What takes more time, the average inhalation or average exhalation?
It’s actually not as obvious as it seems at first. When you inhale, air comes in really
quickly and steadily, but the breath slows down a little bit at the end of the inhalation. Then you
pause for a moment. Then as you exhale, you force air out of your lungs really quickly at first,
but like before the rate tapers off at the end. Which phase of breathing takes longer depends
on where you’d place that break between inhale and exhale. And as it turns out, the division
cuts the inhalation shorter — most healthy people take longer to exhale than to inhale.
And we know this thanks to spirometry — spiro for breath,
metry for measurements. It’s a test that uses data about time and the amount of air you breathe
to draw a graph that looks like this. And each one of these peaks and valleys tells us something
about our lung function and breathing pattern, and of course, can be used to find pulmonary disease.
So today, we’re going to use spirometry to look at an average breath in a healthy person,
plus, we’ll see how things like age and disease might affect that graph.
[intro]
Hello and welcome. If you’re new here,
my name is Patrick and this channel is all about anatomy and physiology.
So from the beginning, there are a few kinds of tests that all fall under the spirometry umbrella.
We could use a flow-volume loop which shows the volume of air on the x axis,
and flow, or volume per second on the Y axis. As someone breathes, this graph measures how quickly
they’re moving air in and out, eventually forming a loop. This kind of graph works
since breathing is cyclical — we breathe in and out repeatedly. And it shows us that you
push out air really quickly at the start of an exhale, then the flow rate slows down.
Meanwhile, an inhalation has a much steadier ramp up and ramp down.
We could also look at volume over time in a graph that looks something like this.
It’s the main one we’ll use today.
On the Y axis is the volume of air in the lungs. On the very bottom is 0 milliliters per kilogram
of body mass. This would be totally empty — which doesn’t happen in real life lungs but
we’ll get to that. At the top is full volume, as much air as you could possibly store in the
lungs assuming you had someone inhale as much as they possibly could. This range is
called the total lung capacity or TLC. As we dissect this graph, each of those
tinier waves tells us something about our lung function. So let’s break it down, piece by piece.
The first thing you notice is the steady up and down waves. Those are your average,
casual breaths. And that volume of air that you move in and out of an average breath is
pretty even and predictable. This is called your tidal volume (sometimes abbreviated as
TV or V(sub T)). And it’s a surprisingly little amount of air — usually about 7
milliliters of air for every kilogram of body weight. I weight about 88 kilograms,
so my average breath is around 616 milliliters. About this much in a balloon.
But, of course, you have the /ability/ to inhale a lot more air than you do during an average breath.
Maybe you’re holding your breath so you can dunk your head underwater or walk through a
locker room without inhaling BO. That volume of air inhaled during a max inhalation is called the
inspiratory reserve volume (or IRV). It goes from baseline all the way up to the top of the graph.
Now, if we add the volume of a normal breath, our tidal volume, with that big big inhale, we get a
value called the inspiratory capacity — it’s the maximum amount of air you could inhale after
a normal exhale. It’s only a little bit bigger than IRV, but it’s an important distinction.
Let’s get back to normal breathing for a second though. Take a few breaths, establish your normal
pattern, take a normal inhalation, then exhale as much as you can. Try to totally empty your lungs.
Did you notice that? You exhaled a greater volume of air than you inhaled.
According to normal spirometry, you had about 2 breaths worth of air just chilling in your lungs
that you could’ve exhaled. This is called your expiratory reserve volume, or ERV.
It’s all the air you could possibly breathe out after a normal exhale.
And that’s a good little feature to have. Like, imagine you’re walking down the street and run
into this cool guy. And he’s super cool, he’s just ripping the fattest, CHUNKIEST cotton.
And you don’t want his second hand vape fumes in your lungs. But if you take a big breath
before you walk past him, you’ll seem rude. Instead, you hold your breath without inhaling,
and keep walking. Your expiratory reserve volume came in handy.
Now, if we were to tell someone to breathe in as deep as possible,
then force all the air out of their lungs, we’d get a number called their vital capacity, or VC.
This number can tell us quite a bit about disease too.
Because in order for the lungs to inflate fully, then empty as much as they can,
they need to be compliant, or squishy and elastic. And if they’re stiff from something
like cigarette smoking, then they won’t inflate or deflate as well, which lowers vital capacity.
But if we go back to the graph, you’ll notice something. Even when you tried to empty
your lungs, your lungs still weren’t totally empty. While we can’t directly measure it with
spirometry, there’s a little bit of air we just can’t breathe out. This is called the residual
volume. It’s exactly what it sounds like, leftover air. It’s only a little bit, about 15 milliliters
per kilogram of body weight My undergrad professor called this the stale air, which I think is appropriate.
But it’s an important value because it ensures that the air sacs in our lungs always stay a little bit
inflated. Think back to the balloon again. It’s harder to blow up a totally empty balloon than
a balloon with some air already in it, so it’s an advantage to keep some air in there at all times.
Now, in day to day life, you probably don’t exhale any more than you need to. You hang
out in that normal tidal volume range. And since we don’t usually push past that normal range,
we can combine the expiratory reserve volume, the air you could voluntarily push out,
with the residual volume, the stale air, into one category. We call it functional residual
capacity. This is the amount of air left in your lungs after a normal exhalation.
And that’s it. Those are all the most common values for our normal graph.
But of course, all kinds of factors influence quote normal values.
Anthropometry is a big one. Taller people tend to have bigger lungs and thus, bigger lung volumes.
People with obesity tend to have smaller lung volumes, maybe due to decreased chest wall
compliance. And men tend to have bigger lung volumes than women of the same age and height.
Speaking of age, lung volume increases from birth through early adulthood,
and matures somewhere between age 20 and 25. They stay pretty consistent for the next 10 years,
but around age 35, the thoracic wall starts getting a little stiffer and less elastic.
This results in a gradual decrease in vital capacity and inspiratory capacity,
and an increased functional residual capacity and residual volume. I know
that’s a lot of terminology, but if we translate it, we’re saying that a stiffer
chest makes it harder to take a deep breath, which leaves more air leftover in the lungs.
Age and anthropometry aside, different diseases change the graph.
Let’s look at two common types of lung diseases: restrictive diseases
and obstructive diseases. Restrictive diseases happen when the lungs can’t expand very well,
so we often see overall smaller total lung capacity, longer inhalation or a
lower volume of inhalation on the graph. But with obstructive diseases like COPD or asthma,
those blocked airways keep the person from exhaling a full breath, so they’ll have a
much larger residual volume — the stale air left in the lungs. On the spirometry graph, exhalation
takes them a longer time and they’ll tend to have a reduced inspiratory capacity as well.
That’s the other interesting thing about this graph — time. We can learn a lot
about someone’s respiratory health once we bring in the X axis. Oftentimes we’ll
just use a flow-volume loop since time is already built into flow.
One of the most common tests is getting someone to breathe out as hard and fast as they can,
and measuring the volume exhaled in the first second. This is called FEV1 for
forced expiratory volume in 1 second. Some people use FEVt sometimes, where you just substitute t
for however long you’re measuring an exhalation. You could also look at those peaks, what’s called
the peak expiratory flow, or PEF which you’ll find on the top half. Then you'll have maximal
inspiratory flow, or MIF, on the bottom. And just a reminder, those are flow rates, not volume.
You could also glean useful information from certain ratios. Like, if you divide FEV1
over Forced Vital Capacity, you’re essentially asking “how much of someone’s entire exhalation
is breathed out in the first second of a hard exhale?”. For most healthy adults, that’s around
70 to 80% of the breath. But for someone with an obstructive lung disease, they won’t be able to
force as much air out in the first second, so their ratio will be lower, maybe around 45%.
On the other hand, someone with restrictive lung disease will have a reduced FEV1 and a reduced
FVC, so their ratio usually stays the same, or even increases because their FVC is lower.
So you can learn a ton about someone’s respiratory health from these graphs. Now,
if you need a refresher on some of the anatomy and physiology that makes the lungs work, check
out my respiratory system playlist where you’ll find even more videos like this one. I’d like to
thank my patrons on Patreon for making this video happen. Have fun, be good, thanks for watching.