Hi, welcome to our series on life-threatening lung conditions.
I want you to understand the role of the alveoli
because this is a foundational concept in understanding anything about your respiratory system.
So let's start with a fun question, I like trivia.
So, how many square feet of alveoli surface area dose the average adult have?
Now, we care about this because remember, the exchange of oxygen and carbon dioxide
only happens at the alveoli level. So what do you guess, how many square feet?
Ready, look at that. We've got about 300 million alveoli per lung,
so since, if you have two lungs, you've got about 600 million total in both lungs,
so that gives you 75 meter squared or more than 807 square feet of alveoli surface area.
You gotta stop for just a minute and appreciate how incredible that is.
Now, we're gonna walk through talking about one of those to give you structure,
but don't lose sight of the fact that you have 600 million in both lungs.
Let's talk about the path to the alveoli.
So I breathe in air in my nose and mouth and it's gonna travel down my airways.
Now, the respiratory bronchioles divide into multiple alveolar ducts,
so breathe in through my mouth and nose, then the respiratory bronchioles divide into multiple alveolar ducts.
Now, these ducts have alveolar sacs and alveoli,
remember, that's where the actual gas exchange takes place.
Now, you can see the sacs here but I wanna take a look at one, a single alveolus, okay?
So we're gonna take one of those, pluck it out and blow it up big for you.
Now, alveoli are typically one-cell thick,
okay, so they have epithelial cells, they're just one-cell thick in their wall,
so they look like these tiny little bubbles that are within these sacs.
The capillaries that surround these alveoli are also one-cell thick, we call those endothelial cells.
Now, this is what allows the CO2 and the O2 to diffuse across the alveolar membrane.
Okay, so why are focused on this one singular sensation?
Because that lets you know, it's very thin, easily permeable,
so we've got diffusion of the CO2 coming from the blood back into the alveoli
and the alveoli is diffusing oxygen from this lung cell into the bloodstream,
so that's why it's important that you keep in mind these are very fragile,
right, these membranes are one-cell thick and very fragile.
Now, the elastin fibers are the elastic support during the stretching and recoil,
the stretching and recoil of your lung tissue.
So here we have it, this is what I promised you,
looking at just one of these little alveoli by themselves.
So, this is an individual one.
I want you look at the special cells that are in the alveoli itself.
Why am I smiling? Because this is so cool and I can't wait for you to get a picture of this.
So the alveolar wall, the connective tissue between the alveoli,
this is something that sometimes gets kinda skipped over in your physiology courses
and this is the root of understanding everything in the respiratory system.
So, look at the picture, okay, there's the alveolus.
Look, we've got epithelial cells, right, and they're just in the single layer
that's what makes up the alveolar wall.
Now, look at the capillaries, there you go.
You got one layer of cells to a single-cell layer in the capillary wall those are endothelial cells.
Alveolus, epithelial cells; in the capillaries, endothelial cells, that's in the capillary wall.
Now, what do you see flowing by there?
Yeah, those are the blood cells, red cells, white cells, platelets,
that's what's gonna be in those capillaries.
But we're most interested in are the red cells because they carry the oxygen and the CO2.
Now, alveoli are in direct contact with the capillaries of the circulatory system.
Now, there's some basement membranes in there, we'll get to those a little bit later,
but know that they bump right up against each other, that's the idea.
They're really fragile, they're one-cell thick and we need to be close to each other
because they've gotta exchange that CO2 and the oxygen.
Now, both alveolar and capillary walls are one-cell thick and easily permeable.
Why does she keeps saying that? Because it's really important.
When we start talking about the way the respiratory system has problems, diseases, infections,
this is what you need to keep in mind to understand what's going on.
Now, diffusion of oxygen from the alveoli to the capillaries occurs
because the concentration of oxygen is lower in the capillaries,
see that's how diffusion works.
So see the oxygen flowing in and it heads right into those capillaries.
It happens by a process called diffusion.
So just circle that word, I want you to keep that in mind, it's diffusion of oxygen from the alveoli,
cuz I just breathe air in into the capillaries.
Now diffusion of carbon dioxide from the capillaries,
see that coming up from the capillaries up through the alveoli?
Because the concentration of carbon dioxide is lower, that's when the diaphragm relaxes.
Okay, let's look at that again.
So, I breathe in, right, oxygen comes into my alveoli,
it transfers down into my capillaries through diffusion.
When I breathe out, I'm breathing out CO2,
so diffusion of oxygen from the alveoli to the capillaries happens
because the concentration of oxygen is higher in the alveoli than it is in the capillaries; same thing with CO2.
This is bringing back, these capillaries are bringing back blood that needs oxygen,
so because there's a higher level of carbon dioxide in the capillaries,
it's gonna diffuse into the alveoli and out through your mouth and nose.
When I talk about alveolar expansion and recoil, now this is a pretty cool drawing here,
but let's look at the bottom. We're looking at breath volumes.
See it has that arch? So, you see the first half is breathing in, the second half is breathing out.
Now, there's intra-alveolar pressures,
look, as I'm beginning to breath in the pressure is zero,
but then you see it goes to negative 1. Why do you need to know this?
Because when I'm breathing on my own, it's negative pressures inside here, right,
when my diaphragm flattens, that makes negative pressure and allows me to breath in.
Now, as I'm breathing out, we go back to zero plus one.
For now, I just want you to have this as a frame of reference.
But have you heard people talk about positive pressure mechanical ventilation?
You see ventilators can't do what my diaphragm can.
It doesn't create negative pressure inside the chest wall so it has to be positive pressure all the way,
that's why mechanical ventilation is harder on your body than if you're able to breathe on your own.
So takeaway points from this slide,
I want you to circle negative one, that's gonna remind you that when you're breathing in,
your diaphragm has helped create negative pressure inside your chest cavity,
that's what allows you to fill your lungs and to allow them to expand.
Write a note for yourself that says, mechanical ventilation is positive pressure.
Now, you can see if we get those pressures off, you can really do some damage to these very fragile alveoli.
So during inhalation, the alveoli expand as the negative pressure
in the chest is created by a contraction of the diaphragm.
Now, when we say contraction, remember your diaphragm is kind of domed,
when it contracts, that muscle it's also flattening to create that negative pressure.
Now, when I'm exhaling, the alveoli recoil, they spring back as the diaphragm relaxes.
Your lung needs to be compliant, it needs to be able to expand and to contract, right?
To expand and to contract.
We think about the heart doing that to empty blood out into other chambers,
but sometimes you don't think about it about your lungs, it's critically important.