Glomerular filtration

Definitions & Key takeaways

Glomerular filtration is a process by which blood plasma is filtered through the glomerular filtration membrane. This is the first step of urine formation, by which the kidney starts to eliminate toxins from blood plasma. The glomerular filtration membrane has tiny pores allowing only small elements to pass, and ideally spare large molecules such as albumin and blood cells.

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Introduction0:00–0:28

The workhorses of the urinary system are the kidneys which are the twin, bean-shaped organs in your body that clear harmful substances by filtering blood - like a water purification plant that helps clean the drinking water for a city.
Blood filtration happens inside the over a million nephrons scattered inside each kidney, and each nephron is made up of a renal corpuscle and a renal tubule.
So let’s zoom in on the renal corpuscle, which is where blood filtration starts. The renal corpuscle is made up of the glomerulus - a tiny bed of capillaries - and the Bowman’s capsule surrounding the glomerulus.

Renal Corpuscle0:28–1:08

Between the glomerulus and Bowman’s capsule there’s a space called Bowman’s space. Blood gets to the glomerulus through the afferent arteriole, but interestingly enough, once the blood leaves the glomerulus, it doesn’t enter into venules.
Instead the glomerulus funnels blood into efferent arterioles which divide into capillaries a second time. These capillaries are called peritubular capillaries - because they are arranged around the renal tubule.
Now, the first step in blood filtration happens at the glomerular filtration barrier. The glomerular filtration barrier is made up of three layers and together they separate the blood inside the glomerular capillaries from the fluid inside Bowman’s capsule.

Glomerular Filtration Barrier1:08–1:36

They work like a sieve, allowing water and some solutes in the plasma like sodium, to pass into Bowman’s space, while keeping red blood cells and plasma proteins in the blood.

Endothelium1:36–2:10

Starting from the capillary lumen, the first layer of the glomerular filtration barrier is the endothelium, made up of glomerular capillary endothelial cells.
These cells have fenestrations, which are like pores in the cell themselves, tiny spots where the cytoplasm isn’t filled in so that solutes and proteins can pass right through.
But the fenestration are tiny so they block red blood cells from passing through. Blood minus red blood cells is plasma - so plasma gets to the second layer of the glomerular filtration barrier, which is the basement membrane.

Basement Membrane2:10–2:26

The basement membrane is a gel-like layer with tiny pores and this layer prevents plasma proteins from passing through. That’s because the pores are too tiny for plasma proteins to slip through, and because the basement membrane has a negative electric charge, which repels the negatively charged plasma proteins.

Epithelial Layer2:26–2:58

The third layer of the glomerular filtration barrier is the epithelial layer, which is made of special cells called podocytes that wrap around the basement membrane like the tentacles of an octopus.
Between these tentacle-like projections are tiny gaps called filtration slits. This third layer works with the basement membrane to block the passage of plasma proteins.
Fluid that makes it past all three of these layers is called glomerular filtrate. Now, the volume of fluid that gets through the glomerular filtration barrier is ultimately governed by Starling forces.

Starling Forces2:58–4:26

Starling forces include both the hydrostatic pressures, which are fluid pressures, as well as oncotic pressures, which are pressures based on the amount of protein, that are present on either side of the barrier.
Since there are no proteins in Bowman’s space, there’s are only three Starling forces at play: the hydrostatic pressure of capillary blood, pushing against the endothelium of the barrier, which we’ll refer to as Pgc; the hydrostatic pressure of filtrate in Bowman’s space, which we’ll refer to as Pbs; and the oncotic pressure determined by the concentration of proteins in capillary blood - referred to as ℼ gc.
Together, these three pressures determine the net ultrafiltration pressure of the glomerulus, which is the difference between the force favoring filtration, Pgc, and the two forces opposing filtration, Pbs + ℼ gc.
So the net ultrafiltration pressure is that gives us Pgc - (Pbs + ℼ gc). Now, let’s think about the net ultrafiltration pressure at different point along a glomerular capillary.

Net Ultrafiltration4:26–5:28

Early on in the capillary, right around where blood flows in from the afferent arteriole, oncotic pressure is at its lowest, because the capillary is still full of fluid and that means that the protein concentration is relatively low.
As blood passes through the capillary, fluid is filtered into Bowman’s capsule, so the concentration of proteins in the capillary increases, and, as a consequence, oncotic pressure increases as well.
By the end of the capillary, right around where blood is ready to flow into the efferent arteriole, oncotic pressure reaches a maximum value, and that makes the net ultrafiltration pressure approach 0.
This point is called the filtration equilibrium, and from this point on, there’s no net shift in fluid in either direction - going towards the Bowman’s space or going towards the capillary.
In other words, once filtration equilibrium is reached, blood is no longer filtered by the glomerulus. Net ultrafiltration pressure directly determines the glomerular filtration rate - which is the total amount of filtrate produced by all the glomeruli in both kidneys in a minute.

Glomerular Filtration Rate5:28–6:17

Glomerular filtration rate is the net ultrafiltration pressure multiplied by the filtration coefficient - or Kf. Kf indicates how permeable a capillary is to fluid, and it depends on the surface area that’s available for fluid filtration as well as the permeability of the glomeruli.
The glomerular capillaries have a relatively high Kf when compared to systemic capillaries, because glomerular capillaries are huddled together like a ball, which increases the total surface area for filtration, and because the glomerular capillaries have fenestrations.

Capillary Hydrostatic Pressure6:17–7:40

The glomerular filtration rate can vary based on changes in any of the three Starling forces. First, let’s look at capillary hydrostatic pressure, which is mainly determined by the resistance of the afferent and efferent arterioles.
If there’s vasoconstriction of the afferent arteriole, this reduces renal blood flow, and, in turn, capillary hydrostatic pressure and glomerular filtration rate.
If there’s vasodilation of the afferent arteriole, there can be the opposite effect - increased renal blood flow, increased capillary hydrostatic pressure, and an increase in glomerular filtration rate.
Now, if there’s vasoconstriction of the efferent arteriole that decreases renal blood flow; however, since the increased resistance is after the glomerulus, there’s more fluid inside the glomerular capillaries, and that leads to an increase in hydrostatic pressure and glomerular filtration rate.
If there’s vasodilation of the efferent arteriole that increases renal blood flow, but since blood encounters less resistance exiting the glomerular capillaries, there’s a decrease in hydrostatic pressure and glomerular filtration rate.
Second, let’s look at oncotic pressure. An increased concentration of proteins will lead to an increase in oncotic pressure - and that leads to a decrease in glomerular filtration rate.

Oncotic Pressure7:40–8:05

A decreased concentration of plasma proteins, has the opposite effect - there’s a lower oncotic pressure and an increase in glomerular filtration rate.
Third, there’s the hydrostatic pressure in Bowman’s space. If there’s a blockage in urine flow, like a stone that’s lodged in the ureter, blocking the drainage of urine, then the back up of urine can cause an increase in hydrostatic pressure opposing filtration.

Hydrostatic Pressure in Bowman's Space8:05–8:38

An increase in hydrostatic pressure in Bowman’s space leads to a decrease in glomerular filtration rate. And typically, there aren’t situations that cause a decrease in hydrostatic pressure in Bowman’s space.

Review8:38–9:10

All right, as a quick recap: the glomerular filtration membrane sits between the blood and Bowman’s space. The glomerular filtration rate is the amount of blood filtered each minute, and it’s determined by three Starling forces: capillary hydrostatic pressure, capillary oncotic pressure and hydrostatic pressure in Bowman’s capsule.
Together, they determine the net ultrafiltration pressure, which is the main determinant of glomerular filtration rate, along with the filtration coefficient - or Kf.