“What makes human breathing so complex?”

This question showed up recently in my email box. I’m a volunteer responder on the website, Quora, an online question-answer site (of somewhat uneven quality). As a retired biomedical scientist, sharing information is a form of payback for my good fortune in having had an excellent education, a reasonably well-paid job teaching medical students, and the opportunity to do interesting research for more than three decades.

Of the many questions from Quora in my email box, I’ve answered several. I’ll share a few of those responses here on this blog. Having recently finished a book on self-help health-care, I’m currently seeking an agent for it. It’s designed for motivated readers who want to understand and care for their bodies over a long and healthy lifetime. It’s also aimed at people who are trying to navigate our complex health-care system more efficiently.

This was my answer to why breathing is so complex

Breathing is a complex process because it must be both automatic (normally controlled by the autonomic nervous system), and voluntary (controlled by the voluntary nervous system). Moreover, unlike most other organ functions except the heartbeat, if it is interrupted for even a very short time (a few minutes), you’re dead.

So the nerve centers that control normal breathing are in the deepest part of the brain, the brainstem (pons and medulla), and they have a double back-up system. This part of the brain – the “reptile brain” – normally directs our unconscious functions.

But the nerve fibers that go to the diaphragm (via the phrenic nerve) are myelinated, which means that they are fast-acting. And they’re under voluntary control, which means you can choose when and how to breath–if you’re awake, that is. You can see the nervous control of breathing muscles illustrated below. Moreover, those muscles that control breathing – the diaphragm and intercostal muscles – are voluntary, striated skeletal muscles, unlike the cardiac muscle that causes the heart to beat, or the smooth muscle that controls peristalsis (movement) in the gut.

Still, when you’re thinking about other things–and especially when you fall asleep–the automatic breathing function takes over and you don’t even have to think about it.

So breathing is a physiologic activity that you can control when you’re awake but that continues to happen under involuntary control while you’re asleep. Breathing is so complex because we must breathe whether we’re awake or not!

I believe that respiration is the only bodily activity that is normally both voluntarily and involuntarily. That’s pretty amazing!

Nerves and muscles that control breathing

Here’s an illustration, from Wikimedia Commons.

Respiratory centers and nerves for breathing