Breath Taking. Michael J. Stephen
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Название: Breath Taking
Автор: Michael J. Stephen
Издательство: Ingram
Жанр: Биология
isbn: 9780802149336
isbn:
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.
32. Thomas Wright, William Harvey: A Life in Circulation (Oxford, UK: Oxford University Press, 2013), xvii–xxi.
33. David G. Ashbaugh, D. Boyd Bigelow, Thomas L. Petty, et al., “Acute Respiratory Distress in Adults,” Lancet 290, no. 7511 (August 12, 1967): 319–323.
34. Giacomo Bellani, John G. Laffey, Tai Pham, et al., “Epidemiology, Patterns of Care, and Mortality for Patients with Acute Respiratory Distress Syndrome in Intensive Care Units in 50 Countries,” JAMA 315, no. 8 (2016): 788–800.
35. Roy G. Brower, Michael A. Matthay, Alan Morris, et al., “Ventilation with Lower Tidal Volumes as Compared with Traditional Tidal Volumes for Acute Lung Injury and the Acute Respiratory Distress Syndrome,” New England Journal of Medicine 342 (May 4, 2000): 1301–1308.
36. Michael A. Mathay, Carolyn S. Calfee, Hanjing Zhuo, et al., “Treatment with Allogeneic Mesenchymal Stromal Cells for Moderate to Severe Acute Respiratory Distress Syndrome (START Study): A Randomised Phase 2a Safety Trial,” Lancet Respiratory Medicine 7, no. 2 (February 2019): 154–162.
37. John B. West, “How Well Designed Is the Human Lung?” American Journal of Respiratory and Critical Care Medicine 173, no. 6 (2006): 583–584.
38. Adrian Bejan and Eden Mamut, Thermodynamic Optimization of Complex Energy Systems (Dordrecht, NL: Springer, 1999), 71.
Chapter 3
An Infant’s Drive to Breathe
The lungs not only facilitated the beginning of terrestrial life on this planet, they also facilitate the beginning of our individual lives. During the final trimester of pregnancy, the lungs are the only organ in the fetus that is not working. The heart is beating away at a fiery 160 beats per minute, the kidneys are making urine, with the baby peeing right into the amniotic fluid (which is then swallowed again by the baby in a repeating cycle). The brain and muscles are awake with kicks and backflips and rolls. But the lungs remain completely silent and nonfunctioning.
This all changes, and abruptly, when the baby emerges from the womb. The lungs must turn on in an instant in order to begin their job of oxygen extraction and carbon dioxide release. To measure the success of this change, all hospitals throughout the world use what is called the APGAR score, named after the distinguished professor of surgery from Columbia University, Virginia Apgar. The first woman to attain full professorial status at Columbia’s medical school, Dr. Apgar devised this beautifully simple and elegant way to assess the health of a newborn in 1953.
At one and five minutes after birth, the Appearance, Pulse, Grimace, Activity, and Respiration are assessed, and a score of 0, 1, or 2 is recorded for each, for a maximum of 10. The majority of babies easily achieve a score of 8 or 9. The purpose of the APGAR score is to identify babies at immediate risk and to take proactive measures to improve whatever deficiency is present. This could mean just agitating the baby until he or she wakes up, or it could mean giving more oxygen or inserting a breathing tube into the lungs. Sometimes in medicine inaction is preferable, along the lines of “First do no harm.” A low APGAR score is not one of these times. A newborn scored at 6 or 7 will usually improve on her or his own. A score under 5 is panic time.
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