Renal Physiology: Overview - Online Tutor, Practice Problems & Exam Prep

The kidneys play a crucial role in urine formation through three processes: glomerular filtration, tubular reabsorption, and tubular secretion. Glomerular filtration occurs when blood pressure forces water and solutes into the glomerular capsule, creating filtrate. Tubular reabsorption selectively transports essential substances like water, electrolytes, and glucose back into the bloodstream, primarily in the proximal tubule and nephron loop. Tubular secretion, the opposite process, removes toxins and maintains acid-base balance by transferring substances from blood to filtrate. Approximately 99% of filtrate is reabsorbed, highlighting the kidneys' efficiency in maintaining homeostasis.

Overview of Renal Physiology

This video is just going to be a general overview of renal physiology. Now, I'm going to preface this by saying that people often assume that the way kidneys make urine is by just taking all the bad stuff out of our blood, and then that's what becomes the urine. But it's actually not like that at all. It's more like the kidneys take almost everything out of our blood, then they have to put all the good stuff back into the blood, and then the leftovers of that process are what ends up being our urine. Though it's much more complex than most people initially think. And nephrons are going to be carrying out 3 distinct physiological processes to help us get to that end goal of creating urine.

So, our first process is called glomerular filtration, and this is the actual formation of the filtrate. Now, as we kind of alluded to in our video on the renal corpuscle, what's happening here is that blood pressure basically forces water and other solutes out of those leaky, permeable capillaries and into that capsular space. And then that fluid is going to be called filtrate. So, as you can imagine, this is happening mainly within our glomerular capsule, which we have drawn for you here in purple. So that's our first step. It's just making the filtrate.

Next, we have our tubular reabsorption. So, tubular reabsorption is the process of reclaiming essential substances from the filtrate. And when I say essential substances, I mean things like water, electrolytes, like sodium and potassium, nutrients like glucose. You know, we don't want to lose all of that to urine, right? We want to keep that. That's really good for us. We need that. So, reabsorption is a really, really important process. And I'll show you, some context in a second for just how intensive the process of reabsorption can actually be. But again, what's basically happening here is the selective transport of substances back into our bloodstream. So we're transporting them from the filtrate back into the bloodstream. Now, reabsorption is going to be happening mainly in our proximal tubule and our nephron loop, but we will see some reabsorption in the distal tubule and collecting duct as well. You can see here in our image, we've colored basically the entire renal tubule and collecting duct in green, but our proximal tubule and nephron loop are in this darker green to indicate that that is where most reabsorption is happening. But again, we're going to see some of that in our distal tubule and collecting duct as well.

And then, our final process is tubular secretion. So, tubular secretion for the most part happens simultaneously with tubular reabsorption, but they are very distinct processes. So, tubular secretion can help us maintain an electrolyte and acid-base balance, and it can also help us remove any toxins or drugs from the blood that were not originally filtered for whatever reason. And tubular secretion is often considered the opposite of reabsorption. So remember, reabsorption is when we basically take substances from the filtrate and put them into blood. Tubular secretion is going to be taking substances from the bloodstream and putting them into the renal tubule or putting them into the filtrate, they're literal opposites of each other. And secretion is going to be happening within our proximal tubule, distal tubule, and our collecting duct, which you can see we have colored on all of those here in blue for you. And as you can see, there is no secretion happening in the nephron loop. The nephron loop is just going to be doing reabsorption.

Now, to give you some context again for how intense this process can be, our kidneys process about 180 liters of blood-derived fluid every single day. That means that we're making about 47 gallons of filtrate every day. And about 1.5 liters of that will actually leave the body as urine. So that can be like approximately 1% or even less than 1% actually turns into urine. So you can see how intense this process of reabsorption actually needs to be. It's about 99% of the filtrate has to go back into the blood. So, very, very fun stuff. We're going to learn all about it. I'm very excited to take this journey with you and I will see you guys in our next video. Bye bye.

Renal Physiology: Overview Example 1

Okay. So the main purpose of tubular reabsorption is to what? So remember the point of reabsorption is to literally reabsorb all of the essential good substances that ended up in the filtrate. We want to pull them back into our bloodstream. So looking at these choices, our answer is going to be b, it's to help us reclaim important substances from our filtrate. Remember that creating filtrate happens in step 1 in glomerular filtration. By the time we get to tubular reabsorption, the filtrate has already been created. And then it can't be C because tubular reabsorption actually helps us decrease urine volume. So if we were not reabsorbing water, for example, all that water would end up in our urine. Meaning we'd have a much higher urine volume. The reabsorption helps us to decrease urine volume if anything. So that's why the answer can't be c, and our answer here is b.

Tubular secretion occurs in the…

• A. Glomerular capsule only.
• B. Proximal tubule, distal tubule, and collecting duct.
• C. Glomerular capsule, proximal tubule, distal tubule, and collecting duct.
• D. Proximal tubule, nephron loop, distal tubule, and collecting duct.

Here’s what students ask on this topic:

What are the three main processes involved in urine formation?

The three main processes involved in urine formation are glomerular filtration, tubular reabsorption, and tubular secretion. Glomerular filtration occurs when blood pressure forces water and solutes into the glomerular capsule, creating filtrate. Tubular reabsorption selectively transports essential substances like water, electrolytes, and glucose back into the bloodstream, primarily in the proximal tubule and nephron loop. Tubular secretion, the opposite process, removes toxins and maintains acid-base balance by transferring substances from blood to filtrate. These processes ensure that the kidneys efficiently filter blood and maintain homeostasis.

How does glomerular filtration work in the kidneys?

Glomerular filtration is the first step in urine formation. It occurs in the glomerular capsule, where blood pressure forces water and solutes out of the permeable capillaries and into the capsular space, forming filtrate. This process is driven by the high pressure in the glomerular capillaries, which pushes fluid through the filtration membrane. The filtrate contains water, electrolytes, glucose, and other small molecules, but larger molecules like proteins and blood cells are typically retained in the bloodstream. This initial filtration step is crucial for removing waste products from the blood.

What is the role of tubular reabsorption in the kidneys?

Tubular reabsorption is the process by which the kidneys reclaim essential substances from the filtrate and return them to the bloodstream. This includes water, electrolytes like sodium and potassium, and nutrients such as glucose. Reabsorption primarily occurs in the proximal tubule and nephron loop, but also in the distal tubule and collecting duct. This selective transport ensures that valuable substances are not lost in the urine. Approximately 99% of the filtrate is reabsorbed, highlighting the efficiency of the kidneys in maintaining the body's fluid and electrolyte balance.

What is tubular secretion and why is it important?

Tubular secretion is the process by which the kidneys transfer substances from the bloodstream into the renal tubule, adding them to the filtrate. This process helps maintain electrolyte and acid-base balance and removes toxins or drugs that were not initially filtered. Tubular secretion occurs mainly in the proximal tubule, distal tubule, and collecting duct. It is considered the opposite of reabsorption, as it involves moving substances from blood to filtrate. This step is crucial for eliminating waste products and excess ions, ensuring the body's internal environment remains stable.

How much filtrate do the kidneys produce daily, and how much of it becomes urine?

The kidneys process about 180 liters of blood-derived fluid every day, producing approximately 47 gallons of filtrate. However, only about 1.5 liters of this filtrate actually leaves the body as urine. This means that roughly 99% of the filtrate is reabsorbed back into the bloodstream. This high reabsorption rate underscores the kidneys' efficiency in conserving essential substances and maintaining the body's fluid and electrolyte balance.