[Nobody's Confidant (OP)]

Is this just an old wives tale? Or does it actually work? I don't see how it could help.

[RD]

Exhaled breath has a much higher concentration of carbon-dioxide (CO2) than fresh air.
Re-breathing CO2-rich exhaled breath from the paper bag can regulate hyperventilation.

Overbreathing as a result of stress causes the delicate balance of gases (especially
carbon dioxide) within our bodies to become disturbed.
A rapid, shallow or irregular breathing pattern causes carbon dioxide to be breathed out
from the lungs, in turn producing chemical reactions within the body. These chemical
changes produce many of the symptoms of hyperventilation such as breathlessness,
palpitations, tingling and feelings of anxiety.

http://www.wsh.nhs.uk/documents/PatientLeaflets/5268-1.pdf

Don't try paper bag breathing for obstructive breathing problems such as asthma, emphysema.

[Andrew K Fletcher]

You are also rebreathing the humidity / moisture breathing high humidity causes they system to slow down. For example, a restless child with croup cough in a crib / cot with plastic padded sides receives a mist spray and within a few minutes stops coughing and goes to sleep. Observed this several times during vists to my son in hospital following surgery for a broken wrist In this case Co2 levels were not increased.

I can explain how this affects the respiration and circulation causing it to slow down after hyperventilating. But you won't find a reference to it in the literature

[Bigjoemonger]

A chemical reaction in the lungs allows the swapping of co2 for o2 to occur. When you hyperventilate you breathe out too much and in too little. So there's oxygen in your lungs but no co2 to trade places with the oxygen.

The oxygen can't get into the blood so your body thinks there isn't enough so it tries to breathe faster to compensate. By breathing in the bag you breathe back in some of that co2 which rebalances the gas exchange process so you get what you need and can get your breathing under control.

[Colin2B]

The oxygen can't get into the blood so your body thinks there isn't enough so it tries to breathe faster to compensate.

In fact oxygen can get into the blood and problems can occur when there is a adequate supply.
The primary trigger for breathing is the rising CO2 level in the blood rather than the falling O2 level, but hyperventilating flushes out CO2 and this low level lead to narrowing of the blood vessels that supply blood to the brain. The reduction in blood supply to the brain leads to symptoms like lightheadedness and tingling in the fingers. Severe hyperventilation can also lead to loss of consciousness particularly in breath hold diving where O2 can be used up before the CO2 rises to the ‘need to breath’ trigger level.
As has been said, breathing into the bag increases the CO2 level in the blood. There are other ways:
Breathe through pursed lips.
Breathe slowly into a paper bag or cupped hands.
Attempt to breathe into your belly (diaphragm) rather than your chest.
Hold your breath for 10 to 15 seconds at a time.

IMPORTANT These are not a cure for all breathing problems as the OP might suggest. Also, hyperventilating can be caused by serious illness eg heart failure or lung disease. If you suffer from this without obvious cause eg extremely stressful situation, please see a doctor.

[chris]

The normal pH - acidity level - of the blood and body fluids is about 7.4; a contributor to this is carbonic acid (H₂CO₃) arising from the carbon dioxide (CO₂) released from respiring cells.

Some of this carbon dioxide is carried back to the lungs dissolved in blood plasma; a small amount binds to proteins in the blood; the bulk enters red blood cells where an enzyme called carbonic anhydrase catalyses the formation of carbonic acid, which dissociates to form bicarbonate (HCO₃^-) and hydrogen ions:

CO₂ + H₂O H₂CO₃ H⁺ + HCO₃^-

This bicarbonate exits the red blood cell and enters the blood plasma, which carries it to the lungs. There, the relatively low concentration of carbon dioxide shifts the above reaction equilibrium back towards the left, regenerating carbon dioxide, which diffuses into the alveoli and is exhaled. The high oxygen concentration also displaces carbon dioxide bound to haemoglobin, and this too moves into the alveoli.

Critically, the concentration of carbon dioxide in the lung dictates the rates at which carbon dioxide (as dissolved CO₂, CO₂ bound to haemoglobin, or CO₂ carried as bicarbonate) can leave the body. So if you hyperventilate then you will promote the loss of carbon dioxide from the body. Since, at rest, CO₂ is being produced in tissues at a relatively constant rate, this will lead to a net reduction in carbon dioxide and hence a loss of carbonic acid from the tissues. The reduction in carbonic acid will lead to a respiratory alkalosis (blood and tissues becoming more alkaline), which produces the strange symptoms we experience, of dizziness, and tingling in the peripheries.

Breathing in and out from a paper bag will help because the oxygen concentration in exhaled air is quite high, so there is no reduction in tissue oxygenation. However, the exhaled CO₂ is returned to the body, reducing the rate at which further CO₂ is lost and also encouraging some more to dissolve back into the blood and reverse the alkalosis.

[Colin2B]

Thanks for the detail @chris .
As I understand it, the medulla oblongata is sensitive to decrease in pH and increase in PCO2 (partial pressure of CO2) when giving the signal that breathing is needed.
There also seems to be a back up system in the carotid body chemoreceptors which are sensitive to pH and PCO2, but only secondarily. Their sensitivity to decreased PO2 is greater when pH is decreased and PCO2 is increased.
Amazing system this simple act of breathing!

[chris]

In humans the carotid body at the bifurcation of the common carotid body detects arterial blood pH. It transmits this information to the medulla oblongata (the caudal brainstem) where it is integrated with other chemoreceptive inputs. This information is used to inform the respiratory centres, which are also nearby, and augment rate of breathing when blood pH falls. Thus, even a metabolic acidosis, such as occurs when a diabetic goes into ketoacidosis, can be compensated, at least initially, by hyperventilation.

[chiralSPO]

Would drinking a carbonated beverage help as well?

Presumably inhaled CO₂ would reach the blood more quickly, but I would imagine that ingested carbonic acid would also be absorbed, either through the small intestine or belched up and absorbed through the nasal passages or breathed in...

[chris]

@chiralSPO - yes, metabolic acidosis from ingested foodstuffs will contribute to total body pH, but the amount you'd need to ingest, and the time it would take to achieve an effect would make this an unhelpful remedy. That and the practical problem of getting someone who's hyperventilating to swallow a can of Coke...

[Monox D. I-Fly]

The oxygen can't get into the blood so your body thinks there isn't enough so it tries to breathe faster to compensate.

In fact oxygen can get into the blood and problems can occur when there is a adequate supply.

Wait, so oxygen-addiction is a thing? Back then when I was a child, there was a tokusatsu episode (forgot whether it was a Metal Hero series or an Ultra series) where the victims got too much oxygen in their body making them addicted to it and it was portrayed as dangerous in the episode. At the time I thought it was silly, never knew that it was in fact Truth in Television.