Excretory System and the Nephron

Captions are on!

Is it true that some body systems are  more familiar? Well-known? For example,

the circulatory system: many automatically know:  it includes the heart. Nervous system: yeah,

the brain and many other things involved with  it. Digestive system? We all know the overall

function and many structures of it. Muscular  system: ok you get the point. We’re adding to

our body systems video collection, but there’s  one system that I notice people aren’t quite

sure about what it includes, nor do they realize  how incredible it is - It's the excretory system.

In this video, we’re going to  introduce the human excretory system,

and then put extra focus on  the kidneys, and then…the

nephron. The nephron we save for the end. SO,  DON’T SKIP THE END OR YOU MISS THE BEST PART!

Ok, so let’s consider two obstacles that  have to be addressed for survival. I mean,

there’s a lot of obstacles  – but here are two big ones.

One - You have to maintain an osmotic balance–  that means you got to find an osmotic balance

by managing the water and the solutes in the body.  And two - You have to get rid of metabolic wastes.

What’s metabolic waste? Well, it could include  carbon dioxide. Or nitrogenous wastes – which

occur from the breakdown of proteins. There’s a  lot of protein breakdown in metabolic processes

after all. The excretory system focuses  on addressing those two major issues.

Many organs and structures have roles in  addressing these two issues and therefore serve

roles in the excretory system. The skin- which can  excrete water and substances. The liver – highly

involved in detoxification and produces urea-  more on urea later. The lungs – which excrete

the gas waste carbon dioxide. And those organs  are also involved in other systems – remember

body systems don’t work in isolation- the skin is  an organ of the integumentary system, the liver is

an accessory organ in the digestive system,  lungs are organs of the respiratory system.

But our focus in our short time is going to be  on another set of organs that play a huge role

in the excretory system – the kidneys. In fact,  we could say the urinary system if wanted to

isolate to these structures: kidneys – there are  two and found in the lower back. The bladder – a

single sac that will hold urine. Ureters- there  are two and these drain the urine produced from

the kidneys to the bladder. Urethra – a single  tube where the urine will travel out of the body.

So urine is produced by the kidneys, and  we’re going to focus on the process that makes

urine. Blood is filtered by the kidney,  producing urine, which is a portion of

the body’s waste products that need to be  excreted. A reminder, we like to show a

general and simplified version of very complex  topics – so explore our description for more!

We start with this beautiful  thing called…the nephron.

Each kidney has tons of them. Like one  kidney can have a million of these things.

They are the functional unit of the kidney.  It has a lot of different parts, but

its overall goal is to process waste  products from the blood to create urine.

The first part of the nephron that we’ll talk  about has something called the glomerulus,

which you could consider a specialized mass  of capillaries. It is surrounded by this,

the Bowman’s Capsule. Blood pressure forces fluid  from the blood in the glomerulus into the Bowman’s

capsule. Once this fluid is in the Bowman’s  capsule, the fluid is called the filtrate. What’s

in filtrate? Well, here are a few major things:  water. Some glucose and amino acids. Salts.

H+ ions. Bicarbonate ions (HCO3-). Other ions.  Some medications- if applicable. Some vitamins.

And urea. A nitrogenous waste produced by the  liver that the body needs to get rid of. So the

nephron is going to take this filtrate through the  ride of its life while it processes it . Some of

the filtrate is reabsorbed, meaning some of the  filtrate will cross the barrier of the nephron

back into the fluid surrounding the nephron (also  called the interstitial fluid), and eventually

circulate again through the body. But in order  to get rid of some components -and eventually

excreting them as urine- those items will be in  the tubes of the nephron eventually to form urine.

Some substances that enter or leave the nephron:  they might travel by diffusion or facilitated

diffusion. Remember, diffusion and facilitated  diffusion don’t require the help of ATP and

those transport types moves with the gradient  from high to low concentration. But sometimes,

substances are transported by  active transport – requiring

ATP. We aren’t really focusing on the specific  type of transport in this video, but that’s a

fascinating concept to further explore and we do  want you to know it varies– see video details.

Let’s go! We go from the Bowman’s capsule to  Proximal Tubule. Proximal can mean “near” and it

is the tubule nearest the glomerulus. Important,  because there is another tubule later on.

In this proximal tubule, NaCl moves to the  interstitial fluid. A little about NaCl:

it is a salt. I’m going say NaCl in this video  a lot but realize that to enter or leave, the

Na+ and Cl- can separate to go through separate  channels. Water will follow by osmosis – which

makes sense, this interstitial fluid is  hypertonic due to NaCl exiting. Therefore,

we say salt and water are reabsorbed because  they’re not staying in the nephron here-

they’re going to the interstitial fluid.  Other substances like glucose, amino acids,

potassium (K+), and bicarbonate (HCO3-) are  also reabsorbed- again meaning, they’re going

to the interstitial fluid by either active or  passive transport. Now when we say reabsorbed,

not all of these are 100% reabsorbed, and so some  concentrations of these remain in the filtrate.

Now what is secreted? Meaning what will  move from outside the tubule (the fluid

surrounding this area of the nephron) into  the proximal tubule? H+ ions and ammonium

ions (NH4+) are some items secreted. With the  substances being reabsorbed and secreted – like

bicarbonate and H+– you can tell the proximal  tubule is important for pH regulation.

Now we move into the loop of Henle. It has a  descending limb – going down – and an ascending

limb – going up! We start with the descending  limb – going down. There’s a lot of aquaporins

here. Remember those channels? Aquaporins make it  easy for water to travel through. So the water can

get reabsorbed here because the water can get out  of the loop and into the interstitial fluid. And

by osmosis, water would travel that way as the  interstitial fluid is hypertonic at this part.

That means the interstitial fluid has  a higher solute concentration than the

filtrate - and remember that water generally  has a net movement towards hypertonic areas.

Now, the descending LIMB of Henle doesn’t have  channels for most solutes – like salt – and so

NaCl is stuck in the nephron. So as you  descend down and water continues to exit,

the solute concentration INSIDE the filtrate  of this descending LIMB continues to increase.

Time to go up the ascending LIMB of Henle. Now  there aren’t aquaporins here meaning the water in

the filtrate can’t get out. But there are proteins  that the NaCl can now travel through to leave and

get reabsorbed. So in the thin segment of this  ascending limb, NaCl will diffuse out. It makes

sense that it would – the NaCl is moving from a  high concentration of NaCl [in the filtrate] to

a lower concentration of NaCl [in the interstitial  fluid]. Now in the thick segment of the ascending

limb, NaCl continues to exit the nephron but this  time it’s pumped out by active transport. So if

you’re losing all this salt, you can imagine it  is making the filtrate very dilute at this point.

Okay, now on to the distal convoluted tubule. In  this area, you’ll see H+, ammonium, potassium,

and more substances secreted – which means into  the filtrate it goes. Whereas substances like

NaCl, water, and bicarbonate will be reabsorbed,  meaning they’re exiting the filtrate to be

reabsorbed into the interstitial fluid. This  distal tubule also contributes to pH regulation.

And now, it is time, for the collecting duct.  It is time for the filtrate to become urine.

NaCl will be able to be reabsorbed. Water,  too, but hormonal control really regulates

the amount of water here. The permeability to  water of this collecting duct is controlled by

hormones. After all, a person who is dehydrated  needs to have as much water as possible

to be reabsorbed into the interstitial  fluid. The filtrate, on the other hand,

will be very concentrated. A person who has  had a lot of water to drink may have less

water reabsorbed into the interstitial fluid – and  thus that person’s urine may be much more dilute.

Now we mentioned what urea was and that  throughout this nephron there would be

times it was reabsorbed and secreted. In the  collecting duct, there is now a significant

amount of urea in this filtrate, but since  there is high concentration, I do want to

mention that some urea will be reabsorbed by  diffusion into the interstitial fluid, too.

Urine that is produced by  the nephrons of the kidneys

will ultimately travel down the two  ureters. Then the urine will be stored

in the bladder before it is expelled  from the body through the urethra.

Now, again, there are far more substances  moving in and out of the nephron than we

focused on. But the overall goal is that you  can see the complexity of this. It’s not just

that kidneys are filtering out stuff from the  blood – no, the nephrons of the kidneys control

the reabsorption of many substances and then  the secretion of many substances and all of

this is influenced by hormonal control and the  osmotic balance that surrounds these structures!

So intricate. In fact, there are medications  used to treat certain conditions that act on

these osmotic values. For example, diuretics.  Diuretics are prescribed for high blood pressure,

congestive heart failure, and other conditions.  While there are different types of diuretics,

one major concept is they tend to increase the  amount of water in the filtrate of the nephron so

more water is typically present in the urine.

What about a situation where a person has  severely compromised kidney function? If a

kidney transplant is not ideal  or possible for the situation,

the person may need regular dialysis.  Hemodialysis or peritoneal dialysis

are options that involve filtering the  blood and assisting with osmoregulation.

If learning about the kidneys makes you  want to explore more, just to emphasize:

there are careers focused on kidney function  alone. A nephrologist for example. Well,

that’s it for the Amoeba Sisters,  and we remind you to stay curious.