The body has a respiratory center in the brain located in an area known as the Medulla Oblongata
and the Pons regions.
They get voluntary information, like when you want to sing or hold your breath, from
an area of your brain called the cerebrum.
On the other hand, involuntary information from an area located in the hypothalamus affects
the respiratory center when you get anxious or are fearful.
Both of these regions affect breathing from within the brain and do not require information
from the body, like say how much carbon dioxide is in the blood.
As for when you’re just going along breathing normally without thought, the respiratory
center gets information from what are known as chemoreceptors.
These receptors are responsible for regulating our breathing so just the right amount of
oxygen gets in, and the right amount of carbon dioxide gets out (despite what we voluntarily
want to do).
There are two main types of chemoreceptors- peripheral and central.
They're made up of a grouping of cells called Glomus cells.
While the purpose of these receptors is to make sure we are getting enough oxygen, most
of them don't react to oxygen levels.
Instead, most, and all of your central chemoreceptors, react to carbon dioxide and pH levels.
Only peripheral chemoreceptors use oxygen levels as a part of how they regulate certain
types of breathing.
So why and how do these chemoreceptors do it?
Almost everything in the human body needs to be maintained in a very narrow range for
the many chemical processes within it to work properly.
This is known as homeostasis.
Every living cell in our bodies has numerous chemical reactions taking place to sustain
life as we know it.
The molecules responsible for these chemical reactions are known as enzymes.
They control everything from food digestion, the making of new cells, to creating DNA strands.
Most of these enzymes are made of protein molecules, and all are extremely sensitive
to changes in many things like pH, oxygen and carbon dioxide levels, and temperature
changes, to name a few.
If you increase or decrease any of these things beyond what is normal for them, different
enzymes will begin to react differently and abnormal processes that cause a wide range
of problems begin.
As an example, if you were to breathe faster than you need for the work your body is doing
at the time, like with an anxiety attack, you could cause what is known as respiratory
alkalosis.
The end result might be numbness and tingling in your fingers, toes and lips, uncontrolled
cramping known as a carpodedal spasm, abnormal cardiac rhythms and low blood pressures.
Central and peripheral chemoreceptors all work similarly to maintain the narrow range
of carbon dioxide to oxygen levels.
Inside our blood and the cerebrospinal fluid around our brains there is a specific amount
of oxygen and carbon dioxide.
This information from outside the cells gets transmitted within the cells.
That information causes a reaction that releases different types of neurotransmitters (the
chemicals that transmit signals from one nerve to the next) to be released.
Those transmitters make the nerves controlling our breathing muscles increase or decrease
their activity.
The end result is faster or slower breathing, depending on the needs of your body at the
time.
The peripheral chemoreceptors are called that because they are located outside of the central
nervous system.
They are in two separate locations, in your carotid arteries, known as your carotid body,
and in the arch of your aorta, known as the aortic body.
When the oxygen levels within the blood get too low, not very much oxygen is forced into
the Glomus cells.
When this happens, the low amount of oxygen causes a signal, called depolarization, to
release the neurotransmitters telling your nerves that control the breathing muscles
to contract.
This isn't the only way your peripheral chemoreceptors work.
The Glomus cells also release carbon dioxide as a result of them working.
When the level of carbon dioxide inside the blood gets too high, the CO2 from within the
cell cannot get out.
This also causes a release of neurotransmitters and you breathe more.
More CO2 also causes the blood to become more acidic.
This low pH is also sensed by the cells and thus pH will also affect the neurotransmitter
release.
In the end, the peripheral chemoreceptors can make you breathe faster, or slower, by
sensing the levels of oxygen, carbon dioxide and pH within the blood.
Central chemoreceptors are located right near the respiratory center within the brain.
These chemoreceptors only react to levels of CO2 and pH within the fluid around the
brain, called cerebrospinal fluid.
They affect CO2 and pH levels slower than peripheral chemoreceptors because it takes
time for the carbon dioxide to get from your blood into your cerebrospinal fluid, through
a grouping of tissues called the “blood-brain” barrier.
That being said, it is these receptors that mainly stimulate inspiration, and consequently
expiration.
Expiration causes excess CO2 to be released, thereby decreasing the amount of CO2 compared
to oxygen.
The result is that perfect homeostasis we talked about earlier.
When it comes to what is most important in affecting involuntary breathing, you can see
that CO2 levels tend to affect breathing much more than oxygen levels.
It is the only level the central chemoreceptors are aware of, and the peripheral chemoreceptors
use it in conjunction with oxygen to determine how they will react.
Bonus Fact: Speaking of carbon dioxide levels, it turns
out female mosquitoes identify you as a potential target for a blood meal (and, yes, only the
females are blood suckers) via the carbon dioxide your body puts off, which they detect
via their maxillary palp organ.
In fact, mosquitoes can detect you this way from up to about 100-150 ft (~40 meters) away.
This is one of the many reasons mosquitoes tend to target people who are in the middle
of exercising more- when you're working out and breathing hard, you're giving off more
carbon dioxide than you otherwise would in your resting state.
This also contributes to overweight or otherwise larger individuals being more targeted by
mosquitoes than their slimmer or shorter brethren- the more body mass, the more oxygen the body
needs to maintain it and the more carbon dioxide the individual will give off.
Further, the higher your metabolic rate, the more carbon dioxide you give off and the more
you'll be (at least initially) attractive to female mosquitoes looking for some blood
so they can make baby mosquitoes.
This is also one of the reasons why children are usually less likely to be bitten by mosquitoes
than adults, and men more likely to be bitten than women.
Another contributing factor to people exercising being more likely to be bitten is body heat-
the higher your body heat, the more likely a mosquito is to notice you.
On that note, this is also a reason pregnant women are more attractive to mosquitoes (statistically
almost twice as likely to be bitten than the rest of the general populace).
Pregnant women exhale on average about 21% more carbon dioxide than when they are not
pregnant.
Their body temperature is also typically slightly higher than non-pregnant individuals, particularly
around the belly.
So that's the initial attraction.
Much like when clubbing, just because an individual is attracted to you from afar, once they get
close up, they may actually decide not to go any further.
You’re just not their type…
Seriously.
It turns out the factors involved in whether a mosquito will decide to land and then whether
they decide to bite essentially comes down to your general scent and then taste, both
in terms of certain chemicals attracting the mosquito even more and others potentially
repelling them.
How sweaty you are at a given moment, and what exactly is in your sweat, will greatly
influence these attractive and repellent factors, with such markers as lactic acid, uric acid,
and ammonia being among the compounds in your sweat and other skin excretions that attract
mosquitoes.
On the other hand, those who naturally excrete the compound 6-methyl-5-hepten-2-one have
been found to be significantly less likely to be bitten owing to this compound acting
as something of a mosquito repellent.
One's blood type also seems to be one of the markers used by mosquitoes to determine whether
you're a good candidate for a blood meal.
How?
Approximately 85% of people emit certain blood type markers, something at least certain types
of mosquitoes seem to be able to detect.
For example, in one study Asian Tiger Mosquitoes, preferred people with Type O blood nearly
twice as much as people with type A blood.
It also appears that the blend of microbes living on your body also effects how attractive
you are to at least certain types of mosquitoes, although research into this is also pretty
scant at this stage.
That said, in one 2011 study, Composition of Human Skin Microbiota Affects Attractiveness
to Malaria Mosquitoes, they found that more diverse colonies of microbes on your skin
actually seems to repel mosquitoes, whereas a high density of just a small number of certain
microbes, such as Staphylococcus spp, seems to attract them.