The Respiratory System and the Lungs - Carbon dioxide-precious waste

In most of our minds, oxygen tends to be the hero of respiration and carbon dioxide the villain or at least the undesirable waste gas. This isn't really a fair picture. While it is true that too much carbon dioxide would act as a poison in our body, it is also true that we must always have a certain amount of the gas dissolved in our tissues. If we did not, two potentially fatal events could occur. First, our blood chemistry, especially its delicate acid-alkali balance, would get completely out of control. Second—and something of a paradox—the body's whole automatic system of regulating breathing would be knocked out.

It is the level of carbon dioxide in the bloodstream that controls our breathing. This level is continuously being monitored by the autonomic nervous system, specifically by the lower brain's “breathing center” in the medulla at the top of the spinal cord. When the level of carbon dioxide in our body goes above a certain level, signals from the medulla force us to breathe. Almost everybody has played, “How long can you hold your breath?” and knows that, past a certain point, it becomes impossible not to breathe. When you are holding your breath, the unexhaled carbon dioxide rapidly builds up in your system until the breathing center is besieged with signals that say “Breathe!” And you do.

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