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Imagine a future intergalactic war where our kidney's glomerulus is a galaxy. In this war, you've got humans pitted against giant ogres. The humans of the Hydrostatic Republic want to escape this galaxy, while the ogres of the Oncotic Empire want to prevent them from doing just that. This type of war of opposing forces plays out in your kidney's glomeruli and influences something important called the glomerular filtration rate.
The glomerular filtration rate is the amount of blood filtered by the kidney's glomerulus into the Bowman's capsule per unit of time. It is sometimes abbreviated as GFR. This rate of filtration is influenced by the interplay of several important forces.
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You should know that the blood running through your glomerular capillaries, as well as the filtrate in Bowman's space, has two main forces. One of these forces is called hydrostatic pressure. In our case, this is the pressure exerted by fluid on the capillary walls of the glomerulus or the walls of the Bowman's capsule. The hydrostatic pressure in the glomerular capillaries is essentially the blood pressure in the glomerulus.
As our blood pressure in the glomerulus goes up, hydrostatic pressure in the glomerulus goes up as well. This favors an increase in the glomerular filtration rate because this pressure forces more blood to be filtered by the glomerulus per unit of time.
All else being equal, as the blood pressure increases, the glomerular filtration rate increases, and hence, the amount of filtrate, including water and solutes, entering the Bowman's capsule increases as well. Likewise, as glomerular hydrostatic pressure decreases, the glomerular filtration rate decreases as well.
As the amount of ultrafiltrate entering Bowman's space goes up, the hydrostatic pressure in the Bowman's space goes up as well and vice versa. More filtrate in the Bowman's space means there is a higher hydrostatic pressure in the Bowman's capsule. This, then, causes the glomerular filtration rate to decrease as higher hydrostatic pressure in the Bowman's space counteracts the hydrostatic pressure in the glomerulus that favors the filtration of blood.
It's like a constant arm wrestling match where one hydrostatic force is trying to beat the other. Basically, whereas higher hydrostatic pressure in the glomerulus favors an increased glomerular filtration rate, higher hydrostatic pressure in the Bowman's capsule favors a decreased glomerular filtration rate.
Another force involved in the glomerular filtration rate is called the colloid osmotic, or oncotic pressure. In the case of the glomerulus, this is the pressure exerted by colloids within the glomerulus. These colloids are principally a protein called albumin. In essence, the oncotic pressure caused by albumin tries to hold water back in the glomerulus.
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Keep in mind that under normal conditions, no proteins should be filtered by the glomerulus. Hence, there should be no proteins, mainly albumin, in Bowman's space. Therefore, the oncotic pressure in the Bowman's space is virtually inconsequential unless the kidney is diseased and allows proteins to leak out of the glomerulus and into Bowman's space.
To summarize, under normal conditions, increased hydrostatic pressure in the glomerulus favors filtration, increased hydrostatic pressure in the Bowman's space opposes filtration, and increased oncotic pressure in the glomerulus opposes filtration as well and vice versa.
Recall our introductory example of the humans fighting evil ogres in a galactic war. The humans of the Hydrostatic Republic want to escape the glomerulus, while the ogres of the Oncotic Empire want to keep the humans in the glomerulus.
The majority of humans are thankfully fighting for the Hydrostatic Republic. Since the majority of humans are stuck in the glomerulus, their hydrostatic force is very large in the glomerulus. There are a few humans, however, who are traitors and are putting up a smaller resistance, or hydrostatic pressure, in the Bowman's capsule.
This smaller human force is trying to oppose the exit of the majority humans out of the glomerulus and into Bowman's space. Likewise, there are ogres of the Oncotic Empire who have infiltrated the glomerulus and are trying to hold back the humans from leaving the glomerulus through a force called glomerular oncotic pressure.
Regardless, you can rest easy tonight because this movie, like most Hollywood movies, ends on a good note. The winner of this war is the Hydrostatic Republic. This is because its glomerular hydrostatic pressure is higher than the combined glomerular oncotic pressure and hydrostatic pressure of the Bowman's space. Since it is higher than the two forces opposing the filtration of blood, your blood is able to be filtered by the glomerulus, and therefore, urine can be produced.
Of course, this is the dynamic under normal conditions right at the beginning of the process of ultrafiltration. These forces shift up and down as the blood is filtered, as the systemic or local blood pressure is altered, or as disease sets in. Things like the surface area available for filtration and the permeability of the capillaries also play an important role. The details of this are way beyond the scope of this lesson or its needs.
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However, there is one thing to keep in mind. If there is a problem in your body that causes the GFR to decrease or increase beyond its normal rate, you have two systems, called tubuloglomerular feedback and the renin angiotensin aldosterone system, which do their best to bring the GFR back up or down into its normal range.
That is to say, the systems in our body that regulate the GFR try to keep it at a constant rate. Hence, when you hear that GFR is increased due to tubuloglomerular feedback or the renin angiotensin aldosterone system, it is indeed being increased; however, it's only being increased back to a normal rate because something brought it down in the first place.
Okay, I'll stop frying your brain with any more information. The humans won, the ogres are dead, and we should all be happy we can produce urine.
Regardless, do recall one more time, that the GFR, also known as the glomerular filtration rate, is the amount of blood filtered by the kidney's glomerulus into the Bowman's capsule per unit of time. This rate of filtration is influenced by the interplay of several important forces.
One of these forces is called hydrostatic pressure. In the case of the kidney, hydrostatic pressure is the pressure exerted by fluid on the capillary walls of the glomerulus or the walls of the Bowman's capsule.
The other force, with respect to the glomerulus, is called oncotic pressure, which is the pressure exerted by colloids (principally proteins called albumin) within the glomerulus.
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