Close your mouth and nose tightly. See how long you can stay like this without breathing. It is two to three minutes at most, isn't it? Or, imagine breathing a gas other than oxygen. How would your body react to it? How would it impact your health?
The human body is built from the elements found on earth. There are approximately 44 kg of oxygen, 14 kg of carbon, 7 kg of hydrogen, 2.1 kg of nitrogen, 1 kg of calcium, and lesser amount of other elements existing in a 70 kg human body. In fact, many of the 92 naturally occurring elements can be found in the human body, which resembles a small universe. Surely, these elements cannot act chaotically and only fulfill their duties in the organs that are assigned to them in the great mechanism of the human body.
Oxygen is one of these natural elements; under normal natural conditions, it exists in gas and molecular form (O2 and O3). It is abundant throughout the world: nearly 65% of the human body, 88% of the waters on earth, and 46.7% of the earth's crust are made from oxygen atoms (in terms of their mass). Since it is an active element, which makes compounds with many other elements, it is found generally in compound form.
When we say oxygen, the first thing that comes to mind is the oxygen we breathe in the air. The volumetric ratio of atmospheric gases are 78% nitrogen (N2), 20.9% oxygen (O2), 0.93% argon (Ar), 0.04% carbon dioxide (CO2), and lesser amounts of water vapor and other gases.
Oxygen is the most vital requirement for organisms that carry out aerobic respiration. To sustain life, nutrients must be converted into energy in our cells. Therefore, oxygen obtained via respiration must be carried to the cells. The amount of oxygen intake and tolerance time without oxygen varies among living things. Studies have shown that humans can live around 3 to 4 minutes without oxygen; the brain begins to shut down after 5 minutes without oxygen.
Respiration is a wonderful mechanism which is dependent on the nervous system; it is autonomous and usually takes place with ease. The respiration rate for adults is 10-15 times per minute; for children, 20-30; and for infants, 30-40. Respiration can simply be defined as the replacement of carbon dioxide gas in the blood with oxygen. Approximately 400 million alveoli in the lungs are where the oxygen and carbon dioxide exchange takes place. The oxygen that passes into the blood here is transported to cells when it binds to the hemoglobin of the red blood cells. The oxygen, which is captured by the iron (Fe) in the hemoglobin (Hb), is then released for the cells.
Roughly 4/5th of the air inhaled during respiration is nitrogen; 1/5th is oxygen. If we only need oxygen, why do we inhale other gases? It is because the integrity of air plays a critical role in breathing (the oxygen-carbon dioxide exchange). The gas ratio of air before and after respiration is given below.
As seen, while nitrogen and other gases remain constant, oxygen levels decrease, whereas carbon dioxide increases. Only 5% of the inhaled air, or oxygen, is consumed. For our cells to receive the sufficient amount of oxygen they need, the total gas pressure, composition, and oxygen concentration in the air must fit these variables. In addition, water vapor that mixes into gases inside the respiratory track assists with the regulation of gas pressure in our lungs.
The rate of gases in the air is finely tuned. If there is less oxygen in the atmosphere, aside from its many negative effects in nature, it would be insufficient for respiration; the amount of oxygen that passes into the blood to bind with hemoglobin would not be enough for our cells, and the excessive oxygen would also be toxic. For example, mountain climbers use oxygen tanks; as elevation increases, air pressure and oxygen decrease. Excessive oxygen, however, may cause loss of consciousness (coma), nausea, cramps, and visual impairment. Concentrated oxygen can be fatal for cells. According to Australian biologist Michael Denton, each extra 1% above the current 21% ratio of oxygen in the atmosphere would increase the possibility of a lighting based forest fire up to 70%.
Under normal conditions, nitrogen is found in nature in its gas form as an inert element. In other words, it is created so that it only reacts under rare and special circumstances. It does not react with many substances, such as water, air, acids, and bases. Is there any function for nitrogen gas in our respiratory system?
When nitrogen gas enters our body via the air passage, it is exhaled back without getting into any reactions in our lungs, blood, or with oxygen. It does not affect the oxygen we breathe in; however, it plays a role for adjusting oxygen's rate. Its presence is necessary for the respiration of oxygen under certain concentrations, flows, and pressure.
What would happen if any of the inert gases – like helium (He), neon (Ne), argon (Ar), or krypton (Kr) – replaced nitrogen? Not only could these gases not substitute for nitrogen with their physical and chemical features, but they could also not carry out its other functions. For instance, their solubility in blood under pressure is different. Therefore, vital problems such as decompression sickness because of different air and water pressure conditions would be seen more often, especially for pilots and divers. Also, because of different absorption and reflection rates in sunlight, our climate and other species would be affected. Furthermore, the nitrogen cycle takes part in terms of the maintenance of life on earth as outlined by natural laws.
It's plain to see that the balance of elements and gases on our planet, and in the whole universe, have been perfectly balanced to sustain life. It seems remarkable that our planet could be created in such a way as to support so many creatures, who require such a specific balance to survive.