Lesson Video: Osmoregulation Biology

In this video, we will learn how to describe the function of the kidney in osmoregulation and creation of urine.

16:49

Video Transcript

In this video, we’ll learn what osmoregulation is and why it’s necessary. Then, we’ll learn how our kidneys vary urine production to balance the concentration of water in our bloodstream. Then, we’ll try some practice questions. And finally, we’ll review what we’ve learned.

Osmoregulation is the maintenance of a normal or ideal concentration of water within the body. Osmo- is a word part that tells us we’re talking about the concentration of a solvent and almost always refers to water. Water is essential for our bodies to function properly, which you may know firsthand if you’ve ever found yourself dehydrated. Without the right amount of water, our tissues have trouble moving materials around, our organs have trouble getting rid of waste, and our body is not able to maintain its ideal temperature.

Water balance is an important component of what we refer to as homeostasis or the maintenance of a constant, normal internal environment. In order for this homeostasis to be maintained, water must enter and exit our bodies in nearly equal quantities. So, how does water get into our bodies and how does it get out? We get water mostly from drinking fluids; water also comes from some of our food. Water is also a byproduct of cellular respiration, which is occurring continuously in almost all of our cells.

We get rid of water mostly by urinating. Water is also lost through the production of feces by the digestive system. Water is removed from our bodies as sweat, and it evaporates from the moist surfaces of our lungs when we exhale. Under normal circumstances, we lose more water through these excretory processes than our cells make during cellular respiration, which is why it’s essential to drink enough water on a regular basis to replenish your body’s supply.

But what happens if this balance is disrupted? What if you challenge yourself to start drinking three liters of water a day? Or what if you take up a new workout regimen that suddenly increases your sweat production? These types of changes have the potential to disrupt homeostasis, and they’ll kick osmoregulation into high gear.

Since urine is the main way that our bodies remove water, osmoregulation is mostly carried out by our kidneys, which we know are excretory organs that filter our blood and produce urine. So when we take in excess water, our kidneys produce more urine to get rid of more water. And when we don’t get enough water or if we lose too much, our kidneys produce less urine in order to conserve the water that’s in our bodies already.

But how do our kidneys know how much urine to produce? The amount of urine produced by the kidneys is regulated by structures within the brain. The hypothalamus of the brain detects the concentration of water in our blood and stimulates the pituitary gland to release a hormone known as antidiuretic hormone or ADH in response. I find this name to be super confusing, so let’s examine it closely.

A diuretic is a substance that increases urine production, and anti- is a prefix that means not or the opposite of. So, antidiuretic hormone is a hormone that does the opposite of increasing urine production. So, ADH decreases urine production. In other words, an increase in ADH leads to a decrease in urine production, while a decrease in ADH will lead to an increase in urine production.

And let’s take a moment to recall that hormones are signaling molecules primarily produced in glands and carried by the blood to their target. So, ADH is carried by the blood from the pituitary gland in the brain to the kidneys, where it affects urine production by changing the permeability of the collecting ducts and renal tubules. In order to understand how this works, we’ll need to review the basics of urine production within the nephron. The relationships that we’ve already learned about are very important to keep in mind as we move forwards. So, I’ve added some notes to the side of our screen.

Here, we have a simplified diagram of a nephron. The nephron is a tiny structure composed of specialized blood vessels and tubules. Each kidney contains about a million nephrons that work together to constantly filter our blood and produce urine. And they empty that urine into collecting ducts that carry it towards the ureter. Blood enters the nephron from the renal artery and passes through a special structure called the glomerulus.

Almost all of the small molecules filter out of the blood through the glomerulus, and this includes most of the water. The filtrate is absorbed by the Bowman’s capsule, which funnels it into a long twisted tubule. As the filtrate passes through the tubule, anything still useful to the body is reabsorbed back into the bloodstream, and this includes most of the water found in the filtrate. The waste is left behind and removed from the body as urine, and the urine is passed from the nephron into the collecting duct. The collecting ducts connect into larger and larger vessels that eventually converge at the ureter, which removes the urine from the kidney.

So, how does ADH impact the urine production process? When the ADH concentration in the blood increases, we’ve already learned that less urine is produced. This change occurs because the ADH changes the permeability of the cells that line certain parts of the tubule and the collecting ducts. Permeability is the measure of the ability of certain materials to pass through a barrier. And in this case, we’re interested in how much water can pass through.

When ADH concentrations are high, the permeability of the cells in the ducts and tubules increases. This means that more water is able to pass through and is reabsorbed into the bloodstream, and less water means that a lower volume of urine is produced. Let’s look at the opposite case. When the concentration of ADH in the blood decreases, the permeability of the cells lining the ducts and tubules also decreases. That means that less water is able to be reabsorbed from the ducts and tubules into the bloodstream. And as a result, more urine is produced.

Let’s return for a moment to our potential lifestyle changes so that we can see this whole system in action. So, I’ve read online that drinking three liters of water a day will give me clearer skin and reduce wrinkles, so I immediately and drastically increased my fluid intake. How does my body maintain internal water balance? Well, first, my increased water intake leads to the increased concentration of water in my blood, and as a result, my blood becomes more dilute. And this change in my blood is detected by my brain, which inhibits the release of ADH by my pituitary gland and decreases the concentration of ADH in my bloodstream.

This decrease in ADH decreases the permeability of the ducts and tubules in my kidneys. This decreased permeability causes less water to be reabsorbed into my bloodstream. This causes an increase in the volume of urine produced by my kidneys, and that urine is going to be pale, dilute, and watery. And this increased urine production will eventually return the concentration of water in my blood back to normal whether I achieve radiant skin or not.

What about the opposite situation? A busy biology student decides to take a 30-minute run every afternoon to increase their cardiovascular health. Their increased physical activity increases their sweat production. How does their body maintain normal internal water balance? Well, their increase in physical activity suddenly increases the amount of water they lose in sweat every day, which decreases the concentration of water in their blood, making it more concentrated with solutes.

Their brain detects this change in their blood and stimulates the pituitary glands to increase ADH production, which increases the concentration of ADH in the bloodstream. The increased ADH increases the permeability of the ducts and tubules in the kidneys, which increases the volume of water reabsorbed, which means that their kidneys will produce less urine than normal and that urine will also be darker in color and more concentrated. And since less water than normal is being used in urine production, the concentration of water in the blood will eventually return to normal.

When a process functions to return a system to normal, we refer to it as negative feedback. And we’ve seen this concept illustrated here in osmoregulation. The water concentration in our bodies is constantly fluctuating. And this complex system of osmoregulation serves the purpose of keeping it as close to normal as possible.

So now that we’ve learned how the kidney and brain work together to maintain normal water levels within the body, let’s try a practice question.

Which of the following statements best describes how ADH helps regulate water balance? (A) If present, ADH increases the permeability of the collecting duct, so more water is excreted in urine. (B) If present, ADH decreases the permeability of the collecting duct, so more water is excreted in urine. (C) If present, ADH increases the permeability of the collecting duct, so more water is reabsorbed into the bloodstream. (D) If present, ADH increases the permeability of the collecting duct, so there are higher concentrations of glucose reabsorbed into the blood. Or (E) ADH has no effect on the regulation of water balance in the body.

ADH is an acronym that stands for antidiuretic hormone. A diuretic is a substance that increases urine production, and anti- is a prefix that means not or the opposite of. So, ADH is a hormone that decreases urine production in the body. ADH is produced in the brain, and it’s carried by the bloodstream to the kidneys, which are the organs in our body responsible for producing urine. And the production of urine is one of the main ways that our body regulates the amount of water contained in the bloodstream, also referred to as water balance.

That being said, we can draw some conclusions about the relationships between ADH, urine, and water excretion. An increase in ADH would lead to a decrease in urine production. And since less urine is being made, less water is being removed from the body. In contrast, a decrease in ADH production would lead to an increase in urine, which would mean that more water is being excreted from the body.

Well, the kidney makes urine in millions of tiny structures called nephrons. And what I’ve drawn is an extremely simplified diagram that we can use as an illustration. So, how the nephron works is that blood enters from the renal artery and then flows through a special structure called a glomerulus.

In the glomerulus, all of the small molecules filter out of the bloodstream, and this includes most of the water. This liquid, now called filtrate, enters the renal capsule, also called a Bowman’s capsule. The filtrate flows from the capsule into a long, twisted tubule. And from the tubule, everything that is not considered waste is passed back into the bloodstream, which includes most of the water. What’s left in the tubule is now considered urine. Several nephrons empty into one collecting duct, which join into larger and larger vessels until they exit the kidney through the ureter.

So how does ADH change how much urine is produced? Well, ADH affects urine production by changing the permeability of the collecting ducts and the tubules. And permeability means the ability of water to pass through the linings of these vessels. When more water is able to pass from the collecting duct and back into the bloodstream, there’s less water present in the urine. So, the volume of the urine decreases.

So, we can summarize what we’ve learned in that if ADH is present in the bloodstream, less urine is produced. That’s because ADH increases the permeability of the ducts and tubules in the nephrons, which allows more water to be reabsorbed into the bloodstream. Since more water is reabsorbed, less water is excreted and we have a lower volume of urine.

Now, we know everything we need to know to choose the correct answer to our question. So, now, we can review our answer choices again and choose the correct one. I’d like to start with choice (D), which refers to the concentration of glucose. This choice doesn’t answer the question of how water balance is regulated. And then choice (E) says that ADH has no effect on the regulation of water balance in the body. But we know that ADH affects urine production, which indeed affects water balance.

So that leaves us with the choices (A), (B), and (C), which each describe relationships between ADH, permeability, and the amount of water either excreted or reabsorbed. If we examine them carefully, we find that the correct choice is (C), which states, if present, ADH increases the permeability of the collecting duct so that more water is reabsorbed into the bloodstream.

Let’s wrap up our lesson by reviewing what we’ve learned. In this video, we learned what osmoregulation is and why it’s important. We learned how the brain and the kidneys communicate through the concentration of ADH, or antidiuretic hormone, and water within the bloodstream. And we learned how ADH changes the permeability of structures within the nephrons of the kidney, which impacts the amount of water reabsorbed into the bloodstream and the volume of urine excreted from the body.

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