Renal Physiology Step 1: Glomerular Filtration - Online Tutor, Practice Problems & Exam Prep

The filtration membrane in the kidneys consists of three layers: fenestrated endothelium, basal lamina, and filtration slits formed by podocytes. This structure selectively allows water, ions, and small solutes to pass while retaining blood cells and large proteins. Glomerular filtration pressure, influenced by hydrostatic and colloid osmotic pressures, determines the net filtration pressure, crucial for maintaining homeostasis. The average glomerular filtration rate is about 125 mL/min, directly correlating with systemic blood pressure, impacting blood volume and electrolyte balance.

The Filtration Membane

In this video, we're going to be talking about the filtration membrane. Now, if you have ever had a urine test done at a doctor's office, you might remember that they tested your urine for trace amounts of blood or excessive protein. And that's because the presence of blood or proteins in your urine can indicate a problem with your kidneys. So your kidneys have to find a way to filter out this huge volume of fluid from your blood while holding back all of your blood cells, platelets, and most proteins. And that is where our filtration membrane comes in. So the filtration membrane is just the membrane between the capillaries and the capsular space, and it's going to allow the passage of water and any solutes smaller than plasma proteins. And the filtration membrane has 3 layers, and you actually already know about 2 of these. So it's going to be a nice easy lesson for you.

So first, we have the fenestrated endothelium of the glomerular capillaries. These are just the big old pores in those leaky capillaries that we've talked about before. As we've mentioned, those fenestrations are going to allow blood components, except blood cells and platelets to pass through. The gaps of these pores are quite large. They're about 70 to 100 nanometers. So, again, pretty much everything except blood cells and platelets would be able to get through the gap.

Next, we have our basal lamina. The basal lamina is a thin layer of extracellular matrix gel between the two other layers. This gel actually has a negative charge which helps it to repel negatively charged plasma proteins like albumin and globulin. The gaps in this layer are quite tiny. They're about 8 nanometers. You can see there's a big jump there between the big pores of those capillaries and the gaps in the basal lamina.

And then finally, we have our, our finest layer of the filtration membrane. We have the filtration slits of those podocytes. Remember, the podocytes have those foot processes or pedicels that wrap around the glomerular capillaries, and they interlace to form those filtration slits. And again, this is the finest layer, and the gaps here are going to be approximately 6 to 7 nanometers. And so the filtrate that will end up in the capsular space is going to contain water, it'll contain ions like sodium, potassium, chloride, nutrients like glucose and amino acids, as well as waste products like urea and uric acid.

If we come down to our image here, you can see what we are essentially looking at is a zoom in of this wall of the capillary here where filtrate is going to be coming out. And so we can see here on the left side of our zoom in, what we're looking at here is the inside of the capillaries. So we have like a red blood cell, a white blood cell, we've got platelets, big old plasma proteins, and then teeny tiny solutes like ions, all kind of floating around in there. And then on the right side of the image, this tan area is our capsular space where our filtrate is going to end up. And then in the center, we have our filtration membrane there. You can see we have the fenestrated endothelium of the capillaries here. And you can see they have those fairly large pores in between them. There's plenty of space for things like the plasma proteins to get through that layer. But then we have this kind of mint green, basal lamina in between. Again, this layer is quite fine, about an 8-nanometer gap, and it's going to have that negative charge that will help repel those plasma proteins. And then we have our finest layer with our filtration slits, and you can see how teeny tiny those openings would be. And really, only the tiniest solute, ions, water molecules, etcetera, would be able to get through there.

Alright. So that is our filtration membrane and I'll see you guys in the next one. Bye-bye.

Renal Physiology Step 1: Glomerular Filtration Example 1

Okay. So which of the following would not be able to pass through the fenestrated endothelium of the glomerular capillary? Keep in mind that this is the layer of our filtration membrane that has the largest gaps. The point of this layer is to allow all blood components except for blood cells and platelets to get through. So our answer here is C, red blood cells. We don't want any blood cells ending up in our urine. They need to stay in our capillaries. Right? And keep in mind that ions, water molecules, and glucose are all tiny enough to end up in our filtrate. So all of these substances are small enough to get through all three layers of the filtration membrane. So our answer here is C, the red blood cells.

The ________________ is the finest layer of the filtration membrane.

• A. Fenestrated endothelium of the glomerular capillaries.
• B. Basal lamina.
• C. Filtration slits of the podocytes.

Overview of Filtration Pressures

Okay. So, now that we're getting into renal physiology, we're going to be talking quite a bit about glomerular filtration pressure as well as glomerular filtration rate. But to understand those, we're first going to just do a quick overview of filtration pressures and how they work in our body. So as you may recall from some earlier chapters, there are two main forces that drive fluid movement in a capillary bed. So the first of these is hydrostatic pressure, and hydrostatic pressure is the force of a fluid on the wall of its container or basically on the wall of that capillary. This is usually about equal to blood pressure. What's happening here is that the pressure of the blood is literally just pushing the blood up against the wall of the capillary. And then if that capillary is leaky enough, we're going to see some water and other small solutes potentially get pushed out of the capillary. If you look at our little cartoon here that's labeled hydrostatic pressure, you can see we have this wave of blood pushing against the wall of that capillary, this green wall, and then we have all of these little solutes like sodium and potassium, as well as water molecules ending up in this kind of tan-colored interstitial space. So that is broadly how hydrostatic pressure works.

Now, our next force is colloid osmotic pressure. Colloid osmotic pressure is basically the pressure created by proteins, mainly albumin, that are in the plasma. What happens here is that the proteins create an osmotic gradient that pulls water back into the capillary. Remember, water tends to move from low solute concentrations toward high solute concentrations. They're attracted to those high concentrations of plasma protein. Remember how we talked about a long time ago, how, like, I always think of ions, like sodium and potassium as like introverts. Like they want to follow that concentration gradient and get out of a crowd. Water is the opposite. Water is an extrovert. It wants to come to join every party that it sees. So it's going to be moving from low solute concentrations toward high solute concentrations. It's attracted to all those proteins in our capillary. If you look at our little cartoon over here labeled colloid osmotic pressure, you can see in the capillary, we have a little plasma protein party. There's plenty of plasma hanging out. And the water molecules over here are like, "Hey, that looks super fun. We want to go over there." And so they get drawn back into that capillary to join that little party. So that is broadly how colloid osmotic pressure works.

Now, when you combine these, you end up with net filtration pressure. Net filtration pressure will determine the direction of fluid movement as well as the force, between the capillaries and the interstitial fluid. Alright. So, that is our quick overview. And after a quick example, I will see you guys in our next video to talk about how this actually works in our kidneys. So I'll see you there.

Renal Physiology Step 1: Glomerular Filtration Example 2

All right. So, water moves out of the capillary if blank is higher than blank. So, remember, between our two forces here, hydrostatic pressure and colloid osmotic pressure, hydrostatic pressure is the one that will be forcing fluids out of the capillary because, basically, the blood pressure is pushing fluid up against the wall of that capillary and so fluid can potentially escape if the capillaries are going to be leaky enough. And so our answer here is going to be water moves out of the capillary if hydrostatic pressure is higher than colloid osmotic pressure. And there you go.

Glomerular Filtration Pressure

Okay. So now that we understand filtration pressures more generally, let's talk about glomerular filtration pressure. Glomerular filtration pressure is determined by three factors. First up, we have glomerular hydrostatic pressure, and the main principle at work here is, of course, hydrostatic pressure. This one will be largely determined by systemic blood pressure. What's happening here is high resistance causes the blood to push against the walls of the glomerular capillary. This force favors filtration. Basically, the blood pressure will be pushing fluid through the filtration membrane. The actual pressure happening is pushing fluid against that membrane. And so you can see in our image, we have our blood, it's pushing against that filtration membrane, and then all of the water molecules, electrolytes, you know, little tiny solutes get through and end up in the filtrate. This is going to be a pretty powerful force. It's gonna be about 50 to 55 millimeters of mercury.

Now, our next force is capsular hydrostatic pressure. This is also just the basic principle of hydrostatic pressure. But what's happening here is that the filtrate in the capsular space builds up its own hydrostatic pressure. This force is actually going to be opposing filtration. Because what's happening is that the hydrostatic pressure of that filtrate pushes fluid back into the capillary or pushing against the filtration membrane from the opposite side. You can see we have this filtrate here and the hydrostatic pressure of that filtrate is pushing on the filtration membrane in the opposite direction. This force is, lucky for us, pretty weak. It's about 10 to 15 millimeters of mercury.

And then our final pressure is glomerular colloid osmotic pressure. The main principle here, of course, being colloid osmotic pressure. What's happening is that the high concentration of plasma proteins, specifically albumin within the capillaries, is creating an osmotic gradient. Remember, water wants to move from low solute concentration toward high solute concentration. And because the blood in the capillaries is now highly concentrated since all the fluid has moved out that is looking very attractive for all those water molecules. And so this force also opposes filtration. The osmotic gradient is drawing water back into those capillaries, or that's where the water at least wants to be. They can see our, in the little cartoon that we just saw in the previous video, we have our little water molecules hanging out in the filtrate, and they're seeing that high concentration of plasma proteins and they're like, "Hey, we wanna go join that party." Right? So they're getting drawn back toward the capillary. This force is gonna be about 30 millimeters of mercury.

Now, when you add all those numbers up together, our net glomerular filtration pressure is going to be about 10 millimeters of mercury favoring movement through the filtration membrane. This number is actually very important. Our body really wants to keep that pretty consistent because if this number changes by even like 10 to 18%, it can actually have devastating effects on the body, which we will talk more about in our glomerular filtration rate video. So I will see you guys in our example that we can learn how to actually solve for this number together. So, I will see you guys there.

Renal Physiology Step 1: Glomerular Filtration Example 3

Okay. So we are going to solve for net filtration pressure together using this equation here. Based on this equation, net filtration pressure is going to be equal to glomerular hydrostatic pressure, so we'll call that 50 minus, and then in parentheses capsular hydrostatic pressure, so we'll just use 10 plus lumaryllar colloid osmotic pressure, which is 30. So 10 plus 30 is 40, and then 50 minus 40, of course, is 10. So that is how we get that 10 millimeters of mercury. And again, we will talk about why that number is so important in our video on glomerular filtration rate. So I'll see you there.

In the process of filtrate formation, which of the following factors creates an osmotic gradient?

• A. Systemic blood pressure.
• B. A high concentration of negative ions in the capillaries.
• C. A high concentration of plasma proteins in the capillaries.
• D. A low concentration of water in the capsular space.

Glomerular Filtration Rate

In this video, we're going to be talking about glomerular filtration rate. Now glomerular filtration pressure, which we just talked about previously, directly impacts glomerular filtration rate. And glomerular filtration rate is just the amount of filtrate formed by both kidneys in one minute. And it's on average going to be approximately 125 milliliters per minute. Now, on average, what we see is that in healthy individuals, blood pressure and glomerular filtration rate are positively correlated. So generally, what's going on here is that systemic blood pressure is the strongest predictor of glomerular filtration pressure. And then glomerular filtration pressure is the strongest predictor of glomerular filtration rate. And so what we end up with is that if systemic blood pressure increases, typically in a healthy person, glomerular filtration rate will also increase. If systemic blood pressure decreases, glomerular filtration rate will also decrease. So those variables are always moving in the same directions. We have a beautiful positive correlation there. And again, this is on average in healthy individuals and that this is the pattern that we would expect to see. Now, glomerular filtration rate is highly regulated by the body. And that is because it has a very strong impact on blood volume, blood pressure, as well as just our general homeostasis and health.

So just to give you some examples of what can happen when glomerular filtration rate kind of gets out of whack. If we were to have chronically increased glomerular filtration rate, we would end up with an increased urine output. Because we're just making more filtrate, and more filtrate means more urine. Because of that increased urine output, we would have a decreased blood volume because water is leaving the body. Right? We have decreased blood volume which would lead to decreased blood pressure. That can lead to things like dehydration as well as electrolyte imbalances. We're going to be losing a lot more electrolytes, in all of that urine. Now on the flip side, if we were to have chronically decreased glomerular filtration, we're going to end up with a decreased urine output where it's not going to urinate as often. We're going to be retaining all of that water in our body which will lead to increased blood volume as well as increased blood pressure. And then, the adverse effects of that could be things like hypertension, edema, or swelling, as well as the retention of waste products and toxins within our body because they're obviously not getting excreted in urine, at the correct rate. So you can see why glomerular filtration rate really needs to be kind of held at a nice constant. And we're going to spend the next couple of videos talking about many of the ways that our body monitors and regulates glomerular filtration rate. So I'll see you there.

Renal Physiology Step 1: Glomerular Filtration Example 4

Okay. So, Caitlin is a 25-year-old woman. She has no underlying health conditions and does not take any medication. When her blood pressure increases, what outcome would you expect to see? So remember, blood pressure, glomerular filtration pressure, and glomerular filtration rate are all going to be positively correlated. They're all going to be moving in the same direction. So if blood pressure increases, we're going to expect glomerular filtration pressure to also increase. So based on that, we can narrow it down to either B or C. And then if glomerular filtration pressure increases, we're going to expect glomerular filtration rate to increase. So it looks like our answer is B. As blood pressure increases, our glomerular filtration pressure increases, and then our glomerular filtration rate increases as well. So there you go.

Which of the following is a possible consequence of a prolonged or chronic decrease in glomerular filtration rate?

• A. Dehydration.
• B. Leukemia.
• C. Edema (swelling).
• D. Hypotension.

Here’s what students ask on this topic:

What are the three layers of the filtration membrane in the kidneys?

The filtration membrane in the kidneys consists of three layers: the fenestrated endothelium, the basal lamina, and the filtration slits formed by podocytes. The fenestrated endothelium has large pores (70-100 nm) that allow most solutes to pass but block blood cells and platelets. The basal lamina is a thin extracellular matrix gel with a negative charge, repelling negatively charged plasma proteins and having smaller gaps (about 8 nm). Finally, the filtration slits are formed by the interlacing foot processes of podocytes, with gaps of approximately 6-7 nm, allowing only the smallest solutes to pass through.

How does hydrostatic pressure influence glomerular filtration?

Hydrostatic pressure is the force exerted by a fluid against the walls of its container, such as blood against capillary walls. In the glomerulus, glomerular hydrostatic pressure, primarily determined by systemic blood pressure, pushes fluid through the filtration membrane into the capsular space. This pressure favors filtration, driving water, ions, and small solutes out of the blood and into the filtrate. The typical glomerular hydrostatic pressure is about 50-55 mmHg, making it a significant force in the filtration process.

What is the role of colloid osmotic pressure in glomerular filtration?

Colloid osmotic pressure is created by plasma proteins, mainly albumin, in the blood. These proteins generate an osmotic gradient that pulls water back into the capillaries from the filtrate. In the glomerulus, this pressure opposes filtration by drawing water back into the blood, counteracting the hydrostatic pressure. The typical glomerular colloid osmotic pressure is about 30 mmHg. This balance between hydrostatic and colloid osmotic pressures determines the net filtration pressure, crucial for maintaining proper kidney function and fluid balance.

How is glomerular filtration rate (GFR) regulated in the body?

Glomerular filtration rate (GFR) is regulated by several mechanisms to maintain homeostasis. The primary factors include systemic blood pressure, which directly influences glomerular hydrostatic pressure, and thus GFR. The body also uses autoregulation mechanisms like the myogenic response and tubuloglomerular feedback to adjust the diameter of afferent and efferent arterioles, stabilizing GFR. Hormonal controls, such as the renin-angiotensin-aldosterone system (RAAS), also play a role by adjusting blood volume and pressure. These mechanisms ensure that GFR remains relatively constant, despite fluctuations in systemic blood pressure.

What happens if glomerular filtration rate (GFR) is too high or too low?

If GFR is too high, it leads to increased urine output, resulting in decreased blood volume and blood pressure, potentially causing dehydration and electrolyte imbalances. Conversely, if GFR is too low, urine output decreases, leading to fluid retention, increased blood volume, and blood pressure, which can cause hypertension, edema, and the accumulation of waste products and toxins in the body. Therefore, maintaining a stable GFR is crucial for overall health and homeostasis.