Today, I’m going to teach you how the oxygen from the air you breathe gets to your
muscles and tissues. It involves anatomy, chemistry, biology, and a bit of physics.
Okay, let’s get into it.
Hello and welcome, my name is Patrick and if you’re new here,
this channel is all about anatomy and physiology.
We’ll start with an overview of respiratory anatomy to get our bearings.
In this diagram, we’re looking at someone from the front, so this is their right side,
this is their left side. And I clarify that because the lungs aren’t symmetrical — the
right lung has three lobes while the left lung has two lobes.
When a breath enters the mouth or nose, goes down the airway until it terminates at microscopic alveoli,
or air sacs. These things are situated right next to really thin blood vessels
called the pulmonary capillaries. This is where gas exchange actually happens,
and it’s where we’ll spend most of our time talking about the movement
of oxygen and carbon dioxide. We have a lot of these alveoli — around 500 million of them.
But breathing happens because muscles let us inhale
and exhale, so let’s talk about those muscles.
Right below the lungs, you’ll find a dome-shaped muscle called the diaphragm.
And this muscle is a huge deal when it comes to breathing.
See, your lungs don’t have any muscles of their own. But healthy lungs are really compliant,
or elastic and bouncy, so if they’re being moved around by a muscle, they’ll just go along for the ride.
During a normal breath, the diaphragm contracts along with the external intercostal muscles — or
muscles between the ribs. There are other muscles that can help us breathe when we’re in trouble,
like the scalenes or the sternocleidomastoid, but for the vast majority of healthy breaths,
we can focus on the diaphragm. When it contracts, it expands the space, or volume, inside the chest.
But when you change the volume a container,
like the lungs, you end up changing the pressure of the gases inside it.
At its core, pressure is the force exerted by the liquids or gases against the walls of their
container. You can increase pressure by increasing the number of collisions in the container,
which you can do by increasing the amount of gas inside or by shrinking down the container.
In this case, the volume of the container — the lungs — increases,
so the pressure on all those air molecules inside
decreases. And vice versa — as the volume decreases when we exhale, pressure inside the lungs increases.
This is where another physics law comes in.
Whenever there’s a difference in pressure between two gases and they’re connected somehow,
their pressures will tend to equalize. That means a gas will move from areas of high pressure
to low pressure. It doesn’t matter if we’re talking about gas molecules in a weather
system or in our physiology. Gases tend to flow from high pressure to low pressure.
You can think of the greater than, less than sign. If the pressure is greater
in container A than in container B, think of that greater-than sign
as an arrow that’s pointing in the direction of movement. A > B is the same as A —> B
This is the entire basis of our lesson today.
When our nose or mouths are open to the outside environment,
the pressure inside our lungs is constantly trying to equalize with the atmosphere.
When you take a breath and expand your chest’s volume, you lower the total pressure of air inside
your lungs. Since air flows from high pressure to low pressure, air flows into your lungs.
Cool, so at this point, we understand how bulk air gets in and out of our lungs.
That’s the process of ventilation. Now we have to get that stuff from the lungs
into the bloodstream, what’s called perfusion.
The air we breathe is a mixture of different gases — mostly nitrogen and oxygen,
but there’s some water vapor and carbon dioxide in there too.
When we measure pressure, we’re measuring all of the collisions in that mixture at
the same time — the collective force exerted by all the different gases.
But to understand how gases get into the bloodstream, we need to know the pressure
exerted by one gas at a time. This is called the partial pressure of a gas.
It’s like asking, how many collisions are solely because oxygen molecules were
bouncing against the walls of the container? We can figure out that number, that pressure,
by multiplying the percentage of the gas in the mixture by the total pressure of the mixture.
For example, atmospheric pressure is 760 millimeters of Mercury at sea level.
Since nitrogen is about 78.6% of air, nitrogen has a partial pressure of 597.4mmHg. Likewise,
oxygen is 21 percent of air, and 21 percent of 760 comes out to 160 millimeters of Mercury.
At 400 parts per million in atmosphere, CO2 has a partial pressure of 0.3mmHg,
and at 4 hundredths of a percent in atmosphere, water vapor has a partial pressure of 3 millimeters of Mercury.
But it depends on the composition of the gases in the air sample you're talking about. The air in our alveoli for instance has
way more water vapor in it than the air around us. So the partial pressure of
water vapor is higher inside our lungs than outside of our lungs.
So let’s zoom into an alveoli and pulmonary capillary to see what we’re working with.
Each alveoli is going to have a mixture of gases in it. The big difference is that there’s going
to be more carbon dioxide and more water vapor than the air around us. Carbon dioxide because
it’s a waste product that we’re breathing out, and water vapor because we want our airway to be nice and humid.
The alveoli does have the same total pressure as the outside air though, so we’ll assume 760mmHg.
But because the composition of gases is different, each gas has a different partial pressure compared
to outside air. If we break down the percentages and partial pressures, the alveoli is 74.9%
nitrogen with a partial pressure of 569 mmHg, 13.7% oxygen for a partial pressure of 104,
6.2% water vapor for 47, and 5.2% carbon dioxide for 40mmHg.
And just like bulk air, gases will flow from areas of high partial pressure to low partial pressure.
In this case, we’re interested in the partial pressure difference between the alveoli and the
pulmonary capillaries around them. If the partial pressure of a gas is greater in the alveoli, it will flow into
the capillaries and vice versa. Although, that depends on how easily that gas dissolves in blood.
Nitrogen gas doesn’t dissolve well in blood, so very little actually transfers from alveoli to
bloodstream. Nitrogen is important for other biological processes like the urea cycle,
but we get that nitrogen from the food we eat, not the air we breathe.
So we’ll focus on oxygen and carbon going forward.
Now, Oxygen gas isn’t super soluble in blood either, but there’s a big partial
pressure difference between the alveoli and capillaries that is does move in.
Let’s say we take a sample of blood from the pulmonary capillaries.
On the left side of the diagram is deoxygenated blood coming from the right side of the heart,
on the right side will be oxygenated blood going to the left side of the heart to be pumped into
systemic circulation. That deoxygenated blood has a partial pressure of oxygen of only 40mmHg.
Compare that to the 104mmHg in the alveoli. That big of a difference in partial pressure
causes oxygen to flow from the alveoli into the bloodstream, even though oxygen isn’t all that soluble.
Once they’re in the blood, those molecules of oxygen bind to proteins in our red blood
cells called hemoglobin. If you want to get more in depth on that process,
check out my video up here or in the description..
Okay, so now oxygen is in the blood and the partial pressure of oxygen jumps to 100mmHg.
That number will stay consistent until we drop oxygen off at the tissues.
Meanwhile, that deoxygenated blood has quite a bit of carbon dioxide in it. And
CO2 is super soluble in blood, so it freely flows from alveoli to blood and vice versa. The partial
pressure of Carbon dioxide in the capillary is 40mmHg in the alveoli, while it's about 45mmHg
in the pulmonary capillary. This causes CO2 to flow out of the bloodstream so it can be exhaled.
Meanwhile, there’s still plenty of CO2 in your arterial blood for a partial pressure of 40mmHg.
About 20% of it is bound to the globin chains in hemoglobin,
but about 70% of it is dissolved in the blood in the form of bicarbonate,
or HCO3. The rest of it is just dissolved in the blood plasma.
Everything that’s happened to this point is called external respiration. This next part is called
internal respiration — we need to drop off oxygen at the tissues and collect the CO2 waste.
So at this point we’re in systemic circulation. The partial pressure
of oxygen in the blood is 104mmHg and the partial pressure of CO2 is 40mmHg.
Let’s say we get to the quadriceps muscle. The partial pressure of oxygen in those tissues
is about 40mmHg, which isn’t a coincidence. The partial pressure of oxygen in deoxygenated blood
is exactly the same as in muscle tissue at rest. The same phenomenon happens with CO2.
The partial pressure we saw at the pulmonary capillaries just reverses.
So the partial pressure of CO2 in the muscle is about 45mmHg.
So now we’ve got some gradients to work with. Oxygen flows from its high partial pressure in the blood
to its low pressure in the muscle, and CO2 flows from muscle to blood.
At this point, I want you to take a breath -- take a common sense check. You know that your
muscles consume oxygen and make CO2 as a waste product. You know — before any
numbers get involved — that oxygen moves from blood to muscle, and CO2 from muscle to blood.
So you know that the partial pressure of oxygen has to be higher in the blood,
and the partial pressure of CO2 has to be higher in the muscle than in the blood.
Remember our greater than arrow from before and you can always do a little logic check.
So if you understand the partial pressure gradients from external respiration,
you’ll have no trouble with internal respiration. But I want to take a closer look at how those
gases are exchanged at the tissue. Our cells use glucose and turn it into ATP for energy.
If we take glucose and oxygen, we turn it into carbon dioxide and water via cellular respiration.
After our cells metabolize that sugar, we dump CO2 into the blood. Like I said before,
a small percentage of it dissolves into the blood plasma, but most of it enters the red blood cell.
Okay, now we’re in deoxygenated venous blood. It flows back to the right side of the heart
where it’s pumped into pulmonary circulation. Then the whole process starts over again. The
partial pressure of oxygen is lower than in the alveoli, so oxygen flows into the blood again.
Now, here’s the big paradox. Partial pressure gradients explain how gases move when they’re
free floating. But those gases are all bound up in hemoglobin, and depending on whether
the blood is at the pulmonary capillaries or at the tissues, we ask it to either bind as
much new oxygen as possible, or let go of as much oxygen as possible. Same thing with CO2.
So I put together a video about the Bohr effect and haldane effect,
the laws you need to know to understand how this phenomenon works. You can check that out right here.
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