As an earth-bound species, our fascination “to be above it all” — to transcend, to soar or to approach the divine — is timeless. The Egyptians, Romans, Greeks, Chinese, Indians, and peoples all across time, have ascribed flight to their deities and embedded flight into their fables and legends retold. To fly was the ultimate expression of power — be it a dragon or one of the yantras detailed in ancient Sanskrit text. To fly was to inspire awe.
But no legends of flight have endured like the story of Icarus, son of Daedalus. As first mentioned by Homer, Daedalus was a mythological sage and egotistical innovator. Credited with the first bathhouses, dance floors and the most life-like sculptures of Athens. Yet in a fit of jealousy, the accomplished Daedalus killed his ascendant nephew and was banished from Athens to Crete. There he built the inescapable minotaur’s labyrinth for King Minos but, as punishment for conspiring with the Minos’ lustful wife, he was sentenced to be forever imprisoned atop one of the island’s highest towers with Icarus, his only son.
While wishfully imagining their escape, Daedalus envisioned a human-sized wing that could be assembled from the dropped feathers of his avian companions. Attached only by thread and wax, Daedalus implored his son to take “the middle way — not to fly too close to the ocean waves nor too close to the intense heat of the Aegean sun.” Daedalus watched in dread as Icarus could not bring himself to heed his father’s warnings — soaring higher and higher until it was too late. A trip “too high” had claimed another.
Surely inspired by these myths and more, human aviators have moved beyond copycat bird wings to eventually harness the buoyancy of lighter than atmospheric gases (at first hydrogen and then helium). In these so-called “balloons” we have soared to life-threatening heights. As one of the world’s longest still standing records, in September 1862, James Glaisher and Henry Coxwell ascended to ~37K feet without the aid of supplemental oxygen. Amazingly, both Coxwell and Glaisher survived thanks to some last-minute luck. As they rose to nearly seven miles high, the temperature dropped well below zero and they began to notice difficulties with vision. “I could not see the fine column of mercury in the wet-bulb thermometer, nor the hands of the watch, nor the fine divisions on any instrument.” While Coxwell climbed out of the basket to free an entangled gas release valve, Glaisher was already losing consciousness.
At the time, neither Glaisher nor Coxwell could have understood the cause of their “balloon illness” that was brought on by the extreme cold and scarcity of oxygen. They may have also suffered from the “bends” as a result of the swiftly falling pressure during their too rapid ascent. The net result was nausea, paralysis and eventually total loss of consciousness.
Since these bold or perhaps foolish ascents, humans have conquered increasingly dizzying heights, but most with the advent of portable oxygen. The current record for a round-trip balloon ascent is 69K feet; while others have used balloons as platforms for skydiving near the very edge of outer space at 128K feet. However, these oxygen-assisted endeavors blur the line of what humans can “personally” accomplish.
Our true limits are set by biology, not engineering. Since we have evolved to live within the specific confines of the earth’s atmosphere, we are keenly dependent on the 20.94% concentration of oxygen that is evenly distributed from pole-to-pole and from sea-level to the edge-of-space. Although the concentration of oxygen is the same everywhere on our planet, pressure and temperatures change dramatically as you go up in elevation and in response to local weather conditions. Per gas laws, this translates to the same percentages of oxygen but decreasingly fewer molecules of oxygen per unit volume as pressure decreases. And although lower temperatures will increase the density of air and thus increase the number of molecules per unit volume, the dramatically lower pressures at higher elevations largely occludes the effect of the colder temperature.
The workhorse behind the scene of our gasping breath are small (~7 microns across) enucleated biconcave cells called erythrocytes (or red blood cells, “RBC”). The most prevalent cell in our blood (and body) representing 25% of all our cells. RBCs are generated within our bone marrow, and upon maturation, live approximately 120 days in circulation. Within each RBC are ~300 million molecules of a specialized gas transportation protein called hemoglobin. Upon traveling through the oxygen-rich capillaries of our just filled lungs, each hemoglobin can bind up to four atoms of oxygen. Then, as RBCs traverse the capillaries of our distinct tissues, a lower dissolved oxygen level prompts the hemoglobulin bound oxygen to release. The free atom (actually a pair of atoms) “O2” is then rapidly whisked into cells for their acute use by mitochondria in aerobic respiration (i.e. making energy). Interestingly, RBCs themselves do not use oxygen but as an exemplary act of “cellular selflessness,” they rely on the less efficient anaerobic metabolic pathways to produce the energy they need.
At sea level or generally below 5,000 feet, all goes well. But as we ascend, the pressure swiftly drops and with it the number of oxygen molecules available per breath. As an example, the average atmospheric pressure is 29.92 inches of Hg at sea level, but only 8.45 inches of Hg at the summit of Mt. Everest (29,035 ft.). As the density of inhaled oxygen declines, our bodies promptly sense the changes and trigger the production of more RBCs. But the process of making more RBCs takes time, and beyond just increasing RBC numbers, our physiology requires several other metabolic tweaks to function efficiently at these higher elevations. Our lungs require an increase in hydration to support the more difficult gas exchange processes; thus our fluid intake requirement increases dramatically. As our blood thickens (polycythemia) due to higher cellularity, i.e., more RBCs, our cardiovascular system has to work faster and at higher steady-state blood pressures.
For those attempting new heights on foot, a slow process of acclimatization is effective in enabling us to adjust to the new conditions, while minimizing the effect (or risk) of acute mountain sickness (AMS). For those that ascend rapidly by balloon, helicopter or as a wheel-well stowaway, their experience is all too often grim. By the time one has risen from sea level to just 14K feet, the percentage of fully oxygenated hemoglobin molecules within each RBC has already dropped by ~10%. For those who rapidly ascend to just 17K feet, greater than 50% will experience AMS and run the risk of pulmonary or cerebral swelling (edema). Of the 110 known aircraft wheel-well stowaways (tracked in the US), over 75% have died. Of those that have survived, some traveling up to 39K feet and into temperatures as low as -81 degrees F, a rapid state of unconsciousness and the extreme cold likely enabled their survival.
Having recently awoken from being face down unconscious at ~20K feet in the icy cold rock scree on a steep Andean mountainside, I am reminded of the stoic words of Glaisher, the balloonist world record holder, “No inconvenience followed my insensibility.”
Yet perhaps more applicable was the flight of Icarus, as fate and foot had enabled it, “to be just one step higher, and then higher, had proven to be irresistible.”