Why do you breathe?

Breathe in, breathe out
Why do you breathe day and night from birth until death? The obvious answer is that if you don't breathe, you will die. But why? Taking it a step further, you might argue that you need oxygen. OK, why? Does all life need oxygen?

I'm going to step away from the oxygen question for a moment and ask you to hold your breath.

After just 15 or 20 seconds most people have an uncontrollable urge to breathe. Why? What is the stimulus? You might be surprised to learn that the stimulus to breathe is not related to oxygen, but to carbon dioxide. The respiratory center in the medulla oblongata at the base of your brain has chemoreceptors that monitor blood pH. Your normal blood pH is 7.4; slightly basic. You maintain that pH very closely--a drop to only 7.35 is considered acidosis, and a rise to only 7.45 is considered alkalosis. How does this related to an urge to breathe?

Let's talk for a moment about carbonated beverages. When you have a new bottle of soda you do not see many bubbles in it, but as soon as you open the container the bubbles magically appear. What happened? The carbon dioxide has dissolved in the water--the high pressure has caused it to combine with the water in the following reaction (I apologize for not knowing how to do superscripts and subscripts):

CO2 + H2O --> H2CO3 --> HCO3- + H+

(In English: carbon dioxide combines with water to form carbonic acid, which then dissociates into bicarbonate ions and hydrogen ions.) This reaction is very reversible.

Normally if you bubble CO2 through water only a very small portion of it will combine with the water--most of the gas will simply bubble right through. As pressure builds, more and more of the gas will combine with the water, leading to the drink becoming more and more acidic. When the pressure is released (by you opening the bottle, for example), the reaction quickly goes the other direction and the carbon dioxide comes bubbling out of solution.

In your body, you are continuously producing carbon dioxide. Like in the soda, most of it would not combine with water spontaneously, and unlike the soda your body is not under high pressure. However, your red blood cells have an enzyme (carbonic anhydrase) that greatly facilitates this reaction, so most of the gas gets converted into carbonic acid and enters your blood in that form. It is this acid (and the resulting hydrogen ions) that causes your blood to quickly drop in pH. It is this change in your blood chemistry that your medulla oblongata is sensing, and this is the stimulus that causes motor nerves to your diaphragm and chest muscles to fire. You don't have to think about it, luckily, and you will breathe automatically in a rhythm to continuously keep your pH in line.

We still have not answered the question of why and how we use oxygen, nor did we answer the question of why and how the carbon dioxide is being produced (so many questions!). However, let's end this little post with a few take-home messages:

• The urge to breathe is triggered in response to rising carbon dioxide levels in your body, not by the falling oxygen levels
• Because your body is not monitoring oxygen levels as carefully, it is possible to lose consciousness (and die) from lack of oxygen without even panicking about it (this is what happens with Carbon Monoxide poisoning).
  • You will panic if drowning or being strangled, but it is because your blood is becoming acidic from elevated CO2 levels and you need to breathe to return pH to normal.
• You breathe hard while exercising because you are producing more CO2 and need to ventilate more frequently to maintain blood pH.
• Overexertion (such as sprinting as hard as you can for as long as you can) will result in not just a high production of CO2 but also a buildup of lactic acid (sorry, not telling you why just yet). You will be left panting hard to "catch your breath" (aka return your blood pH to normal). Many people think that feeling sore the day after exercise is due to lactic acid, but in fact by the time you are once again breathing normally your body has processed all of the lactic acid and your pH is back to normal.
• Like seals or dolphins, humans do have a diving response that can be trained, allowing you to hold your breath for several minutes.

Sorry about Oxygen--I kind of left it out of the story. Next time I will talk not just about why your cells need oxygen, but about the whole Oxygen Revolution!