Respiratory system

 

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.