Did you know that what you breathe out is just as important as what you breathe in? We have learned that inhaling oxygen in each breath is vital to our existence– we die within minutes without this precious gas.  In this article we’ll unravel how what we exhale (breathe out) is just as critical for our survival.  

         The average American produces around 4.4 lbs. of trash daily.   That means a small family of 4 generates around 120 pounds of trash weekly. (1) Have you ever wondered what would happen if garbage collectors of your city went on strike?  The city of Seville, Spain didn’t have to wonder, as efforts to reduce wages resulted in garbage collectors going on strike.  (2)  As you can see, it wasn’t a pretty picture.  Their streets were clogged with mounds of trash!  If there’s anything we’ve learned in the history of mankind, it is you can’t ignore the trash problem. 

What does trash have to do with our topic of “What’s In A Breath?  Our cells are like small households, each producing “trash” that needs to be removed.  The “trash” we’re talking about it is a waste gas called carbon dioxide (CO2).  Why do our bodies make CO2?  Whenever you burn anything, say a car burning gasoline, CO2 is released as a byproduct of that combustion.  Each cell in our body produces this gas when we use the oxygen we breathe in to “burn” the food we’ve eaten in order to convert it into ATP molecules. Without these our cells will die. The average adult body consists of 40 trillion cells which produce a combined total of 2.3 lbs of carbon dioxide (CO2) each day! (3)  That’s a lot of trash to get rid of! Solving the trash problem means designing an efficient system to pick up and safely dispose of trash from 40 trillion customers, not just once per week, but non-stop, every second of the day.

         But, why is CO2 considered “trash”?  Under normal conditions CO2 is not toxic to us, as our body has an efficient method of handling and removing it so it doesn’t build up.  When this system is disrupted, just like when garbage collectors go on strike, the concentration of CO2 can quickly increase to toxic levels. This problem has a name; acidosis. CO2, normally a gas, becomes carbonic acid (H2CO3) in liquids like blood.  This weak acid, the same one which causes the “fizzy” in soft drinks, donates a hydrogen ion (H+), causing our pH (acid concentration) to decrease.  This pH issue is the toughest problem to be solved in taking out the trash.  Why?  Our blood pH must remain within a very narrow window: 7.35 and 7.45.  Death can occur if it drops below 7.0 for any length of time because our body chemistry simply stops working as it should. (5) (6) We better get rid of that trash!

See Activity 1 to learn more about carbon dioxide in solutions.  

Take a deep breath.  Those oxygen molecules you just breathed in are already traveling through your blood on their way to your cells.  As the oxygen molecules are passing into the cells, CO2 molecules are making their way out.  Do they “know” that if they don’t find a way out of our body they will poison us?  No, of course not.  Each gas molecule obeys its own concentration gradient, meaning it always moves from areas of higher concentration to areas of lower concentration.  The concentration of CO2 made in the cells will build up until its partial pressure is greater than the partial pressure of CO2 in the blood.  Oxygen moves from a partial pressure (PO2) of 95 mmHg in the blood towards the PO2 of 40 mmHg in cells.  CO2 moves from a PCO2 of 45 mmHg in the cells towards the PCO2 of 40 mmHg in the blood.    

Here in the US we practice the “manifest system” when disposing of hazardous materials.  This is referred to as the “cradle to grave” method in which the facility that generates the waste is responsible to track the waste until it has been safely disposed of.   There are three ways that hazardous waste is transported to its final resting place.  The most common method is by truck, as it can more easily gain access to industrial sites and TSDFs (Treatment Storage Disposal Facility).  The two other methods are by train (usually only very large shipments) and very rarely by air. (6)   CO2 molecules entering the blood also have 3 travel options as they make their way to the lungs.  How many CO2 molecules are we talking about?  On average each of your 40 trillion cells dump around 16.5 million (16,500,000) molecules of CO2 into the blood every 4 seconds.  Since you’re alive and reading this, we have good evidence that your body has been handling this trash problem in an efficient manner. Let’s track 100 of these molecules in order to better understand their amazing journey out of your body.

Travel option number one is to swim.  CO2 molecules are around 20 times better at dissolving in liquids than are oxygen molecules. Still only around 5-10 of our 100 molecules (5-10%) are able to dissolve in the plasma of the blood because as each one enters the blood PCO2 increases. When the PCO2 reaches 45 mm Hg, no more floaters are accepted, as that partial pressure equals that within the cells. These “floaters” travel to the lungs in the blood plasma just as they are, for the most part (see Notes #4) whereas the rest of our CO2 molecules we’re tracking must assume a different “identity” in order to make it to the lungs.  Well, that takes care of 5-10 CO2 molecules, but we still have 90-95 CO2 molecules (90-95%) without a ride.

       Our second group of CO2 travelers, consisting of 10-15 (10-15%) reach the lungs via a more complex method.   These CO2 molecules are able to bind to proteins, specifically hemoglobin proteins found within red blood cells (and to a lesser degree plasma proteins). The presence of CO2 molecules entering the red blood cells induces the hemoglobin proteins to let go of their remaining oxygen molecules.  Now the CO2 binds to the hemoglobin and travels disguised as a compound called carbaminohemoglobin.  Try saying that fast 5 times! What’s so cool about that?  Since the CO2 molecules are now traveling through the blood in a disguised form they no longer can perform their toxic work of lowering our pH.

        That still leaves us with around 75-85 (75-85%) of the CO2 which must be transported to the lungs.  Here is where our sense of wonder will really get stimulated.  As these 75-85 CO2 molecules enter the bloodstream, they also enter red blood cells where they are met by an enzyme called carbonic anhydrase (CA).  Remember that enzymes serve as catalysts which speed up chemical reactions, but don’t themselves join in the reaction.  Under the influence of the carbonic anhydrase catalyst the CO2 reacts with water to become carbonic acid (H2CO3). Though this reaction happens without CA, as we discussed earlier, it can occur at speeds up to 1 million times faster under its influence. Without this valuable enzyme only 15% of the CO2 building up in cells would be eliminated (7). As mentioned earlier, the carbonic acid breaks apart into H+ and HCO3- ions. 

This process creates some good and bad news, followed by some great news.  The good news is that the nasty CO2 can now travel to the lungs disguised as HCO3- ions. Uh-oh, but there’s bad news, also.  All the H+ ions just released will make our blood more acidic, lowering the pH.  What’s the great news?  Our bodies come equipped with a buffer system which is able to handle incoming H+ ions so they don’t cause havoc.  The first line of defense is our built-in pH and gas analyzing sensors called chemoreceptors (see article # 6) which trigger our respiratory muscles to breathe faster to get rid of more CO2.  It has been found that doubling the breathing rate for as little as 1 minute serves to increase the blood pH by 0.2. (8) Secondly, hemoglobin, the amazing oxygen transporter in red blood cells, binds excess H+ ions to itself before they can create a problem, just like a hazmat technician safely disposing of toxins. That’s great news!  Problem solved, right?

      But, what if you are dumping loads and loads of CO2 into the bloodstream, say as when you are working out or playing basketball?  A third line of defense involves our kidneys.  A low pH triggers two responses.  First, they can reabsorb more bicarbonate ions (HCO3-) to place back in the blood which react with excess H+, reforming the weak carbonic acid (H2CO3).  Secondly, they can begin to pull more H+ ions out of the blood and send them into the urine to be eliminated.  Nothing like having a backup system to help in emergencies!

Let’s summarize what we’ve discovered so far.  If we focus on just 100 of the trillions of CO2 molecules leaving our cells each second, we learned they travel to the lungs using 3 different methods.  The first group of 5-10 just “jumped in” and floated as CO2 dissolved in the blood with little effect on pH (without carbonic anhydrase only a small amount of the CO2 dissociates into H+ and HCO3-).  The second group of 10-15 travelers were attracted and bound to proteins (mostly hemoglobin proteins with some plasma proteins).  They make the journey to the lungs disguised as special carbamino compounds.  Our third group of 75-85 CO2 molecules were transformed by the carbonic anhydrase enzyme to travel as bicarbonate ions (HCO3-).  By the way, these bicarbonates can’t remain within the red blood cell, or excess water would enter and rupture the cell (9).  However, a special protein called an anion exchanger pulls off a prisoner exchange by extracting the bicarbonate ion out of the red blood cell into the plasma in exchange for a chloride ion (Cl-).  Now that our largest group of CO2 molecules is safely tucked away as HCO3- in our plasma we’re ALMOST ready to take out the trash. 

One last problem – you can’t breathe out bicarbonate (HCO3-) as it isn’t a gas.  Oh, no!  Evolution did not plan for this, and now we’re doomed to acidosis and untimely death!  Just kidding, the Designer of this system didn’t bring it this far only to fail in the end.  He has solved the trash problem completely! You are about to witness the final drama in solving the trash problem, and it better happen quickly, as the blood carrying the CO2 in its various forms is only in the unloading zone of the lungs for less than a second!          

 When the blood carrying all our 3 groups of CO2 passengers nears the lungs, rising pH levels and higher levels of oxygen signal a flurry of activity.  The anion exchanger gets prompted to quickly reverse the process, exchanging the bicarbonate ion in the plasma with the chloride ion in the red blood cell.   At the same time the presence of increased oxygen coming from the lungs induces the hemoglobin protein to release both its carbaminohemoglobins and H+ ions in favor of oxygen.   The H+ is reunited with the HCO3- to form H2CO3, carbonic acid.  Now, it’s carbonic anhydrase which flips back into action, quickly converting the carbonic acid back into water and CO2 gas.  Now that the CO2 is a gas again, it oozes through several membranes to find itself in one of the lung’s small alveoli, driven by the lower partial pressure of CO2 in the air just breathed in.  Finally, we’re ready to get rid of that nasty trash!       

Okay, you can BREATHE OUT now.  What seemed like an insurmountable problem, removing the toxic waste of 40 trillion customers and safely disposing of it, has been solved. Relax! The air you just breathed out has 100 times the concentration of CO2 compared to what you breathed in (4% compared to 0.04%).  By “taking out the trash” you are keeping your blood pH in that very narrow window we discussed earlier. Little did you know that your physical life is continuously being balanced like a tightrope walker on a tightrope. Each breath maintains that balance without you ever having to think about it. True, we can’t live without oxygen, but our life also depends on taking out the trash.   Every breath you take is a gift from God!

Activity - pH Changes Due to Carbon Dioxide

1. Make a batch of red cabbage juice by adding 1 cup of red cabbage with 1 cup of boiling water.  Blend for 30 seconds and strain out the pulp.  Let cool.  Note the color.

2. Add several drops of a base like dish soap or baking soda solution. What happens to the color? Now add an alka seltzer tablet.  The tablets contain citric acid and baking soda which react to form CO2, all the bubbles you see.  If you don’t have a tablet you can mix 1/2 teaspoon of baking soda with 2 oz of vinegar. this reaction also produces CO2.

3.  What happens to the color of the red cabbage juice?

4. Use your cabbage juice to test the pH of other liquids in your home

Questions for thought:

Could our bodies have evolved a buffer system that reacts to pH levels, and known how to handle emergencies? How did kidneys “learn” to respond to low pH levels? How did we eliminate CO2 before carbonic anhydrase evolved? How did we get “lucky” enough to have an atmosphere with enough CO2 for plants, but not enough to poison us?  How incredible is it that plants and animals have a wonderfully balanced symbiotic relationship where we exchange gases? Would you still be alive if you did not have a multitude of anion exchanger proteins in your bloo ources:

1. https://www.dumpsters.com/blog/us-trash-production

2. https://www.dailymail.co.uk/news/article-2274467/Striking-rubbish-collectors-cause-stink-Seville-letting-6-000-tons-waste-pile-citys-streets.html

3. https://www.sciencefocus.com/planet-earth/how-much-does-human-breathing-contribute-to-climate-change/

4. https://www.ncbi.nlm.nih.gov/pubmed/6804193

5. https://opentextbc.ca/anatomyandphysiology/chapter/26-5-disorders-of-acid-base-balance/

6.  https://www.britannica.com/technology/hazardous-waste-management/Transport-of-hazardous-waste

7. https://courses.lumenlearning.com/suny-osbiology2e/chapter/transport-of-gases-in-human-bodily-fluids/

8. https://opentextbc.ca/anatomyandphysiology/chapter/26-4-acid-base-balance/

9. https://academic.oup.com/bjaed/article/5/6/207/331369

Notes:

1. 5. C6H12O6 + 6O2 -------------------> 6CO2 + 6H2O + ~38 ATP

2.  2.3 lbs x 454g = 1,044 g / 44 g/mole = 23.7 moles x 6.022 x 10^23= 1.43x 10^25 / 40 trillion cells = 357 billion CO2 molecules generated by a resting cell / 21,600 breaths = 16.5 million CO2 molecules leaving a cell every 4 seconds.

3.  CO2+H2O⟷H2CO3(carbonic acid)⟷HCO3- +H+

4. A small percent of this CO2 dissolved in the plasma do react with the water to form carbonic acid, but without the presence of carbonic anhydrase it is a much slower process.