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 know the overall function and many structures of it.
Muscular system? Okay, you get the point. We're adding to our body system's 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 later, so don't skip too much or you miss the best part. Okay, 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 gotta find an osmotic balance by managing the water in the solutes in the body. And two, you have to get rid of metabolic waste. What's metabolic waste? Well, it could include carbon dioxide or nitrogenous waste, 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, which is the liver, the liver, highly involved in detoxification and produces urea, 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 we want to isolate these structures.
Kidneys, there are two, 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 kidneys, 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.
The nephron 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 the 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 plus ions, bicarbonate ions, 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 will process 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 excrete 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, and those transport types move with the gradient from high to low concentration.
But sometimes substances are transported by active transport, requiring ATP. Let's go! We're going 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 to say NaCl in this video a lot, but realize that to enter or leave, the Na and Cl can separate to go through separate proteins. Water will follow by osmosis, which makes sense as this area is hypertonic. due to the NaCl.
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, and bicarbonate 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 a outside the tubule into the proximal tubule.
H plus ions and ammonium ions are some items secreted. With what's being reabsorbed and secreted, like bicarbonate and H plus, 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 ascending limb going up.
We'll 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 you descend down and water can get out of the 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 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 are 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 secrete it, 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 this 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 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, and thus that person's urine may be more dilute. Now we mentioned what urea was and that throughout the nephron there would be times it was reabsorbed and secreted. In the collecting duct, there is a significant amount of urea in this filtrate.
But, since there is a 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 coming 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 Mipa Sisters, and we remind you to stay curious.