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.
