Hello, in this video, we're going
to focus on lung physiology, focusing on gases and pH.
And we'll also look at the oxygen dissociation curve.
So air enters the respiratory system in the upper airways,
proceeds to the conducting airways
and then onto the respiratory airways made up
of the branches of the bronchi, bronchial, terminal bronchioles
and finally ends in the alveoli.
You can think of the alveolarus, which is singular of alveoli,
as the building blocks of the lungs where
gas exchange occurs--
gas exchange occurring between the lungs and the bloodstream.
The gas exchange in this case is between oxygen and carbon
dioxide, or CO2.
So just recapping, the basic physiology--
the heart will pump deoxygenated blood
from the right ventricle towards the lungs.
deoxygenated blood means blood containing a higher
concentration of CO2 with lower amounts of oxygen.
In each alveolus, the carbon dioxide will be dropped off
and exhaled out of the body, so removing carbon dioxide
from a body.
And then oxygen will be inhaled and will then
reoxygenate the blood.
This newly oxygenated blood or reoxygenated blood
will then go back to the left side of the heart.
The heart will then pump this oxygenated blood
around the body to the other organs, such as the brain.
The deoxygenated blood delivers oxygen to organs, tissues.
The organ tissues then uses up this oxygen
to help create energy in the form of ATP.
Now as a process of creating this energy ATP, the organs,
the tissues will form a byproduct,
which is carbon dioxide.
Carbon dioxide is then made and is actually
expelled into the bloodstream as exchange, you can say.
Now because carbon dioxide is in the bloodstream,
the blood is now deoxygenated once again.
And this deoxygenated blood will return
to the right side of the heart.
The right side of the heart will continue this cycle.
It will pump blood to the lungs to reoxygenate the blood.
Oxygen itself travels in the body mainly in red blood cells.
Oxygen binds to these molecules called hemoglobin,
and we have millions of these in each red blood cell.
In deoxygenated blood, with deoxygenated hemoglobin,
or deoxyhemoglobin, the structure of hemoglobin
here is referred to as a tight structure.
Hemoglobin-- just recapping--
is a protein made up of four subunits, each of which
contain a heme [INAUDIBLE] attached to a globin chain.
And each globin chain contains iron where oxygen normally
binds to.
Each of the four iron atoms in hemoglobin
can reversibly bind one oxygen molecule.
Let's see what happens when deoxyhemoglobin receives
oxygen. Well, the reaction is rapid,
requiring less than 0.01 seconds.
Remember in deoxyhemoglobin, the globin units
are tightly bound in a tense configuration,
in a t configuration, which means that it actually
reduces the affinity of the molecule for oxygen.
When oxygen is first bound, the bonds holding the globin units
are released, producing a relaxed configuration, or an r
configuration, which exposes more oxygen binding sites.
So to put it simply, you can imagine it like cascade.
Firstly, the hemoglobin is in a tense configuration
with no oxygen. And then when oxygen starts to bind,
it will become more relaxed, a relaxed configuration,
which actually increases the affinity for oxygen even more--
kind of weird.
This whole thing is called positive cooperativity.
So by the end, the hemoglobin molecule is oxyhemoglobin,
because it is filled with oxygen.
And it is in a relaxed form.
So from the lungs, you now have heaps
of oxyhemoglobin, which means your blood is reoxygenated
really.
And this reoxygenated blood is ready to be
delivered to the body tissues.
It's a good idea now to briefly talk
about partial pressures of oxygen and carbon dioxide
in deoxygenated blood and also in oxygenated blood,
because it will be relevant to what we'll talk about later.
But you can imagine that when blood is oxygenated
or reoxygenated from the lungs, it
would have a higher partial pressure of oxygen, so
higher amounts of oxygen, 100.
And carbon dioxide is low, 40.
The deoxygenated blood I am talking about
are found in your arteries, such as your aorta.
Deoxygenated blood, on the other hand, is very different.
When we talk about deoxygenated blood,
we are talking about the veins.
So oxygen can be up to 40 millimeters mercury and carbon
dioxide about 45.
The changes in partial pressure is
important to understand because it helps explain the hemoglobin
dissociation curve.
And we will talk about that now.
The hemoglobin dissociation curve, or the oxygen hemoglobin
dissociation curve, relates a percentage saturation
of the oxygen carrying power of hemoglobin
to the partial pressure of oxygen. Now
that might sound crazy, but let's just
try to understand it together.
So on the x-axis, you have the partial pressure
of oxygen, 25 to 100 millimeters of mercury.
And on the y-axis, you have the saturation
of hemoglobin, which what we're looking at specifically
is oxyhemoglobin in percentage.
This saturation of hemoglobin means
how much oxygen is bound to the hemoglobin basically.
The more oxygen bound to hemoglobin,
the higher the oxyhemoglobin and therefore,
the higher the saturation.
So the oxygen hemoglobin dissociation curve
has a characteristic sigmoid shape
due to the tense and relaxed configuration inter-conversion.
There is a plateau here where the partial pressure of oxygen
is nearly high.
The plateau signifies that high partial pressure of oxygen
does not cause large changes in oxygen
saturation of hemoglobin.
So at the plateau, you can see that the partial pressure
of oxygen of 100 already has an oxygen saturation of 98%.
So there is not much change whereas before the plateau--
so a partial pressure of oxygen or 50, let's just say,
already has an oxygen saturation of 85%, so a dramatic change.
So note that small changes at low partial oxygen pressure
leads to large changes in oxygen saturation.
Another example is that a low partial pressure of oxygen,
such as 26, has already oxygen saturation of 50%.
The reason for this sigmoid shape
and why a low partial pressure of oxygen
has a quick increase in oxygen saturation
is because of positive cooperativity When
the first heme in the hemoglobin molecule
receives oxygen, the affinity increases for the second
and the third, and so on.
Now let's look at factors which can shift this curve, which
are essentially factors affecting
the affinity of hemoglobin for oxygen.
Three important conditions affect the oxygen hemoglobin
dissociation curve.
These are the pH, the temperature,
and the concentration of 2, 3-diphosphateglycerate, or DPG.
Here is the curve normally.
We will look at examples of what shifts the curve to the right
and what shifts the curve to the left.
When the curve shifts to the right,
it means that there is reduced hemoglobin oxygen affinity.
And this occurs in tissues.
So basically, the hemoglobin affinity for oxygen
is decreased.
Remember the normal curve, the partial pressure
of oxygen of 50 had an oxygen saturation of about 83% to 85%.
In this new curve, the partial pressure of oxygen of 50
has an oxygen saturation of 70%, so reduced oxygen saturation.
And this is because there is reduced affinity for oxygen.
Things that cause this to happen include an increase
in carbon dioxide, which really means a decrease in pH.
Other things that causes the shift to happen
include an increase in temperature
and an increase in DPG, or BPG.
It's good to remember that the reduced hemoglobin
oxygen affinity occurs in tissues in general.
Because when you think about it, you
want to give your oxygen to the body tissues
so that they can use it.
These tissues include the placenta,
because you want to offload oxygen to the fetus,
and muscles cells, especially during exercise.
Now the curve can also shift to the left here in red.
What happens here is that we see higher hemoglobin oxygen
affinity.
And for this example, you can think of the lungs,
because it is in the lungs where oxygen comes and binds
to hemoglobin.
And so you would expect to see higher hemoglobin
oxygen affinity here.
Later when the blood travels to the body tissues,
such as the muscles, the affinity
reduces, as we have learned.
The other factors which contribute
to the high hemoglobin oxygen affinity, so this red curve,
are reduced carbon dioxide, which means higher pH,
reduced temperature and also reduced
DPG, which is essentially opposite to the curve
when it shifts to the right.
So let's compare now.
In this red curve, so a high hemoglobin oxygen affinity,
if the person has a partial pressure of oxygen of 50,
the oxygen saturation is already 90%.
So in this video, we talked about how
oxygen binds onto hemoglobin from a deoxygenated state
in a tense configuration into a relaxed r configuration.
And we also talked about the oxygen dissociation curve
and factors which caused the curve to shift to the right,
as well as factors which caused the curve to shift to the left.
Hope it made sense.
Thank you for watching.