Strange Mechanisms: How, Exactly, Do We Breathe?

How we breathe is not something most of us ever stop to contemplate. Breathing mechanics are also not particularly intuitive. Theoretically, there are two ways air could get into the lungs. Air could either be pushed in (positive-​​pressure breathing) or pulled in (negative-​​pressure breathing). In both cases, air flows from an area of high pressure to an area of low pressure. In the former, a high-​​pressure area is created, forcing air to a lower-​​pressure area. In the latter, a low-​​pressure area is created, pulling air from the higher-​​pressure area. For instance, frogs are positive-​​pressure breathers. Frogs actively gulp air into their mouths, squeeze their mouths tight, and push air from their mouths into the lungs. Forcing air into the lungs is also how mechanical ventilators work.

On the other hand, humans are negative-​​pressure breathers. Our primary breathing muscle, the diaphragm, sits underneath the lungs. In its rest position, it forms twin domes, one underneath each lung. When we inhale, the domes contract and flatten. The diaphragm pulls the lungs down with it, causing the lungs to expand. Air rushes into the low-​​pressure cavity created by the expanded lungs. During exhalation, the diaphragm relaxes upward. The pressure inside the lungs becomes slightly positive as compared to atmospheric pressure, and air rushes out.

Take a breath in, and now let it out. Stop at the point where it feels natural to stop. We call the volume of air left in the lung at this exact spot functional residual capacity (FRC). For an average adult, there is about 2.5 liters of air in the lung at FRC. During most of the respiratory cycle, the lungs act like a rubber band pulling inward. The tendency of the lung to collapse inward is called elastic recoil. At the same time, the chest wall also has a recoil force directed outward. FRC occurs at the exact point where these two opposing forces are perfectly balanced.

Where FRC occurs matters because in diseases like emphysema, in which the lungs lose their elasticity, the balance point at which FRC occurs is greater and the lungs become too big. Unfortunately, this pushes the diaphragm down. When a person with emphysema breathes in, the diaphragm is already so low, it has nowhere to go. They are already flat, making it very difficult for patients to breathe. On the other hand, for patients with diseases that scar the lungs, sometimes called pulmonary fibrosis, the forces drawing the lung in become too great and the lung shrinks (lower FRC).

There are other muscles besides the diaphragm that participate in breathing, including the intercostal muscles between the ribs and many of the muscles in the chest, neck, back, and abdomen that create a girdle around the chest cavity. When the lung is diseased, patients must rely more on these accessory muscles to help with the extra burden of breathing. These other muscles also come more into play during forceful inhalations and exhalations. For instance, an even greater exhalation can occur by contracting the accessory respiratory muscles, including the intercostal muscles, to increase pressure in the abdominal cavity, force the diaphragm up, and push additional air out. However, it is impossible for anyone to force all the air from their lungs. Even at maximum exhalation, the average adult has about 1.2 liters of air left in their lungs, called residual volume (RV).

Breathing is so vital to life that the respiratory control center is housed in one of the most primitive parts of our brain, the medulla oblongata. This part of the brain is capable of continued functioning despite loss of consciousness, ensuring that critical functions like breathing are always maintained. The brain’s respiratory center communicates with the diaphragm via the phrenic nerve. This allows the brain to either speed up or slow down our breathing. So even when we are sleeping or unconscious, as long as the brain stem’s connection to the diaphragm via the phrenic nerve is intact, breathing continues.

One might envision that if we were designing this system, we might have little sensors that would tell the brain when oxygen levels in the blood were getting low. The brain could then send signals back to the lungs to tell them to breathe more quickly and deeply. The body’s solution for this problem is even more complex. It includes a backup system housed in the brain, as well as input from peripheral receptors that monitor not only oxygen but carbon dioxide and pH levels in the blood as well. When carbon dioxide dissolves into the blood, it combines with water to become bicarbonate, which serves an important role in regulating the acid-​​base balance in the blood (measured in units of pH).

The body is one highly orchestrated symphony of thousands of simultaneous chemical reactions, all finely tuned to optimal performance at a very specific pH: 7.4. A pH of 7.0 is perfectly neutral, with 7.4 being slightly alkaline. So when breathing alters the concentration of carbon dioxide in the blood by a significant amount, the pH in the blood also changes. Too much carbon dioxide and the blood becomes acidic. Too little and the blood becomes too alkaline, or basic.

We can think about the lungs like a conveyor belt, picking up oxygen and dropping off carbon dioxide with every breath. Chemoreceptors housed in the carotid artery monitor oxygen, carbon dioxide, and pH to determine how fast or slow breathing should occur. If all is functioning properly, then the body has enough oxygen and just the right amount of carbon dioxide to maintain a pH of 7.4. But if the system is perturbed, this conveyor belt really only has two options, to speed up or slow down. This system can handle “normal” perturbations such as exercise. When we exercise, we typically use more oxygen but also generate more carbon dioxide that we need to get rid of. So speeding up the conveyor belt helps with both problems.

What if, however, you want to take an airplane trip. Every time you board a flight, you inadvertently walk into an evolutionary blind spot. The interior of a plane is typically pressurized to the equivalent of roughly 6,000 to 8,000 feet above sea level. While the percentage of oxygen in inspired air is constant at various altitudes, the drop in total atmospheric pressure at higher altitudes results in a drop in the partial pressure of oxygen. At 8,000 feet, the partial pressure of oxygen is roughly 34 percent of what it is at sea level. Partial pressure refers to the air pressure contribution from any single gas within a mixture of gases. This reduction in the partial pressure of oxygen reduces the driving pressure of oxygen into the blood, resulting in decreased oxygen content of the blood. The body responds by trying to breathe more, to speed up the conveyor belt. But unlike exercise, where oxygen demand and carbon dioxide generation more closely match each other, in this case speeding up the conveyor belt to deliver more oxygen also has the unfortunate consequence of ridding the body of too much carbon dioxide, thereby increasing blood pH.

If we reach higher elevations more slowly, the kidneys, which also play a key role in maintaining the body’s pH balance, can over days to weeks bring the blood pH back into equilibrium. On the plane, however, the body settles on a compromise with suboptimal oxygenation but less severe acid-​​base disturbances than might otherwise occur. If we instead travel quickly to high altitude and stay there, the combination of low oxygen compounded by pH disturbances can lead to symptoms such as headaches, fatigue, and nausea—​​otherwise known as “acute mountain sickness.”

As opposed to the effects we notice from this involuntary control over our breathing, conscious control over our breathing patterns, particularly slow, deep breathing, may also have profound effects on the rest of our body. Controlling one’s breath to enhance health has been practiced by Eastern cultures for thousands of years. While the exact mechanisms are not well understood, slow breathing has the ability to alter brain activity and nerve signals going to other organs, such as the heart. Deep breathing exercises have been shown to lower heart rate, blood pressure, and even cortisol levels, which are a measure of the body’s stress response. Slow breathing has well-​​documented effects on improving mood, reducing stress, and triggering a relaxation response. Slowing down the breath can also improve our attention span and even lower pain levels. Hence, the lungs not only provide life-​​giving oxygen and rid our bodies of potentially toxic carbon dioxide, the act of breathing itself contributes to our day-​​to-​​day sense of well-​​being.

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Reprinted from Breathing Lessons: A Doctor’s Guide to Lung Health by MeiLan K. Han. Copyright © 2021 by MeiLan K. Han. Used with permission of the publisher W. W. Norton & Company, Inc. All rights reserved.