- [Voiceover] We're gonna talk about
the transport of sperm.
I just mean how it gets transported
out of the male reproductive tract,
and into the female reproductive tract,
where it can hopefully fertilize an egg,
and result in a pregnancy.
To do this, we're gonna first
look at a sagittal view
of the male reproductive system.
I'll just show you what a sagittal view is.
Basically, if we were to look
at this blue guy over here,
and if we said this is his left side
and this is his right side,
we would make a cut
down his midline, like so,
and we would sort of cut away everything
on one of the sides.
In this case, we're gonna cut away
everything on his left side.
Then we're gonna look,
here's an eye here,
we're gonna look at
his right side of his cut surface
on the right side here.
When we do that,
we find that we see
something like that.
This is what we call a sagittal view.
Just to remind you,
this would be his right leg.
The bottom line is that
sperm needs to be transported
out of our bodies.
We do that, in males,
via a two-step process.
The first step is called erection.
An erection is basically
when the penis goes
from having very little blood within its core,
a state called flaccidity,
and I'll write that out,
to a state where it's filled with blood,
and we call that an erection,
when it's filled with blood.
The second step in our two-step process
is called ejaculation.
Ejaculation is basically
the expulsion of sperm
out of the reproductive tract.
Let's go into the mechanisms
of how this happens.
Before we talk about
how an erection happens,
let's briefly discuss
why an erection happens.
Let's just clear off
some of this stuff here.
Believe it or not,
an erection actually
starts in your brain.
Here we have a brain.
When you're physically
or mentally sexually stimulated
by sights or sounds,
or smells, or even thoughts,
your brain sends signals
to your penis,
and it can cause an erection.
It does that by sending signals
to the blood vessels in the penis,
and those signals cause
those blood vessels to open up,
and allow blood into the penis.
So it's this filling
of the penis with blood
which results in an erection.
We'll expand on that in a moment.
First, we'll talk about
some regional anatomy of the penis.
The base of the penis, here,
that's called the base, or the root.
This area here,
is known as the body.
This area here is called the head,
or, the glans.
What we'll do next
is look at a more detailed view
of the anatomy of the inside of the penis.
The best way to visualize that
is if we did something called
a transverse view.
Transverse view is basically a cut
that goes this way across the penis.
We'll put our little eye here
and we'll look up
at this cut surface here.
When we do that,
you'll find that
it looks something like this.
Again, this is called
a transverse view.
Just to orient you,
this transverse view
is of two different penises,
side by side.
The one on the left is a flaccid penis,
It's not filled with blood.
You can tell because
it's got a lot more
visible blue veins,
which you don't see in erection.
Over on the right we have
an engorged, erect penis.
You can see the veins
have been compressed to the sides.
So that one's erect.
I'll explain all that in a minute.
In this view, it's quite easy to see
the three cylindrical vascular compartments
that get filled with blood during an erection.
These vascular compartments,
or chambers,
are called the corpora cavernosae,
and you have two of those,
one on each side here,
and the corpus spongiosum,
which you have down here.
By the way,
this is the bottom of the penis,
the underside,
and this is the top of the penis.
Also, this structure here in the center,
this tube, is the urethra.
Blood normally flows
into these vascular chambers
via both dorsal arteries
and cavernosal arteries.
Here's a dorsal artery,
and here's a cavernosal artery here.
When the penis is filling with blood,
blood actually leaks out
of these cavernosal arteries
and into these purple circles
that you see called lacunar spaces.
These lacuner spaces run
the entire length
of these vascular chambers,
so there are a lot of spaces there
for blood to leak out into.
That's what causes the penis
to get engorged.
Blood is normally drained out
of these chambers
by veins with the same name.
So, dorsal veins
and cavernous veins.
But under normal conditions,
i.e, when you do not have an erection,
the blood flow into the penis
equals the blood flow out of the penis,
so there's no actual
net change in erection status.
Let's just label these here.
This one on the left
here is flaccid.
This one over here
on the right is erect.
The reason why the one
on the left is flaccid,
is because it has arterials
that are constricted.
These red arterials,
when they're constricted,
they don't actually allow
much blood into the penis.
Over here on the right,
the erect penis is sexually excited.
So you can see visually
that the arterials on this side
are much more dilated,
that is, they let in a lot more blood,
thus can cause an erection.
What's keeping this penis flaccid?
What is keeping these arterials
from opening up?
Well, It turns out that it has
to do with your brain.
Normally, little chemical signals
from the brain,
called norepinephrine,
cause the arterials
to stay constricted.
We can see a neuron here,
leaving the spinal cord,
and sending a signal to the arterial,
to keep it small.
By the way, a neuron is a cell
of the nervous system
responsible for sending messages.
Over on the other side,
on the erect side,
there's another neuron,
actually a different type of neuron,
that actually sends
a different signal to the arterials,
and results in them opening up
and allowing blood into the penis.
The signal that this neuron is sending,
it's called norepinephrine.
What norepinephrine is,
it's a little chemical signal
from a division of the nervous system
called the sympathetic nervous sytem,
or the SNS.
That'll become important later on.
The signal that this neuron is sending
is NO, or nitric oxide.
This neuron is actually from
a division of the nervous system
called the parasympathetic nervous system,
or the PSNS.
As these arterials dilate
and allow lots of blood
into these sinuses here,
they actually fill so much
that they push outward
against the edges
and compress the veinous drainage
of the penis.
That basically prohibits flaccidity,
thus, it results in an erection.
Just to be complete,
an erection is reversed
when the inflow of blood is stopped,
and the veins are then allowed
to open up and allow the blood
back out of the sinuses.
Then you go flaccid again.
That might happen when the nitric oxide
from those green parasympathetic neurons
stop being produced.
One last thing before we move on.
You might be thinking,
"Vish, you told us that
"these arterials dilate and allow blood
"into the vascular chambers."
Well, what's to stop the penis
from filling up indefinitely?
We actually have,
if you look at these yellow circles here,
the three of them,
they surround our vascular chambers
and they prevent them
from over-expanding.
These wrappings are called
the tunica albuginea,
and they're just
a supportive structural tissue.
Let's just clear off some of this stuff
and move on to the second phase,
which is ejaculation.
We'll start with a definition.
What is ejaculation?
Ejaculation is the discharge
of semen from the penis.
You normally discharge about
three to five milliliters
per ejaculation.
In that three to five milliliters,
you actually pack about
300 million sperm.
An ejaculation happens when, basically,
a critical level of sexual excitement
has been reached.
Sexual stimulation actually causes nerves
in the penis to send chemical signals
to the spinal cord and brain.
The brain and the spinal cord
send messages back to the penis
to cause ejaculation.
There's two phases to ejaculation in itself.
We'll start with our erect penis
because now we're sexually excited.
Remember, now that we're erect,
we've filled these
vascular chambers here with blood.
The first part is
sympathetic nervous system stimulated.
Remember, in red we drew these neurons,
the sympathetic nervous system neurons.
They're gonna release norephinephrine,
which I'll abbreviate as NE,
onto all of the following structures:
the epididymis,
the vas deferens,
the accessory glands,
and the ejaculatory duct,
which is here sitting inside
the prostate gland.
To respond to that norepinephrine,
these structures that I mentioned here,
actually contract and emit semen
into the beginning part of the urethra.
In the second phase,
the semen is now sitting here
at the beginning part of the urethra.
The smooth muscle of the urethra itself,
all along its length,
and this muscle at the base
of the penis called the
bulbospongiosum muscle,
they then contract and
expel the semen from the urethra
out of the tip of the urethra,
called the meatus.
That's basically the process of ejaculation.
All of these muscular contractions
are associated with a feeling
of extreme pleasure.
You also get full body physiological changes.
For example, you see a decrease in heart rate
and blood pressure after ejaculation.
In some, the process of ejaculation
and the whole body physiological changes,
is called an orgasm.