Now, if I ask you this question, "Hey, Sue, can you tell me what surface tension is?"
What would you respond?
If you find yourself avoiding eye contact with me in this video, don't worry about it,
because you're probably wondering,
"Ah, why are we talking about the surface tension of water?" That's a fair question.
Surface tension applies to all types of liquids
and it's important for you to understand the impact of surface tension on the alveoli in the lungs.
See, surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible.
So, water, shrinks together to form those droplets,
that's why water droplets are a good example, so surface tension
is what causes those droplets to come together into the smallest surface space possible.
We're talking about water because water has a relatively high surface tension.
Let me give you an example.
Do you have a water bottle anywhere close to you, just take it and, before you shake it
put the lid on it and then shake the bottle. What happens?
Well, while you're shaking it, while you're agitating it,
you'll see bubbles that come up in the water.
Take a look at these bubbles in your water.
They don't stay open or inflate it with air for very long, it's because of the surface tension.
The water's gonna be attracted to itself,
it's gonna shrink to the smallest space and those bubbles gotta go.
Now, alveoli are lined with a thin film of water, get it?
Yeah, take a look at this picture.
Now, we know that the water is going to try to shrink into its smallest space,
because each alveoli is lined with a very thin film of water, look what happens.
Oxygen is dissolved in this water so it diffuses more easily
and more readily across the alveolar wall into the capillary
so that's why we have the water, that's a good thing.
But water surface tension will cause it to shrink into its smallest size so look at where the water is located.
Remember the example of the water droplets?
Think of them on the leaf. Remember how the water all came together?
Well, there's a force that pulls them together, surface tension.
So look at the pressure, the force that's gonna be applied by the water in the alveoli.
Because it's around the outside of the walls, that water is trying to get together, right?
Water wants to be together so what's gonna happen
to the shape of the alveoli with the surface tension of water?
It's gonna tend to collapse. So we've gotta do something about it.
Alveoli are round-shaped and the tension will pull inward,
that's why it matters that you understand surface tension.
You know that it's gonna be attracted to itself into within a spherical shape,
it's gonna be attracted to the center and cause the alveoli to collapse.
So surface tension of the water lining creates a force pulling it towards the center or collapse of the alveoli.
Now, without the counteraction of surfactant, the alveoli would collapse.
Again, I've used the word collapse multiple times
because I wanna make sure that point is really driven home.
So what do we do? Well, it's our friends, the type II alveolar wall cells to the rescue.
See them there as they come flying in like a superhero?
Let me explain how surfactant actually works, how it decreases the surface tension of water.
Now, I want you to think about your water bottle,
remember when we shook that up how quickly the bubbles disappeared?
They didn't stick around for very long, that's because it was pure water.
But what if, we went to the dollar store and we got a big jar of bubble juice.
Now if you add something to pure water that's gonna decrease the surface tension, what happens?
Well, then, I could take that same liquid, I could dip a bubble wand in it
and if I blew bubbles, what happens to those bubbles?
Yeah, they're gonna stay open and full of air for a lot longer.
It's a very relaxing thing to do, in fact, I wish I could send each of you a bubble wand
to help you do some deep breathing and calm yourself as you're studying for exams
but we'll have to use this as an example as we walk through.
So the bubbles in the water are unstable.
It won't stay inflated with air because the surface tension of the water is so strong,
but bubbles that have surfactant, that's a different story.
Because when surfactants are introduced in the water its surface tension is reduced by a factor of what?
Yeah, three or more, so bubbles can remain inflated with air for much longer because of the surfactant.
They're interrupting that power of water of surface tension.
Let me show you what it looks like.
These little round thing on sticks, yeah, this is surfactant.
It gets its name because surfactant means surface-actant agent, that's why we call it surfactant.
Now, these are compounds that lowers the surface tension between two liquids,
so you don't just have surfactant in your lungs, we have them in everyday life.
So surfactants are compounds, remember they're made of more than one thing,
they're compounds that lower the surface tension between two liquids or between a gas
and a liquid or between a liquid and a solid,
so, they lower surface tension between two things.
Now, surfactants can act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.
They can have all those different roles,
but what we're focusing on is pulmonary surfactant,
and it is what lowers the surface tension in the alveoli.
Oh, okay now this gets more interesting.
So, you see the alveoli, we got the type I cells, type II cells,
one that secrete this deliciousness, we call surfactant,
but look at those little tiny circles with the tails?
That's the surfactant that's getting in between those water molecules
and breaking up that surface tension. Why is that a good thing?
Remember surface tension of the water is gonna be pulling inward intending to cause collapse of the alveoli.
Get enough surfactant in there and we diminish that power of the surface tension
of the water we have a less risk for alveoli collapsing.
See surfactant is pushy. Surfactant molecules push themselves in between the water molecules,
see them there? I have this blown up for you so you can kinda get an idea of how these works.
So, when you're studying this, remember, surfactant is pushy.
Now, it has a hydrophilic head and it has a hydrophobic tail.
Hydrophilic means love, hydrophobic means I don't like it,
so hydrophilic heads bind to the water film
and the hydrophobic tails float in the air in the alveolus, is this not cool?
So, see they're prepositioned that way in our graphic so you understand.
One part of the surfactant binds to the water,
the other part hangs out in the air and inside the alveolus, pushes its way through,
binds to the water and helps break up the surface tension of water.
Now, water isn't able to bind together
and this is how surfactant molecules decrease surface tension, pretty cool, huh?
So, take at the alveoli size and shape.
When I take a breath, it's bigger, when I expire, it's a little smaller; when it's collapsed.
Okay, you can see how we're not really have good gas exchange.
So, I want you to know first of all, the lung alveoli need to be able to expand and get bigger,
they also need to be able to recoil, that's called compliance.
If it's collapsed, it's useless for someone, for CO2 and O2 exchange.