Lets learn about glomerular filtration.
Your kidneys produce 180 liters of filtrate every day.
All of this filtrate is produced in a microscopic structure called the nephron.
The part of the nephron where filtration occurs is called the glomerulus.
Here is a picture of an actual glomerulus.
Blood goes into the glomerulus and filtrate comes out.
Heres a diagram of a glomerulus.
The glomerulus contains special structures that help to filter the blood.
Lets take a closer look at these structures.
The capillaries inside of the glomerulus are fenestrated meaning they contain holes.
Special structures called podocytes surround the capillaries.
Podocytes have filtration slits in their foot processes.
The fenestrations, podocytes and a basement membrane between them together form the filtration
membrane.
The filtration membrane only allows very small substances through---examples include water,
glucose, electrolytes and amino acids.
Larger substances such as blood cells and plasma proteins do not fit through the filtration
membrane.
Lets take a closer look at a model of the filter.
There are 3 pressures involved in controlling the filter.
Glomerular capillary hydrostatic pressure is the fluid pressure within the capillaries.
It runs about 50 mm Hg.
Glomerular capsular hydrostatic pressure is the fluid pressure within the glomerular capsule.
This pressure runs about 10 mm Hg.
Colloid osmotic pressure runs about 30 mm Hg.
Glomerular capillary hydrostatic pressure favors filtration.
Glomerular capsular hydrostatic pressure opposes filtration.
Colloid osmotic pressure also opposes filtration.
Colloid osmotic pressure is produced by plasma proteins that act like a salt in the blood.
They help to keep the plasma in the blood by creating a pulling force.
All 3 pressures combine to form the net filtration pressure the pressure that drives the filter.
We calculate net filtration pressure by subtracting the 2 opposing pressures from the glomerular
capillary hydrostatic pressure that favors filtration.
The net filtration is about 10 mm Hg.
Net filtration pressure is proportional to glomerular filtration rate which is the amount
of filtrate produced by all of the glomeruli in both kidneys in each minute.
Glomerular filtration rate is controlled by a number of mechanisms.
One of these is the juxtaglomerular apparatus located at the junction of the distal convoluted
tubule of the nephron and the afferent arteriole.
The juxtaglomerular apparatus has special cells that help to regulate the diameter of
the afferent arteriole by vasoconstricting or vasodilating it.
The juxtaglomerular apparatus can also trigger another system called the renin-angiotensisn
system which can help to increase blood pressure.
There is also a myogenic mechanism within the afferent arteriole that causes increased
vasoconstriction when blood pressure is high and vasodilation when blood pressure is low.
This helps to regulate the amount of filtrate produced.
The sympathetic nervous system also has an effect on the filter.
In times of stress, the sympathetic nervous system shunts blood away from the kidneys
via vasoconstriction.
Even though the millions of glomeruli in the kidneys produce around 180 liters of filtrate
every day, most of it is reabsorbed back into the blood by another process known as tubular
reabsorption.
We hope you have learned something about glomerular filtration and see you next time.