Breathing seems like a simple concept: take a breath in and blow it out, then repeat. But there's a lot going on when air is moving in and out of the body. To understand why certain things cause shortness of breath, we have to understand what happens when we breathe.
Balancing Oxygen and Carbon Dioxide
We need some of each gas (oxygen and carbon dioxide) in our blood to function properly, but we also need to maintain a balance. We naturally keep quite a bit of extra oxygen in the blood for emergencies, so a short-term lack of oxygen isn't that big of a deal. We need that reserve in case a bear decides to chase us through the cul de sac, or we have to teach a teenager how to drive.
We also need it in case of sudden cardiac arrest.
We get oxygen by inhaling oxygen-rich air into our lungs, where the oxygen is picked up by red blood cells in the bloodstream. When the body's tissues burn oxygen as part of their fuel, they create carbon dioxide as a waste product. We get rid of carbon dioxide when the same blood that's flowing through the lungs picking up oxygen drops off molecules of carbon dioxide.
Carbon dioxide, on the other hand, builds up pretty quickly, and we need to get rid of it in a timely manner. In fact, it's the accumulation of carbon dioxide that makes you feel like you desperately need to exhale when you hold your breath. If something interferes with our reaction to too much carbon dioxide, we can still survive for a long time without breathing.
Not having enough oxygen or having too much carbon dioxide can result in a feeling of shortness of breath. High school coaches are rather fond of running "wind sprints" until players are gasping for air, unable to talk. In fact, feeling "winded" is another way to say you're feeling short of breath. Besides the fact that the players' muscles need more oxygen, all that exercise has caused them to build up lots of carbon dioxide they have to get rid of.
We can break down the physiology of breathing into two main parts:
- Moving air in and out of the lungs (ventilation)
- Moving gases -- mostly oxygen and carbon dioxide -- in and out of the bloodstream (respiration)