hiya welcome to biomed sessions with me
Ruz today we're going to be discussing Glomerular filtration
here we have our renal corpuscle which is a structure
essential for the filtration of blood in the nephrons of the kidney
one part of the renal corpuscle is the glomerulus
which is basically a network of capillaries the other part
is the surrounding bowman's capsule most capillaries
have an arterial end and a venous end
this is not the case here as blood flows through the glomerulus
from the afferent arteriole to the efferent arteriole
and as this occurs, components of the blood are filtered out
The fluid that enters the capsule is called glomerular filtrate
and filtration occurs across or to filtration barrier
and filtration occurs across an ultrafiltration barrier. A good analogy for the ultrafiltration barrier
is a kitchen strainer. Now if you pour a nice herby broth of vegetables
into the strainer you will see that large vegetables
are left behind whereas water and anything that dissolves in it
plus tiny particles are able to pass through.
with our glomerulus, molecules less than one point
eight nanometers are freely filtered out whereas molecules more than 3.6nm
are not filtered. Let's take a look
at a zoomed in section of the ultrafiltration barrier .
As you can see there are three layers.
Our bottom layer is the endothelium of the capillary
which contains these pores known as fenestrations.
This layer basically lets everything through
except for blood cells. Our middle layer
is the basement membrane which prevents the filtration
of large proteins. And our outer layer
consists of podocytes -- part of the bowman's capsule.
These look like monsters wrapping their arms around the layers below
and they themselves have many finger-like projections called pedicels
are so close to each that there are just narrow filtration
slits between them
which allow only small molecules to pass through.
One thing to note is that the ultrafiltration barrier
As all 3 layers contain negatively charged glycoproteins
it is difficult for negative molecules to pass through
Hence serum albumin is not filtered
(despite being in the size range). Ultrafiltration of blood
to form glomerular filtrate depends on a balance between the forces that
and those that oppose it. In general
we can refer to these forces as starling forces
in order to fully understand
glomerular filtration you need know about hydrostatic
and oncotic pressure, so I'm going to give you a very simplified explanation.
Hydrostatic pressure refers to the force
a fluid exerts on the walls of it's compartment (this would be either the walls
capillaries or the bowman's capsule). I like to think of it
I like to think of it as 'pushing' because it is kinda like the way water pushes on the inside of a
as it's being filled up but in this instance the fluid can be pushed
out. Oncotic pressure is pressure exerted by plasma proteins
on the walls of the compartment in which they are contained. It kinda has a
encouraging fluid to be drawn in, therefore I like to think of
pressure as 'pulling'. The major driving force for filtration
filtration is the hydrostatic pressure of the glomerulus
which forces fluid out of the capillary. This is opposed by hydrostatic pressure
of the bowman's capsule and the oncotic pressure
of the glomerular capillary protein. [NOTE:
[NOTE: we tend to ignore oncotic pressure of the bowman's capsule
as only tiny amounts of protein are usually present
in the glomerular filtrate] Our Net filtration pressure
(NFP) equals to the pressures favouring filtration
minus the pressures opposing filtration i.e.
hydrostatic pressure of the glomerulus minus hydrostatic pressure of the
minus oncotic pressure of the glomerular capillary protein
which is equal to ten millimeters of mercury
There are many nephrons and hence there are many renal corpuscles
in each kidney. Glomerular Filtration Rate
GFR for short, is the total amount of filtrate formed by
all the renal corpuscles in both kidneys per minute
it can be used as a clue to assess whether an individual
has kidney impairment. GFR
not only takes into account NFP but also Surface
area available for filtration and permeability
of glomeruli . In fact,
equals to the product of Surface area
and permeability multiplied by NFP
which can be condensed to the filtration coefficient
multiplied by NFP. The permeability
and surface area of glomerular capillaries
tend to be greater compared to other capillaries due to the fenestrations
and extensive branching and looping, hence the filtration coefficient
is high and so there is a high degree of filtration
in the glomerulus. It is important to note
when considering GFR we must also take into account the number of
and how effectively the glomeruli filter blood.
So How can GFR be changed?
Well of course by altering either the filtration coefficient
or NFP. For example if we constrict the
afferent arteriole, the Hydrostatic Pressure of the glomerular blood
will decrease due to the reduction of blood available for filtration.
as this pressure is associated with NFP
will also decrease and hence GFR
will decrease. In a later video, we
will go into more detail about the control of GFR
and how it is estimated. But for now
I would like you to understand that the glomerulus alongside the Bowman's capsule
is highly specialized in the filtration of blood
& the Glomerular filtration rate, GFR,
is a good indicator of how well the kidneys are working.
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