Breath Taking. Michael J. Stephen

Breath Taking - Michael J. Stephen


Скачать книгу
alt=""/>

      Figure 2:The human respiratory system.

      A byproduct of oxygen use and cellular respiration is carbon dioxide (CO2), which diffuses out of the cell, into the blood, and back into the capillaries, which now flow to veins. Not used by our body, CO2 is shunted back to the lungs by our venous system, where it diffuses into the alveoli. From there, the air, with its new mixture of gasses, is pushed out through the network of bronchioles and bronchi as the diaphragm relaxes with exhalation, and ultimately expelled out of the mouth or nose and back into the atmosphere. The CO2 disperses easily into the air, where its levels are very low at 0.04 percent of atmospheric gas. Levels of oxygen in the atmosphere stand at a comparatively robust 21 percent, so on our next breath we are able to fill up again with this molecule of life. (The rest of the atmosphere is almost all nitrogen, harmless to us but also useless.)

      Figure 3: Gas exchange at the alveolar level.

      We are attuned so closely to this process of inhalation and exhalation in our slumbering loved ones because we have an instinctive understanding of its urgency: while we can skip meals, breathing must be continuous. The system must be finely coordinated as gas levels need to be kept within a very narrow range in our blood. Receptors in our aorta and carotid artery continuously monitor levels of oxygen and carbon dioxide and send feedback to the respiratory center in our brainstem. Even the slightest change in the levels of gas will trigger more signals or fewer to go out to our muscles of inspiration. The activity in the respiratory center of our brainstem also feeds back to our cerebral cortex, or higher brain, making us aware of any impending danger. This creates the alarming sensation we are all familiar with if our brain senses something wrong with our oxygen or carbon dioxide levels, as when we hold our breath.

      Carbon dioxide is what causes most of the initial problems when we hold our breath, because as it builds up in our blood during a breath hold, it begins to break down into acid. This acid is toxic to our cells, especially as it begins binding with proteins and other molecules it shouldn’t, impeding the normal function of cells. As the breath hold continues, lack of oxygen also becomes a problem, and as cellular respiration in our mitochondria ceases with the dearth of oxygen, cellular death ensues. The cells of the heart muscles are especially sensitive to this, and cardiac arrhythmias can ensue in extreme cases of too much carbon dioxide or too little oxygen. Breathing is the most important thing we are aware of doing, and the body regulates it tightly.

      Some five hundred years after Hippocrates, Claudius Galenus was the next great figure to change how we think about breathing and circulation. Better known simply as Galen, he was born in September 129 CE, in the Aegean Sea town of Pergamon, part of modern-day Turkey. His father, a wealthy patrician, originally had plans for his son to become a philosopher and statesman. These plans changed when the father dreamed that the mythical physician Asclepius visited him with a decree that his son study medicine. Galen’s father spared no expense, and Galen was educated at the best institutions throughout the Roman Empire.

      When he finished his studies, Galen settled into practice in Pergamon. He became the personal physician to the gladiators of the high priest of Asia by performing an act of daring. According to his own report, he eviscerated a monkey and then challenged the other physicians to repair the damage. When none stepped forward, he did the surgery himself, successfully restoring the monkey and winning over the high priest. He later moved to Rome and became the personal physician to several emperors, most notably Commodus, who reigned from 161 to 192 CE.

      Although Galen’s theories on blood flow would be proven to be partially wrong, he was important, like Hippocrates, because of his methodology. Galen cemented the notion that medicine and disease were not the products of divine intervention from the gods but could be discerned from empirical evidence and deduction based on observation and cause and effect. Nonetheless, more than a thousand years passed before his ideas on the movement of oxygen within the blood’s circulation were corrected.

      Upon establishing himself in England after his Italian internship, Harvey published De Motu Cordis et Sanguinis (On the Motion of the Heart and Blood) in 1628, cementing his reputation as a giant in the history of medicine. This work was seminal in our understanding of the basic physiological principles of how blood travels in the body. Harvey had two breakthrough insights. He noted that he had learned from his Italian mentor that the veins all had one-way valves, pointed away from the tissues and organs and toward the heart. Why the venous system, which was postulated by Galen to bring blood to the organs as the arteries did, had back valves to keep blood away from those parts of the body was not easily explained.

      Harvey’s second important observation was made through painstaking dissections of humans and animals. He calculated that the output of the heart was a lot greater than previously thought, about five liters per minute. He correctly reasoned that the tissues could not consume that volume of blood each minute, as proposed by Galen. He needed a more reasonable explanation, a system that was simple yet elegant—and that went against fifteen hundred years of dogma. So he proposed something often found in nature: a system of reuse and recycle,


Скачать книгу