whereas, urine is just very different.2085

Before we talk about what happens to the filtrate now that it is here entering the proximal convoluted tubule,2097

I am going to talk a bit more about the blood supply to the kidney.2105

So, blood components that have not passed into Bowman’s capsule: of course, some of the water, the red blood cells, the white blood cells, the proteins.2110

These components that have not made it into the filtrate continue out and leave the glomerulus via the efferent arterial.2122

In addition, the efferent arterial branches off to form capillaries that surround the proximal convoluted tubule and the distal convoluted tubule.2135

And there needs to be a blood supply around these tubules because items are being2146

picked up to put back into the blood or secreted from the blood into the nephron.2152

The nephron's work involves taking things from the bloodstream and putting things into the bloodstream.2161

So, there needs to be a blood supply closely associated with the renal tubule.2166

There is also a set of vessels called the vasa recta, and these are a group of capillaries, so the vasa recta.2172

These are a group of capillaries that are associated with the loop of Henle.2180

They involve one of these counter current exchange systems that we have talked about earlier on, and it actually allows the kidney to concentrate urine.2187

And I am going to talk about the loop of Henle's role in concentrating urine when we get to that part.2197

So, right now, what we left with was filtrate had been formed. It contains water and these substances, various ions.2203

And it enters the proximal convoluted tubule. The proximal convoluted tubule is a very important site for the reabsorption of products and for secretion.2214

So, I am going to clear out some space here, and we are going to talk about what is reabsorbed, what is secreted, what happens here.2226

Water and certain solutes are reabsorbed, and then, other solutes are secreted; so let's start out with reabsorption.2237

And I am going to use red. This is going to be active reabsorption, and then, I will just use blue for passive reabsorption.2247

The filtrate is here in the tubule.2263

And sodium chloride diffuses from the lumen of the tubule, from the space inside the tubule into these epithelial cells of the nephron that line the tubules.2266

So, filtrate is passing through, and there is sodium; and there is chloride, and there are these cells, and this sodium chloride is taken up by these cells.2278

From there, the epithelial cells actively transport the sodium into the interstitial fluid. Excuse me, interstitial fluid, so out here.2297

Outside of the tubule is the interstitial fluid, which plays an important role in the function of the nephron.2310

And the sodium is going to be actively transported from these cells in the tubule outside to the interstitial fluid. Chloride follows passively.2316

Now, glucose, amino acids and other substances are also actively transported outside to this interstitial fluid.2331

And then, from the interstitial fluid, these items are picked up by blood vessels.2343

So, reabsorption involves taking substances from the tubule, putting them back in the bloodstream, and that occurs via a couple steps.2349

The substances end up in the interstitial fluid, and then, they are picked up by these capillaries that are surrounding the nephron.2357

So, sodium that is actively transported, chloride follows passively.2364

There are a number of other items that are actively transported, so remember that glucose, we do not want glucose to be lost in the urine.2369

That is something our bodies could use, the same with amino acids.2379

So, these things are all reabsorbed: glucose, amino acids, vitamins. We do not want to just urinate out important vitamins.2384

Those are actively transported.2398

Now, where solutes go, water follows. Sodium is particularly important in osmolarity, and in fact, where sodium goes, water usually follows by osmosis.2403

So, as the sodium leaves, and the chloride follows through passive transport, water is also being reabsorbed.2413

In summary for reabsorption, we have sodium and chloride with water following.2426

We have glucose, amino acids and vitamins all being reabsorbed in the proximal convoluted tubule.2431

Now, let's talk about secretion, substances that are secreted, active secretion of - that is not the active color, let's change that to red - hydrogen ions.2437

Remember that the kidney plays an important role in homeostasis, and this includes the regulation of pH.2454

Secreting hydrogen ions into the tubule, those hydrogen ions will leave the body, and that will help maintain the pH of the body.2462

In fact, bicarbonate - one more thing here for reabsorption is bicarbonate - is passively reabsorbed. Hydrogen ions are secreted.2473

So, what you can see is that already the filtrate is much different than when it entered.2489

A lot of things have been picked up. Some things have been added.2493

In order to just keep the picture clear, I am going to go ahead now as we go into the loop of Henle, and let's start with a fresh picture of the nephrons.2500

So, we have done filtration in the glomerulus. Here is the proximal convoluted tubule.2508

Here is the loop of Henle.2514

The next segment that the filtrate is going to pass through is the descending loop of Henle, so descending loop of Henle right here.2518

And what is going on here is that there is additional water reabsorption. There are water channels called aquaporins, and these are water channels.2539

These are found in the descending loop of Henle, so water is being reabsorbed all along this descending loop of Henle.2554

Now, what we have here is H₂O is reabsorbed. It is being put into the interstitial fluid, into the interstitium.2568

And ions and other substances are not reabsorbed.2583

The descending loop of Henle is fairly impermeable to ions and various other substances, so sodium is not reabsorbed, chloride, glucose- none of that.2588

The descending loop of Henle is permeable to water. Other substances such as ions are generally not reabsorbed here.2599

As you go deeper from cortex to medulla, there is a concentration gradient.2618

And this concentration gradient is in the interstitium. It is surrounding the renal tubule.2629

There are solutes out here, and as you go deeper from cortex to medulla, this solute concentration gets greater.2638

So, it starts out up here. There are solutes and then, greater and greater and greater and then, down here at the bottom, it is very concentrated.2647

What is driving the reabsorption of water in the kidney, or in the descending loop of Henle here in the collecting duct, as well, is this concentration gradient.2660

Without this concentration gradient, there would not be this drive for water to be reabsorbed.2673

As you go deeper and deeper into the medulla, the concentration of these solutes is greater.2684

So, even though water is leaving the tubule, and therefore, the fluid inside the tubule is becoming more concentrated,2697

water still wants to leave the tubule because the environment outside is still hyperosmolar to what is going on inside.2705

So, this filtrate is becoming more and more concentrated- a higher osmolarity.2712

However, out here is also becoming more and more concentrated, as well,2719

because of this gradient that has been generated that puts more solutes down into the medulla and fewer up by the cortex.2725

So, due to this concentration gradient, water is pulled towards the hyperosmolar environment outside the tubule.2740

Now, this situation for the ascending loop of Henle is very different than in the descending loop of Henle.2745

In the ascending loop of Henle, water is not reabsorbed. However, sodium chloride ions, are absorbed.2759

The channels here that make this permeable to water, those are not found here in the ascending loop of Henle.2773

The descending loop of Henle is permeable to water. The ascending loop of Henle is impermeable to water, permeable to sodium ions and chloride ions.2781

And in fact, the ascending loop of Henle has a very important job of maintaining. It maintains the concentration gradient from cortex to medulla.2798

Sodium chloride is transported passively. Sodium and chloride are transported passively down in the cortex, near the bottom of the loop, so the medulla.2819

So, down at the bottom of the loop of the Henle deep in the medulla, sodium and chloride are passively reabsorbed.2841

They are transported from the renal tubule to the interstitial area.2849

This movement of sodium chloride helps to maintain the concentration gradient that drives the reabsorption of water.2858

And as we go up the ascending loop of Henle in this region that is thicker...2867

It is thicker because these cells actually use more energy because the transport here of sodium is active as we go up here.2873

So, sodium and chloride are still transported, but now, we are talking about active transport of sodium as we go up the ascending loop of Henle.2883

Things to remember because I noticed a lot of information is that the descending loop of Henle, water is reabsorbed, ions are not.2895

The ascending loop of Henle maintains the concentration gradient going from cortex to medulla, and that is maintained by this reabsorption of sodium.2905

We are going to talk in a second about other ways in which this is maintained.2917

But, this sodium and chloride maintain the solute concentration out here in the interstitium.2920

Now, we get to this fluid that has made it through. We are now in the distal convoluted tubule.2926

What happens in the distal convoluted tubule? Well, some of the functions are the same as the proximal convoluted tubule.2932

Once again, we have sodium and chloride are reabsorbed. Potassium, I have not mentioned yet.2939

Potassium is, it can be reabsorbed or secreted in the proximal convoluted tubule.2952

Here in the distal convoluted tubule, potassium is actually passively secreted, and that removes excess potassium from the body.2960

Also, just like in the proximal convoluted tubule, hydrogen can be secreted.2972

One thing to note is that in the distal convoluted tubule, there is no reabsorption of glucose, amino acids, vitamins.2979

Those should have already been reabsorbed in the proximal convoluted tubule. Here, though, we have sodium chloride.2997

We have water following. We have secretion going on of things like hydrogen and potassium.3003

So, it is doing some of the same things such as a proximal convoluted tubule, but not all.3008

Finally, we get to the last segment of a nephron, which is the collecting duct.3013

Well, the permeability here, as far as water, varies, whereas, I said "oh, the descending loop of Henle is permeable to water".3019

The ascending loop of Henle is impermeable to water. The collecting duct is sometimes permeable to water.3027

And we are going to talk in a few minutes about the hormone ADH that controls3037

whether or not this collecting duct is reabsorbing water and how much it is reabsorbing.3043

But, assuming that the hormone, which is ADH, is acting on the collecting duct, then, these aquaporins are also found here in the collecting duct.3052

And if this ADH is acting on it, then, there is more aquaporins that end up in the cell membrane, and water can be reabsorbed.3064

If a person is dehydrated, then, the collecting duct can control the reabsorption of water, whereas here, it is not really a choice.3078

Here, impermeable to water, water reabsorption does not occur.3088

It occurs here. It does not occur here.3091

Here, it is a variable. It depends on the person's situation.3093

If they are dehydrated, they will absorb more water.3095

If they are fluid overloaded, then, these ducts will be closed, and they will not reabsorb water; and instead, that water will go out as part of the urine.3099

Another thing to note is that the latter part of the collecting duct is permeable to urea. Urea is reabsorbed here.3112

Now, you wonder why would urea be reabsorbed. Urea is a nitrogenous waste.3124

That is something we want to get out of our bodies. What is the point of reabsorbing that?3128

Well, there is very important reason, and that is because urea acts as another osmotically active substance,3133

as another solute essentially, to help maintain this concentration gradient.3141

I keep mentioning the concentration gradient because it is essential in concentrating the urine.3145

Without this concentration gradient, we could not produce hyperosmolar urine- urine that is more concentrated than our body fluids,3150

because this gradient is driving the reabsorption of water that I have been talking about.3157

And in addition to the sodium chloride that is reabsorbed at the ascending loop of Henle,3163

this urea reabsorbed at the collecting duct acts as a solute to help maintain this concentration gradient.3169

And actually, the urea is secreted. It is actually retaken up by the ascending loop of Henle.3177

You can think of it this way, the urea is reabsorbed. It ends up in this interstitial space here in the medulla.3189

It contributes to the osmolarity of the medulla, and then, after hanging out there a while, it is picked back up by the ascending loop of Henle.3198

And, it just makes this loop. Although, of course, some of it leaves the body in the urine.3208

The urine is going to contain hydrogen ions and ureas, potassium, other waste products, some water.3214

So, some urea will make its way out, but others, sometimes, are just recycled.3222

After going through all this, what we finally have is the urine. Remember that the urine leaves via the collecting duct.3229

It exits the kidney at the ureter, the two called the ureter, one in each kidney.3240

Those lead to the bladder, storage of urine in the bladder and then, exit from the body excreted.3245

The urine is excreted via the urethra.3254

In this next slide, I am going to focus more on the function of ADH.3257

And I am going to talk about another hormone that helps to regulate water and sodium uptake by the kidney.3262

Antidiuretic hormone is a hormone that is secreted by the posterior pituitary, which is up in the brain.3272

It is actually produced by a gland called the hypothalamus that we are going to cover in the endocrine section.3280

So, it is produced by the hypothalamus. However, it is stored in the posterior pituitary, which is right below the hypothalamus.3288

And it is secreted by the posterior pituitary. The trigger for secretion of this is an increase in osmolarity of the blood.3295

So, remember, the kidney has a very important role in the homeostasis including maintenance of the osmolarity- osmoregulation.3306

Talking a little bit about the function and the name here, it is antidiuretic hormone or ADH. You may hear this referred to also vasopressin.3316

What is a diuretic? Well, a diuretic is a substance that increases urination, increases urine output.3328

Caffeine, caffeinated drinks, coffee, caffeinated sodas, are diuretics. They increase the urine output.3338

Antidiuretic does the opposite. It decreases urine output, so it is antidiuretic.3346

It is against urine output, so it is decreasing urine output.3353

And what happens is if the body fluids become too concentrated, then, let's say for example, you had a bunch of salty chips.3359

You ate a bunch of chips. They have a lot of sodium in them, and then, you are going to have increased solutes in your bloodstream.3374

And therefore, your blood will have increased osmolarity.3383

The increased osmolarity will trigger the release of ADH by the posterior pituitary gland.3389

This is going to enter the bloodstream, so hormones. So they will enter its bloodstream, and it will act on the kidney.3399

What the kidney is going to do is kidney produces more concentrated urine, so less water will be lost in the urine- less water loss.3406

So, more concentrated urine meaning less water is lost, and therefore, if water is conserved, that will dilute out these solutes.3429

And the result will be that the osmolarity of the blood decreases.3440

And what is the mechanism by which this happens? Well, ADH acts on the collecting ducts and the distal convoluted tubule.3451

Recall that I said that the collecting ducts contain water channels- the aquaporins.3475

And what ADH does is it binds to receptors on the collecting duct.3483

And it triggers a cascade that results in an increase in the number of channels in the collecting duct.3489

The result is going to be increased water reabsorption and conservation of water, and their osmolarity will go down.3499

So, it is not triggered by just a loss of blood volume. It is specifically an increase in a concentration of the solutes in the blood:3521

increased osmolarity, release of ADH, kidney produces more concentrated urine and the osmolarity of blood will, therefore, decrease.3528

The second hormone that is very important in osmoregulation is aldosterone. Aldosterone is released by the adrenal cortex.3540

The adrenal glands, which in case you have not watched the endocrine section yet, are glands that are located right on top of the kidneys.3554

And one part of these glands is the adrenal cortex. It produces/releases aldosterone.3562

The trigger is a decrease in blood pressure or decrease in blood volume.3569

And aldosterone is one part of a system called the renin-angiotensin system, which is an important system in maintaining blood pressure.3574

So, let's just start from talking about this activation of this system.3587

There are sensors near the kidney, and they monitor blood pressure and blood volume.3592

If we have decreased blood pressure or decreased blood volume, then, this is detected by the sensors.3609

And in response, renin is released from the kidney.3619

There is a complex cascade that you do not need to know every step of or anything.3630

But, renin is part of a cascade that eventually allows for the cleavage of angiotensinogen, which is a precursor to angiotensin I.3635

Angiotensin I is converted to angiotensin II by an enzyme called ACE- angiotensin-converting enzyme.3661

Now, angiotensin II has two major effects. One is that it directly affects blood pressure.3679

It causes vasoconstriction. In other words, it causes the arteries to constrict.3687

That is going to increase the blood pressure directly.3694

So, if blood pressure is low, this system will trigger vasoconstriction - constriction of vessels - that will raise the pressure in the arteries.3698

A second effect that angiotensin II has is it triggers the adrenal cortex to release aldosterone.3708

What does aldosterone do? Well, aldosterone increases sodium reabsorption by the distal convoluted tubule.3721

Remember that where sodium goes, water will follow, so we have our nephron here, glomerulus.3739

And here in the distal convoluted tubule, if more sodium is being reabsorbed, chloride is going to follow, and water will follow.3750

This increased reabsorption of water is going to increase blood volume and blood pressure.3761

So, we are focusing on aldosterone here because we are talking about the excretory system.3775

But, this whole system helps maintain homeostasis also by angiotensin II vasoconstricting the vessels.3778

Now, one fairly common condition in westernized countries is high blood pressure/hypertension.3786

In other words, high blood pressure, which is a risk factor for stroke, for example.3797

So, in order to control high blood pressure, medications are sometimes used.3803

One of these is an ACE-inhibitor, so it is a medication that is blocking this enzyme.3806

Well, if you block this enzyme, and you block this step, angiotensin II will not exist.3814

It will not vasoconstrict, so blood vessels will remain dilated, and the pressure will be lower.3820

And aldosterone will not cause the retention of sodium in the fluid, so blood pressure will be lower.3825

There is another group of medications, and these are called ARBs, angiotensin receptor blockers, and what these do is they prevent the binding.3832

An ARB would act here, and it would prevent the binding of angiotensin II to receptors on the arteries. This is also a medication to treat hypertension.3846

So, you can see that this basic physiology actually has applications in medicine.3856

Now, we are going to review the lesson for today starting with example one: the figure below illustrates the nephron of an aquatic mammal.3863

And use the figure to explain why the loop of Henle in this mammal is shorter than the loop of Henle in terrestrial mammals.3874

The loop of Henle I showed before would be more like a human loop of Henle, so here is the aquatic animal, a shorter loop of Henle.3882

And then, the human or a typical terrestrial mammal loop of Henle would dip down farther into the medulla.3890

So, thinking about this, let's think about the environment of an aquatic animal. An aquatic mammal is exposed to water all the time.3900

So, it does not need to concentrate its urine as much as a mammal that lives on land especially a land animal that lives in a very dry environment.3915

In a desert environment, an animal will have an even longer loop of Henle, and the reason is,3923

remember that the ascending loop of Henle maintains the concentration gradient going from cortex to the medulla that allows urine to be concentrated.3928

To be more concentrated, it allows for this reabsorption of water. This concentration gradient allows for the reabsorption of water.3960

And the longer the loop of Henle is, the steeper the gradient can be. There is more distance here to pump out sodium chloride.3968

A mammal that has a lot of exposure to water may not need to concentrate its urine as much.3980

Whereas, especially an animal in a very dry environment like the desert, it is going to have to really minimize water loss and have a longer loop of Henle.3986

Example two: why do animals that live in a dry environment secrete, or actually it is excrete, nitrogenous waste in the form of uric acid?3998

Well, remember that there is a couple reasons.4010

In general, uric acid is less toxic than ammonia, so that is why it would be uric acid versus ammonia, but why not urea?4015

Well, dry environment is the key here.4022

Remember that uric acid is not water-soluble, and the big challenge in a dry environment is going to be water conservation.4024

Therefore, uric acid is excreted as a paste. It is not excreted in solution in urine.4035

By excreting it as a paste, the result is minimized water loss. Get rid of the nitrogenous waste with very little water loss.4050

Through what mechanisms do fish living in saltwater maintain their osmolarity below that of surrounding water.4064

Remember that a fish in saltwater is in a hyperosmolar environment. The challenge is going to be not getting dehydrated.4070

Water is trying to leave. Salt is trying to enter, so what this fish needs to do is conserve water, get rid of salt.4086

They do that a few ways. They drink a lot water.4097

Of course, this is seawater, so they are getting the water they need; but they are getting all the salt too, so they get rid of the salt.4101

They excrete salt via their gills, and this is through active transport. They also excrete salt via their kidneys, and they urinate only small amounts.4109

So, ways to prevent dehydration, ways to hold on to water and get rid of salt: drink a lot of water, and you are taking in a lot of salt, too,4131

so get rid of the salt by the gills, get rid of the salt via the kidneys, and kidneys reabsorb most of the water.4141

There is very little urine, and it is very concentrated.4150

Example three: on the figure below, indicate the following: 1. the section of the tubule that maintains the concentration gradient from cortex to medulla.4155

Here, we have the glomerulus, the proximal convoluted tubule. Now, here is the loop of Henle.4166

Remember that the loop of Henle maintains the concentration gradient, and particularly, it is the ascending loop of Henle. Therefore, no. 1 is this section.4172

It maintains this concentration gradient that allows for the reabsorption of water and the production of a hyperosmolar urine.4190

Two: the structure responsible for the reabsorption of glucose.4199

So, initially, blood is filtered by the glomerulus, and filtrate enters the proximal convoluted tubule.4203

In the proximal convoluted tubule, there are some things like glucose and amino acids and vitamins that we do not want to lose in the urine.4213

So, those are reabsorbed right here at this first section of the nephron.4223

The proximal convoluted tubule right here, no. 2, is the structure where reabsorption of glucose occurs.4229

Example four: a child contracts a gastrointestinal infection that causes severe diarrhea. As a result of this fluid loss, his blood pressure drops.4242

So, vomiting, bleeding, diarrhea, these can all cause fluid loss, and fluid loss, in turn, can cause a drop in blood pressure.4254

What hormone will be released to help maintain his blood pressure? By what mechanism will increase the child's blood pressure?4265

Well, remember those two hormones we talked about.4273

And the one that is triggered by a drop in blood pressure or blood volume, actually, is aldosterone.4275

Aldosterone is released by the adrenal glands in response to a drop in blood pressure or blood volume.4282

What is the mechanism of action? Well, aldosterone, remember, acts on the distal convoluted tubule.4291

And what it does is it increases the reabsorption of sodium, and water follows by osmosis.4301

So, more sodium is removed from the tubule back into the blood, and where a solute goes, water will follow.4317

That concludes this section of Educator.com on the excretory system.4327

Thank you for visiting.4332