What Is Glomerular Filtration Rate (Gfr)?

Glomerular Filteration

The renal corpuscle is a structure that is essential for the filtration of blood in the nephrons of the kidney. The glomerulus is one part of the renal corpuscle is, it can basically be defined as a network of capillaries. The surrounding bowman’s capsule is the other part. Most capillaries have an arterial end and a venous end. This is not the case here though, components of the blood are filtered out as blood flows through the glomerulus from the afferent arteriole to the efferent arteriole, this happens as the components of the blood are filtered out. The glomerular filtrate is defined as the fluid that enters the capsule. And filtration occurs across or to the filtration barrier.

A kitchen strainer is a good analogy for the ultrafiltration barrier. Now if we pour a nice and healthy broth of vegetables into the strainer, you will see that the large vegetables are left behind. Whereas water anything else that dissolves in it plus tiny particles are able to pass through. Now, with our glomerulus, molecules less than 1.8 nanometres (nm) are freely filtered out whereas more particles more than 3.6nm are not filtered.

Ultrafilteration Barrier

If you take a look closely at the ultrafiltration barrier you will notice the following. There are three layers, you would notice that if you look closely. Our bottom layer is the endothelium of the capillary which contains these thing called fenestrations, that are these pores. This layers basically lets everything except blood cells through. Our middle is the basement membrane which prevents the filtration of large proteins. And our outer layer consists of podocytes, that are a part of the bowman’s capsule. These seem to look like monsters that have arms which have many finger-like projections called pedicels, and they are wrapping these arms around the layers below. Pedicels are so near to each other that there are only narrow slits for filtrations between them. These only allow small molecules to get through. The ultrafiltration barrier is the other thing to note, it has the all 3 layers containing negatively charged glycoproteins, and because of this it is difficult got negative molecules to pass through. Hence serum albumin is not filtered, even though it still is in the size range. Depending on a balance between the forces that favour filtration and those that oppose it, ultrafiltration of blood to form glomerular filtrate occurs. These forces can be referred to as starling forces in general.

Hydrostatic And Oncotic Pressure

In order to fully understand glomerular filtration, we need to know about hydrostatic and oncotic pressure. Now a very simplified explanation would be – The force a fluid exerts on the walls of its compartment is referred to as Hydrostatic pressure, this would either be the bowman’s capsule or the walls of the capillaries. You can think of it as pushing, as it is kind of like the way water pushed on the inside of a water balloon as it’s being filled up, but in this instance the fluid can be pushed out. Oncotic pressure is the pressure exerted by plasma proteins on the walls of the walls of the compartment n which they are contained. It kind of has a sponge-like effect. Encouraging fluid to be drawn in, therefore you can think of oncotic pressure as pulling. The major driving for filtration is the hydrostatic pressure of the glomerulus which forces fluid out of the capillary. This is opposed by the hydrostatic pressure of the bowman’s capsule and the oncotic pressure of the glomerular capillary protein. Note that, as only tiny amounts of protein are usually present in the glomerular filtrate, we tend to ignore the bowman’s capsule’s oncotic pressure. Our NFP or net filtration pressure equals to the pressures favouring filtration minus the pressure opposing filtration. That is, hydrostatic pressure of the glomerulus minus the hydrostatic pressure of the bowman’s capsule, minus oncotic pressure of the glomerular capillary protein. Which is equal to 10mm of mercury.

= 55mmHg – 15mmhg – 30mmHg

= 10 mmHg

Glomerular Filtration Rate

there are many renal corpuscles in each kidney as a result of there being many nephrons. GFR for short or glomerular filtration rate, is the amount of filtrate formed by all the renal corpuses in both kidneys per minute. If an individual has kidney impairment, this can be used as a clue to asses it. GFR not only taken into account NFP but also surface area available for filtration, and permeability of glomeruli. GFR is the product of permeability and surface area, which is then multiplied by NFP. Which when condensed, will be – NFP multiplied by the filtration coefficient.

compared to other capillaries and due to the fenestrations and extensive branching and looping, the permeability and surface are of glomerular capillaries tend to be grater and thus, the degree of filtration has a chance of being very high in the glomerulus, since the filtration coefficient is high too. It is important to note that when considering GFR, we must also take into account how effectively the glomeruli filter blood and the number of functioning nephrons.

GFR can be changed by either altering the filtration coefficient or NFP. For example, the hydrostatic pressure of the glomerular blood will decrease due to the reduction of blood available for filtration, if we constrict the afferent arteriole. As this pressure is associated with NFP, this will also decrease and hence, GFR will decrease.

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