Chapter 2

      We Must Inhale and Exhale. But Why?

      Sometime after midnight, one deep January night, I remember walking into my bedroom and staring down at my first child in her crib, born on New Year’s Eve and now barely two weeks old. The silver moonlight poured in through the window, illuminating her outline. Her eyes were closed tight, her arms thrown over her head as if in an eternal stretch, her head tilted slightly to the right. Her intoxicating newborn smell brought rapture and calm.

      Like millions of parents before, I considered the serenity of a newborn’s sleep, its restorative depth, but I also instinctively checked on function, to make sure life still blazed within despite the outward calm. This meant checking her belly, to make sure she was breathing. Of course she was: her chest and abdomen moved up and down under her blanket in the rhythmic flow we all recognize as life.

When we observe our loved ones sleeping, old or young, human or pet, we are instinctively drawn to their breath. There is something essential in it we are all attuned to, something we both automatically and unconsciously equate with life. Each time we check on each other, we are validating the words of the Roman philosopher Cicero, dum spiro, spero, “As I breathe, I hope.”26

      Physiologically, what we are observing is the miracle of gas exchange, taking from the atmosphere an invisible element and bringing it into our body to be consumed. The process begins with a signal from the brainstem, the primitive part of the brain at the base of the skull, which travels through nerves down to the muscles of inspiration, instructing them to contract. The biggest and most important of these muscles is the diaphragm, a thin, dome-shaped sheet of skeletal muscle that separates the thorax (chest cavity) from the abdomen.

      With each signal, the diaphragm contracts downward, pulling the thoracic cavity and the lungs with it. This creates negative pressure in the trachea and lung tissue, and with this negative pressure air rushes in, just as water flows down a river. Entering through the mouth or nose, the air travels down the back of our throat, past the vocal cords, and into the trachea. About halfway down the sternum, the trachea divides into the left and right bronchi, which divide again and again into lesser bronchi, termed bronchioles. The air moves through the bronchioles, which extend deep into the lungs, like tendrils from a star erupting in space, until finally it penetrates into cave-like recesses, deep in the lungs. Known as alveoli, and resembling cells of a honeycomb, these grapelike clusters at the end of the increasingly narrow breathing tubes are where gas exchange occurs.

      Continuing its natural flow from an area of high concentration to one of low concentration, oxygen moves effortlessly through the thin surface of alveoli, just a single cell thick, to the adjoining capillaries. Here, thousands of hungry red blood cells grab oxygen, and together they are pumped by the heart to the arteries and then to the tissues of the organs, which are infiltrated with a vast network of capillaries. At the tissue level, oxygen hops off the red blood cell and diffuses through the capillaries into the cells of whatever organ or muscle is nearby.

      Within each cell are mitochondria, the specialized organelles in which cellular respiration takes place: oxygen joins with glucose to produce carbon dioxide, water, and adenosine triphosphate (ATP). ATP is our primary source of energy, the molecule that drives many of our bodily reactions, including muscle contraction, enzyme production, and the movement of molecules within our cells. ATP causes these reactions by breaking off one of its phosphate groups, whose electrons are in a high-intensity state, and transferring this energy to drive the necessary processes of the cell. Now adenosine diphosphate, it gets recycled back to the mitochondria to become high-energy triphosphate again through the ongoing process of cellular respiration.