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Take a deep breath. And let it out. Isn’t it remarkable? The human respiratory system,
I mean. The system that lets us do that – an exchange of gases.
Now don’t confuse the respiratory system with cellular respiration. If you watched our cellular
respiration video, you learned about why our cells need oxygen. Your cells need oxygen to make ATP,
an energy currency, and the gas byproduct produced is carbon dioxide which the body
must remove. This is part of the equation in aerobic cellular respiration done by your cells.
But your respiratory system which takes in the oxygen and expels the carbon dioxide – working
closely with the circulatory system and other systems to do so – is how we get that oxygen
into human body in the first place. And that oxygen will be needed for cellular respiration.
So you inhale. Air passes through your nasal cavity. The air is warmed, humidified,
and filtered. This involves mucus and hairs. Nasal hairs that you can see and then microscopic cilia
which are similar to hair-like structures. Now, we come to the pharynx. A junction if you will
of both food and air. From the pharynx, we go through the larynx (often called the voice box).
Then the trachea. By the way, food should be traveling down the esophagus not the trachea.
We mention in our digestive system video that an epiglottis keeps food from going down the trachea.
The trachea is a pretty fascinating cylinder tube with rings of cartilage. That cartilage helps
support the trachea and keep it open for that air to travel through. The trachea goes down, down,
down to the primary bronchi. One on each side as this branches to the lungs. Just to mention
a bit about the lungs. There are two. Each lung has sections called lobes. Three lobes
on the right and two on the left. There’s a cardiac notch on the left lung side where it's
a little indention to give the heart some room. The left lung is generally smaller than the right.
Now our main focus is going to be what’s happening inside the lungs so let’s continue to go through
the primary bronchi. Primary bronchi divide into secondary bronchi then tertiary bronchi and then
smaller bronchioles. And, you know, it kind of looks like an upside down tree. I like trees.
So a general recap of where we’ve gone: nasal cavity -> pharynx -> larynx  trachea  primary
bronchi  secondary bronchi  tertiary bronchi  bronchioles.
Diameter is getting smaller as you go through these different areas.
Beyond the terminal bronchioles, there will be branching into respiratory bronchioles
and then on to alveolar ducts. Each alveolar duct is surrounded by alveolar sacs. Alveolar
sacs look a lot like…a bunch of grapes. I’m not the only one to think that. Each of these
alveolar sacs contain alveoli and this is where the gas exchange will actually occur.
That’s because these alveoli are made of thin walled cells, have a lot of surface area, and they
have direct contact with capillaries. We mentioned that other body systems work closely together:
the circulatory system works closely with the respiratory system here. Red blood cells in
the capillaries can pick up the oxygen that was inhaled to deliver it throughout the body and
also bring carbon dioxide -a waste gas that needs to be removed- so that it can be exhaled.
Besides the circulatory system, there are other body systems working with this respiratory system.
The skeletal system includes the ribs that protect the lungs like a cage around them.
But muscles of the muscular system are involved too. Muscles involved in respiration includes
muscles between your ribs called intercostal muscles. It includes a major muscle under
your lungs called the diaphragm. It includes abdominal wall muscles. All of these are part of
the muscular system – and they are involved with helping to expand or contract the thoracic cavity.
While you can take voluntary control of your breathing,
you’ll notice that most of the time your breathing is involuntary:
that is, you aren’t consciously controlling it. The nervous system regulates this, and here’s
something pretty cool: it uses pH to do so. The pH scale is based on hydrogen ion concentration
(H+). Acidic substances – shown here as lower numbers on this pH scale - have a higher
H+ concentration compared to bases - which have a lower H+ concentration. Ultimately, the increase
of carbon dioxide concentration in the blood increases the concentration of H+. If you want
to learn more about how that happens – fascinating chemistry- check out our further reading links.
So as the carbon dioxide concentration increases in the blood, the blood pH falls
slightly lower on the pH scale – it is becoming more acidic. The increasing acidity is detected
and sent as signals to the brain. The brain can then control the intercostal muscles,
diaphragm, and abdominal muscles in order to increase the rate and depth of breathing. This can
restore the blood to a normal blood pH and keep the blood pH stable. Around 7.4. Great example of
keeping homeostasis. Just think about when you’re exercising and how amazing it is to have such a
fine-tuned system so your breathing rate and depth can increase as needed. And while we’re really
trying to give general examples to emphasize that body systems don’t work in isolation,
keep in mind that there are other systems involved with the respiratory system to explore.
Before we go, there are 2 final notes I want to mention. First, we want to remind you we
focused on humans. But obviously it’s not just humans that have gas exchange.
Earthworms actually have gas exchange through their skin. Fish can use gills for gases to
diffuse, insects can have a tracheal system which means they can have little openings on
their body- called spiracles – that connect to little tubes inside. It’s fascinating to
learn about all these different systems for getting oxygen in and carbon dioxide out.
Second, understanding how the respiratory system works can help us understand
treatments for respiratory illnesses or respiratory problems that may arise.
There are many careers that focus specifically on the respiratory system – two examples include
pulmonologists and respiratory therapists. They may be involved in the treatment of respiratory
conditions like asthma or emphysema, and, an example I’d like to end with: they might be
involved in the treatment for premature babies that might not have fully developed lungs.
So to expand on this: remember we were talking about the alveoli – we mentioned alveoli have a
large surface area? Ideal for gases to diffuse. But without something called surfactant inside
them, alveoli can be prone to collapse due to the surface tension of water inside the alveoli.
Surface tension being a great thing to review in our properties of water video. So, type 2 alveolar
cells makes surfactant, a substance that includes phospholipids and proteins. Surfactant interferes
with the bonding of water which contributes to lowering the surface tension, making it easier
for alveoli to inflate. But sometimes, babies that are premature may not yet have enough surfactant
in their lungs. This can make it difficult for the alveoli to inflate properly; it can
cause collapse. This can result in respiratory distress syndrome (RDS). But now…due to better
understanding of this, artificial surfactants can be used to treat premature infants and it’s
saved the lives of many. Well, that’s it for the Amoeba Sisters, and we remind you to stay curious.