00:01
So external respiration
or pulmonary gas exchange
is going to involve
the exchange of oxygen
and carbon dioxide across
our respiratory membranes.
00:12
This exchange is going to be
influenced by several things.
00:16
First, the partial pressure
gradients of each of these gases
as the gas solubilities.
00:23
Second, the thickness
and the surface area
of our respiratory membranes,
and finally a
phenomenon known as
ventilation and
perfusion coupling
which is going to match
alveolar ventilation
with pulmonary blood perfusion.
00:39
So taking a closer look
at these influences
and starting with
partial pressure,
recall that there is a steep
partial pressure gradient for oxygen
that exists between the
blood and the lungs.
00:52
Your venous blood are
the blood returning
from your tissues has a
partial pressure of oxygen
of 40 millimeters of mercury.
01:02
This is compared to the alveolar
partial pressure of oxygen
which is much higher
at a hundred and four
millimeters of mercury.
01:10
Because of this,
oxygen is going to flow down
its concentration gradient
out of the alveoli
and into the venous blood.
01:20
We reach an equilibrium
between these two
across the respiratory
membrane at about .25
or a quarter of a second.
01:29
However,
it takes our red blood cells
about three-quarters of a second
to travel from the start
to the end of a
pulmonary capillary.
01:39
Because of this delay
and the travel of
the red blood cells.
01:43
It allows for an adequate
oxygenation of the blood cells
because of this like
slowing our bottleneck.
01:53
So the partial pressure gradient
for carbon dioxide is a less
steep than that of oxygen.
01:59
In the venous blood
or the blood returning
from the tissues.
02:03
The partial pressure
of carbon dioxide
is 45 millimeters of mercury.
02:08
And the alveoli,
this is going to be
a little bit lower
at 40 millimeters of mercury.
02:15
Although this gradient is not
as steep as that of oxygen.
02:19
The carbon dioxide is
still going to diffuse out
an equal amounts.
02:24
And the reason why is
because carbon dioxide
is much more soluble
and plasma than oxygen
and is therefore held
on to a little bit more
so it doesn't require
a steep of a gradient
in order for it to move out.
02:40
So another factor that
is going to influence
our external respiration
is the thickness
and the surface area
of our respiratory membranes.
02:49
So recall that our respiratory
membranes are extremely thin
and there are only about a
half to one millimeter thick.
02:57
This allows for efficient
diffusion across this membrane
into and out of the alveoli.
03:05
Also the alveoli have
a huge surface area.
03:09
In fact, the surface area
of all of our alveoli
put together is
more than 40 times
the surface area of
all of your skin.