Welcome to urine formation step one, Glomerular Filtration.
The first step of urine formation is glomerular filtration.
The part of the nephron Involved is the glomerular capsule.
The primary blood vessel involved is the glomerulus.
However both the afferent and efferent arterioles play a role.
Let's take a closer look.
You can see here that blood will be entering into the glomerulus,
this knot of capillaries, through the afferent arterial.
So the afferent arterial leads into the glomerulus, and
the efferent arterial leads out.
Now, blood originally is pumped from the heart,
down the aorta into the renal artery and
then comes into the afferent arterial from there.
It's really important to note the diameters of the afferent and
efferent arterioles.
The diameter of the afferent arteriole is much larger than that
of the efferent arteriole.
This is important because it means that more blood is getting pumped into
this knot of capillaries, the glomerulus than can leave at any given time.
This creates very high pressure in this knot of capillaries and
the pressure in fact is so high that substances flow
out of these capillaries and into the glomerular capsule.
So the pressure is so high it forces small molecules to leave
the blood and enter into this first tubule of the nephron.
Now, one thing to point out is that there are podocytes
which are part of the glomerular
capsule that attach to the blood vessels of the glomerulus.
So when we look here, you can actually see the podocytes in yellow
wrapping around that knot of capillaries, the glomerulus.
And the reason it does that is to actually make sure that the walls of the glomerulus
do not rupture from the high blood pressure.
Now, between the podocytes,
You kind of see that the podocytes do not cover every where.
Between the podocytes are little slits or pores and that will
allow the small molecules to leave the Glomerulus and
enter the tubule of the nephron the Glomerular capsule.
Now, let's talk about what gets filtered out.
So what actually enters into the glomerular capsule?
I like to call these small molecules NWs.
And the reason why I call them NWs is because those letters are the first
letters of all of the substances.
So, we have nitrogenous wastes,
that's the first thing
That we want to talk about, and we'll look at what those wastes are.
So, nitrogenous wastes entered.
There's several different types.
So, NW.
We have nutrients Sorry.
My pen is not working as nice as I would like.
Then we have water so another pair of NWs, and lastly, we have salts or ions.
So all of these NWs form the glomerular filtration which
is what enters into the glomerular capsule.
So all of those are pushed out of the blood in the glomerulus and
enter the glomerular capsule.
So, nitrogenous waste, nutrients, water, and salts or ions.
The nitrogenous waste that we look at are urea,
uric acid, creatinine and ammonia.
The nutrients are sugars like glucose and amino acids.
There are a variety of salts or ions such as sodium,
potassium, calcium, chlorine, magnesium, bicarbonate and hydrogen ions.
So the filtrate formed from all of those is called glomerular filtrate.
There are large molecules that are not filtered out that will
travel through the glomerulus and then continue in the efferent arterial.
So any, there will be water that remain as well but
the large molecules that we talk about never leaving are the formed elements.
So we have red blood cells, white blood cells and
platelets that remain in the blood vessels at all times, as well as plasma proteins.
So those molecules are all too large to leave the blood vessels.
They cannot go through those pores, they are too large.
So only small molecules end up leaving.
Now, one thing to note when we talk about glomerular filtration is that more
substances are forced out of the blood than our bodies want.
So there are things that we would, our body would like to keep.
If we look at the list over here, things like our nutrients.
So the glucose and amino acids are both things that our body wants to keep.
In addition, more water and salt are filtered out than our body would like.
So, our body wants to get those substances or molecules back and
that will actually lead to the second step of your information, tubular reabsorption.
To summarize, Glomerular Filtration Occurs between the glomerular capsule, which is
the tubule of the nephron, and the glomerulus, which is the blood vessels.
Water, salts, nutrients, and nitrogenous wastes move from the glomerulus
to the inside of the glomerular capsule.
These small molecules, once inside the nephron,
are called the glomerular filtrate.