We are often not aware that space actually begins at the surface of the Earth . We have become accustomed to living in a peculiar environment in that space, where there is a thin layer of gas that is held together around a rocky planet. As we ascend through that atmosphere the conditions begin to look more like those in outer space.
A human being cannot stay alive for more than a few seconds without a source of oxygen at minimal pressure. Because we have evolved in a single atmosphere, the pressure and composition in a space module should resemble conditions on Earth at sea level . The human body is a heat engine that uses carbohydrates, fats and proteins as fuel in the process it uses to keep itself running: oxidation. Hence, it needs the oxygen provided by the lungs. At rest it consumes almost half a liter of oxygen, but can reach 4 liters while running.
At the beginning of the 19th century, the chemist and author of the first atomic model in the history of science, John Dalton, discovered that in a mixture of gases enclosed in a container, each of the gases behaves as if it alone occupied the entire volume. and exerts a pressure equivalent to the percentage it occupies in the mixture: it is called partial pressure. This means that an air mixture containing 20% oxygen with a total of 1013 millibars (mbar) of pressure (the standard atmospheric pressure) means that the oxygen exerts a pressure of 202.6 mbar. In 1878 the French physiologist Paul Bert pointed out that it is the partial pressure that determines the physiological effects of a gas .
Pressure measurements made inside human lungs have shown that the CO 2 on the blood capillary side of the alveolar membrane is about 53 mbar. This is a quarter of the partial pressure of oxygen under normal conditions of pressure and temperature. Because the partial pressure of oxygen is greater than that of CO 2 , oxygen flows across the membrane and replaces CO 2 as CO 2 crosses the membrane in the opposite direction and is expelled out of the body with respiration.
With these concepts in mind we can understand one of the most critical dangers that threaten humans when they are in space: hypoxia, the lack of oxygen in the body. It must be distinguished from anoxia, the almost total absence of oxygen to sustain life. The hypoxic condition can be maintained for several hours while anoxia is fatal in a matter of minutes . Because of the importance of pressure, both situations cannot occur without hypobaria (reduced pressure) and abaria (no pressure) also appearing.
Hypoxia is related to partial pressure. As altitude increases, the partial pressure of oxygen decreases. As the density of air also decreases, there are fewer oxygen molecules occupying the same amount of volume. This means that a human being breathing in these conditions will inhale fewer oxygen molecules (low density) and with a much lower amount of available energy (low partial pressure).
Hypoxia is one of the most insidious dangers that we can find in space because the victim is not aware of what is happening. The first part of the body that is affected by it is the most important, the brain. Although it only has 2% of the total mass, it consumes 25% of the inhaled oxygen. Interestingly, the early symptoms of hypoxia are deceptively similar to mild alcohol intoxication . Also, important things stop seeming important because hypoxia has a calming effect. Motor control and coordination are next to fall. The person becomes clumsy without being aware of it. Hypoxia causes feelings of well-being, numbness, carelessness, and a false sense of security. The last thing a person in these conditions thinks is that they need additional oxygen.
An example of what happens is in smokers, since nicotine has a greater affinity for red blood cells than oxygen. Thus, the smoker subjects his brain and body to a weak form of hypoxia: simply by smoking a cigarette a person automatically places his brain at an altitude of 2,700 meters .
If you do not fly in pressurized cabins, you need to use oxygen cylinders and masks. The problems begin to appear at 5,000 meters , halfway through space. At 10,000 meters the partial pressure of oxygen is equal to that of CO 2 in the lungs, which means that oxygen does not have enough energy to replace it and must be supplied under pressure. Under these conditions, CO 2 is exhaled against the pressure of the environment, contrary to what happens on the Earth’s surface, which makes breathing an extremely tiring act.
At altitudes above 13,000 meters the differential pressure between the oxygen in the pressure breathing system and the ambient pressure is too great and makes it impossible for a person to exhale : pressure breathing with a mask above that height is impossible. Therefore, the entire body and its surroundings must be pressurized by creating an artificial atmosphere (which happens on the International Space Station) or by applying physical pressure to the entire surface of the body (when putting on the astronaut suit).
