Breath Taking. Michael J. Stephen
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like an artificial lung. It is not a long-term solution and only serves to buy time for the lungs to heal themselves. Studies of this treatment in adults with ARDS have had conflicting results, but it is an option for those who fail on the traditional ventilator.
Looking further into the future, the promise of stem cells is not just a dot on the horizon, but a viable therapy that is now making its way through clinical trials. Stem cells have the ability not only to transform into different cell types but also to mitigate inflammation. A phase 2 study, which analyzes mostly safety, was recently completed in patients with ARDS and showed positive findings.36 Further studies are underway, and the entire pulmonary community is holding its breath to see whether the first treatment to improve outcomes for ARDS patients is on its way.
John B. West thought a lot about the issues of oxygenation and ventilation throughout his long career, from the quiet of his laboratory to the windy heights of cold Mount Everest. For the first five decades of his career, Dr. West studied the same class of animal—mammals. But then he turned his attention to a completely different species—birds—and brought awareness to important aspects of breathing.
Today, some ten thousand species of birds exist on Earth, about twice the number of mammal species. They colonize many different habitats and are able to maintain incredibly high workloads, or metabolic rates. One species that stands out is the hummingbird, which, with a wing beat frequency of up to 70 beats per second and a heart that can go to over 1,200 beats per minute, has a metabolic rate thirty times higher than that of humans. Another remarkable bird is the bar-headed goose, which is able to fly up to thirty thousand feet. These are feats of physiology that humans could not think of matching, and Dr. West believes it is their bird lungs, radically different from human lungs, that allow them to sustain these very high workloads.37
Dr. West’s interest in birds was piqued in 1960, when he spent six months with a team of researchers on Mount Everest. The project was dubbed the Silver Hut Expedition for the tin house in which they lived, on the Mingbo Glacier, at nineteen thousand feet (the nearby peak of Mount Everest is at twenty-nine thousand feet). From this perch, West and the other scientists investigated the effects of high altitude on the human body. After some time in this environment, West had become frightfully tired and thin from the stress of altitude. One morning, as he struggled to get going, he looked out of the window of the Silver Hut, drawn by a quacking noise. Way above his head, at about twenty-one thousand feet, was a gaggle of twelve rather ordinary-looking tan geese flying effortlessly in skies normally reserved for jet airplanes. How could West explain the difference between his own extreme fatigue and the bird’s easy flight?
The answer to his question lay in the design of their lungs. Despite all the obvious differences between birds and humans, one of the most important distinctions is not immediately apparent, though it is the likely key to birds’ success colonizing so many habitats: their lungs have separated out the jobs of oxygenation and ventilation. Our lungs are simple in that they have combined these jobs into a single unit. They expand and contract to provide movement of air, or ventilation, much like a fireplace bellows. The same areas that expand and contract to provide ventilation also house the gas exchange areas that allow oxygen to move into the blood and carbon dioxide to be released.
Figure 5: Anatomy of a bird’s lung with air sacs for ventilation and air capillaries for gas exchange.
But as West’s observations led him to point out, if an engineer was designing a breathing machine, the functions of gas exchange and air movement would be separated. Birds have such a system. With each breath they take, air moves into air sacs, which are large, easily distensible organs in which no gas exchange takes place. This air then gets shunted to a separate area for gas exchange, termed air capillaries. There, because there is no need for the gas exchange units to bend when a breath occurs, the distance between the air and the blood vessels is incredibly thin, much less than the one third of a micron in mammals, making the exchange even easier. A final difference is that air movement in the bird lung goes around in a circle, much as blood does, so birds get fresh air with both inspiration and expiration. We, in contrast, are limited to getting fresh air only with inspiration.
With all of these differences, Dr. West argued that a bird’s breathing system is more efficient than a human’s. At the same time, our form follows our function. For most of our needs, the lungs we have do an excellent job. It’s only when we follow the bar-headed goose up Mount Everest with no supplemental oxygen, as some have attempted, that a set of bird lungs would come in handy. Biology has set limits, which some people try to ignore, at times to their own detriment.
The design structure of our lungs ties into our method of locomotion and basic needs of survival, which are much different from those of birds. But for all the variance between our classes, the standard blood levels of oxygen attained in a mammal and a bird are, surprisingly, exactly the same—about 95 mmHg in both species. Both bird and mammal systems seem to be tied into this optimal level of oxygen, not too low but not too high.
We know the problem at low levels of oxygen, but at higher levels, above 100 mmHg, oxygen may become toxic by grabbing electrons we don’t want it to grab, in a process similar to the oxidation that produces rust on a car. The laws and limits of chemistry are at work in all of our biological systems, including the gross anatomical structure of the lung.
It’s not just the laws of chemistry that govern our physiology. Other forces of nature are at work as well. As described in the prologue, our lungs resemble a tree branching up into leaves or down into roots. The lungs could also be compared to the tributaries of a riverbed merging to a main waterway. The neurons of the brain, spreading out into tendrils from the main axon, follow a similar configuration. The human body itself is another example, dividing from a main trunk into limbs, which then divide into fingers and toes.
This branching configuration is so familiar because it appears to follow a law of physics first described in 1996 by Duke University physicist Adrian Bejan. Termed the constructal law, it states that, “for a finite-size system to persist in time, it must evolve in such a way that it provides easier access to the imposed currents that flow through it.”38 The best structure for this happens to be how our lungs are designed, with many small branches connected to one big branch. The lungs are tied into the universe of physics like no other organ, perfectly using the space allotted to maximize flow. And optimizing flow, and movement, is clearly one of the purposes of life from a biological perspective.
Figure 6: The airways of the lung, designed to maximize flow.
26. Merriam-Webster Online, s.v. “dum spiro, spero.”
27. Roy Porter, The Cambridge History of Medicine (New York: Cambridge University Press, 2006), 78.
28. Daniel L. Gilbert, Oxygen and Living Processes: An Interdisciplinary Approach (New York: Springer-Verlag, 1981), 3.
29. Paula Findlen and Rebecca Bence, “A History of the Lungs,” Stanford University website, Early Science Lab, https://web.stanford.edu/class/history13/earlysciencelab/body/lungspages/lung.html.
30. Andrew Cunningham, The Anatomical Renaissance (Abingdon, UK: Routledge, 2016), 61.
31. Saul Jarcho, “William Harvey Described by an Eyewitness (John Aubrey),” American Journal of Cardiology 2, no. 3 (September 1958): 381–384.