picture this you're getting ready to
give a big presentation in front of like
a lot of important people you're
practicing in front of your mirror and
then just for a second you forget how to
speak suddenly you feel that familiar
sting of anxiety like an icy hand on the
back of your neck you look at yourself
in that mirror and you start imagining
some of the worst worst case scenarios
like what if you totally lose your train
of thought up there what if you barf
what if everybody gets up and leaves now
you're really nervous i'm getting
freaked out just talking about it so you
start taking quick shallow breaths and
you're feeling light-headed and seeing
stars and now you my friend are
hyperventilating when we talk about
respiration we tend to focus on oxygen
and who could blame us it's easy to
forget the equally important role that
carbon dioxide plays in maintaining
homeostasis your internal balance
between oxygen and carbon dioxide
factors heavily into all sorts of stuff
especially in your blood where it can
affect your blood's pressure its ph
level even its temperature and now at
like t minus five minutes to your
presentation all those things are out of
whack because you're exhaling more co2
than you should you're just about to
faint when a friend suddenly hands you a
paper bag to breathe into and you've
never been so grateful for a lunch bag
in your life because somehow it does the
trick within seconds
you're back to normal the drop in co2
that occurs in your blood when you
hyperventilate is called hippocampnia
and it signals a breakdown in one of the
most complex and important functions
that your respiratory system performs
that is the exchange of gases inside
your blood cells where the stuff your
body doesn't want is swapped out for
what it desperately needs this exchange
between carbon dioxide and oxygen is
regulated by a whole series of
biological signals that your blood cells
use to communicate with your tissues
about what they have what they want and
what they need to get rid of it's almost
like a code one that's written into your
blood's chemistry in the folding of its
proteins even in its temperature and
acidity it's what allows you to perform
strenuous physical tasks like climbing a
mountain it's also what lets you reboot
your whole respiratory system with
nothing more than a paper bag
[Music]
i'll admit it when we've talked about
the chemistry of your blood so far we've
tended to keep things pretty simple like
hemoglobin contains four protein chains
each of which contains an iron atom
since iron readily binds with oxygen
that's how hemoglobin transports oxygen
around your body
but the fact is hemoglobin's affinity
for oxygen isn't always the same in some
places we want our hemoglobin to have a
high affinity for oxygen so it can
easily grab it out of the air and in
others we want it to have a low affinity
for oxygen so it can dump those
molecules to feed our cells so how does
your hemoglobin know when to collect its
precious cargo and when to let it go
well a lot of it has to do with a
principle of chemistry known as partial
pressure one of the things that fluids
always do is move from areas of high
pressure to low pressure and molecules
also diffuse from areas of high
concentration to areas of low
concentration but when we talk about
gases in a mixture we need to combine
the ideas of pressure and concentration
see air is a mixture of molecules and
when you're studying the respiratory
system you often need to focus on the
oxygen which makes up about 21 of it but
that doesn't actually tell us how many
oxygen molecules there are for that we
need to know the overall air pressure
since more molecules in a certain volume
means more pressure so partial pressure
gives us a way of understanding how much
oxygen there is based on the pressure
that it's creating example the pressure
of air at sea level is about 760
millimeters of mercury but since only
about 21 of that air is oxygen oxygen's
part of that pressure or partial
pressure of oxygen is 21 of 760 or about
160 millimeters of mercury now that's
just outside at sea level when that air
mixes with the air deep in your lungs
including a lot of air that you haven't
exhaled yet the partial pressure of
oxygen drops to about 104 millimeters of
mercury and in the blood that's entering
your lungs after most of its oxygen has
been stripped away by your hungry
muscles and neurons the oxygen partial
pressure is only about 40 millimeters
this big difference in pressure makes it
easy for oxygen molecules to travel from
the outside air into your blood plasma
because as a rule dissolved gases always
diffuse down their partial pressure
gradients this is why it's so much
harder to breathe at higher altitudes
when you climb a mountain the
concentration of oxygen stays at about
21 but the pressure gets lower which
means the partial pressure of oxygen
also decreases to about 45 millimeters
of mercury at the top of mount everest
so the partial pressure of oxygen at the
top of the highest peak in the world is
almost the same as the deoxygenated
blood that's entering your lungs so
basically there is no partial pressure
gradient which makes it really hard to
get oxygen into your blood but let's get
back to the red blood cells remember
that the globin in your hemoglobin is a
protein and when proteins bind to stuff
they tend to change shape and that shape
change can make the protein more or less
likely to bind to other stuff when an
empty hemoglobin runs into an oxygen
molecule things are a little awkward
it's like a first state bonding isn't so
easy but once they finally bind
hemoglobin suddenly changes shape which
makes it easier for other oxygen
molecules to attach like friends
gathering around the lunch table that
affinity for joining in or cooperativity
as it's known continues until all four
binding sites are taken and the molecule
is fully saturated now your hemoglobin
is known as oxyhemoglobin or hbo2 and it
is not
not why the cable network is called that
that's the home box office anyway by the
time the blood leaves the lungs each
hemoglobin is fully saturated the oxygen
partial pressure in your plasma is about
100 millimeters and now it is ready to
be delivered to where it is needed most
active tissues like the brain heart and
muscles are always hungry for oxygen
they burn through it quickly lowering
the oxygen partial pressure around them
to about 40 millimeters so when the
blood arrives on the scene oxygen moves
down the gradient from the plasma to the
tissues to feed those hungry cells that
makes the oxygen partial pressure in
your plasma drop so your hemoglobin
starts to give up more of its oxygen to
the plasma but partial pressures are
only part of what's prodding your
hemoglobin to give up the goods all that
metabolic activity going on in your
tissues is also producing other triggers
in the form of waste products
specifically heat and co2 both of those
things activate the release of more
oxygen by lowering hemoglobin's affinity
for it so you're climbing that mountain
again and your thighs are fueling the
burn red blood cells saturated with
oxygen are going to the muscle tissue in
your quads where the hemoglobin can dump
a bunch of o2 because of the lower
partial pressures of oxygen in your
muscles but a hard-working quad will
also heat up the surrounding tissues and
that rise in temperature changes the
shape of hemoglobin and it does it in
such a way that it lowers its affinity
for o2 so when those red blood cells hit
that warm active tissue they release
even more oxygen like 20 percent more
beyond what partial pressures would
trigger but wait there's more carbon
dioxide triggers the release of oxygen
too because it also binds to the
hemoglobin changing its shape again
lowering its affinity for oxygen still
more and as oxygen jumps ship the
hemoglobin can pick up more co2 finally
just in case the hemoglobin isn't
getting the message at this point
there's one more trigger that your
respiratory system has up its sleeve the
spike in co2 that's released by your
active muscle tissues actually makes
your blood more acidic since your blood
is mostly water when co2 dissolves in it
it forms carbonic acid which breaks down
into bicarbonate and hydrogen ions those
ions bind to the hemoglobin changing its
shape yet again further lowering its
affinity for oxygen so now at last your
tissues have the oxygen they need and
your red blood cells are stuck with all
this co2 that they need to get rid of
your red blood cells ride the vein train
back to the lungs where they encounter a
new wave of freshly inhaled oxygen and
when that o2 binds to the hemoglobin
which again is hard at first it
eventually changes its shape back to the
way it was when we started which
decreases its affinity for co2 so the
hemoglobin drops its carbon dioxide
which moves down its partial pressure
gradient into the air of your lungs so
you can exhale it and the whole thing
can start all over again now if that
isn't enough to make hyperventilate i'm
not sure what is but this brings us back
to that unfortunate episode you had
before your big presentation this whole
complex code of chemical signals that i
just described well it assumes that what
your cells and tissues are telling each
other is actually true but as we all
know sometimes our bodies don't mean
what they say
thanks buddy like when you're freaking
out about your presentation your
sympathetic nervous system makes your
heart race and your breathing increase
to prepare you to fight or flee the
problem is there's nothing to actually
fight or flee from so your muscles
aren't actually doing anything so
they're not using all the extra oxygen
you're breathing in and they also aren't
producing the extra co2 that you're
suddenly exhaling all over the place so
when you start to exhale co2 faster than
your cells release it its concentration
in your blood drops and with less
carbonic acid around your blood's ph
starts to rise and you know what else
while low blood ph does things like
change the shape of your hemoglobin to
deliver oxygen high ph causes
vasoconstriction normally this is
supposed to divert blood from the parts
you're not using during times of stress
like your digestive organs to the parts
that you are using but when you
hyperventilate this constriction happens
everywhere which means less blood is
delivered to your brain which makes you
lightheaded luckily that trick with the
breathing into the paper bag it really
does work it works because it lets you
breathe back in all the co2 you just
breathed out so the partial pressure of
carbon dioxide in the bag is higher
which forces that co2 into your blood
which lowers its ph and you get back to
homeostasis and of course homeostasis is
the key to life and you know also to a
successful presentation if you were able
to remain calm today you learned how
your blood cells exchange oxygen and co2
to maintain homeostasis we talked about
partial pressure gradients and how they
along with changes in blood temperature
acidity and co2 concentrations change
how hemoglobin binds to gases in your
blood and you learned how the thing with
the bag works of course we must say
thank you to our patrons on patreon who
helped make crash course possible
through their monthly contributions not
just for themselves but for everyone if
you like crash course and want to help
us keep making videos like this one you
can go to
patreon.comcrashcourse this episode was
filmed in the dr cheryl c kinney crash
course studio it was written by kathleen
yale the script was edited by blake t
pastino and our consultant is dr brandon
jackson it was directed and edited by
nicole sweeney our sound designer is
michael laronda and the graphics team is
thought cafe
[Music]
you