What causes body permutations in humans

breathing

The first oxygen in water and air

Where does the oxygen in our air come from?

Almost every child knows it: from plants. These produce oxygen and the animals consume it.

Well, today that is still roughly true, apart from the fact that today we can no longer speak of net production. The oxygen produced by the plants is used up again the moment the plants are eaten and digested or burned. That means oxygen production and consumption are balanced.

If you produce a cabinet from the wood of trees, a net production of oxygen remains, but as soon as it is burned or in some other way to CO2 and water is broken down, the oxygen once produced is also used up again.

So where do the huge amounts of oxygen in the air come from that are there today? That is a very long story now. Originally the earth's atmosphere was reducing and did not contain oxygen. Reducing means that there were predominantly gases such as methane, ammonia and hydrogen sulfide in the earth's atmosphere. Sulfur bacteria use the hydrogen sulfide for their photosynthesis. The time came when that gas ran out. What to do? Evolution is inventive. Photosynthesis, which is still carried out by plants today, developed, in which not hydrogen sulfide but water is used. Oxygen is created as a "waste product".

The living beings of that time liked it less because it was extremely poisonous to them. Since there were only single-celled organisms that lived in the oceans at that time, that was not so bad at first, because the oxygen was distributed in the water. In addition, there was a lot of bivalent iron in the sea at that time, which was then oxidized by the oxygen and sank to the sea floor (today's rust formation). Hence the iron stores that are exploited today. But at some point all iron had precipitated and the organisms tasted it less and less because they were now poisoned by the oxygen that had accumulated in the sea. There were still no organisms that could use oxygen positively. A huge environmental catastrophe was looming (although such a catastrophe can of course only be caused by humans! The unicellular organisms didn't know that yet because the "greens" were not yet alive). All organisms now either had to withdraw, died out or they had to protect themselves against the oxygen.

Unfortunately, little is known about the extinct at this time, and no graves have been discovered. Those who protected themselves developed a kind of hemoglobin that bound the oxygen and prevented it from penetrating inside.

It is likely that this substance later developed into the oxygen-binding and transporting hemoglobin. But there are still bacteria today, e.g. the nodule bacteria, which grow on the roots of legumes and fix the nitrogen in the air, which in turn contain the so-called leg hemoglobin, which protects them from contact with oxygen. Initially, the oxygen content of the oceans increased and organisms that were too sensitive or could not develop any protection had no chance of surviving.

After there was enough oxygen in the water, it naturally got into the earth's atmosphere. There the gases, which have a reducing effect, have now been oxidized. In addition, there was also bivalent iron in the ground as it had been in the sea before. This has now been oxidized. This is where the red coloration comes from in many desert areas.

After all components were finally oxidized, oxygen eventually accumulated in the earth's atmosphere. It is estimated that 800-700 million years ago the oxygen content of the atmosphere reached about 10% of today's content. By now there were of course many species in the sea that could use the oxygen for their breathing. So now there was a possibility that these organisms would evolve into land creatures.

It is not easy to say how high the oxygen content was at different times. George, the author of the film series "The desert wins", said that there was a higher oxygen content in the Middle Ages and that the insects were much larger in size. Back then there were dragonflies with a wingspan of 1.5 m. It will not be easy to clarify whether this is true, because this may also have had completely different reasons. I suspect that because of the lack of competition from birds, the insects grew larger (the reason for this is a bit complicated, which is why I won't give it here).

The fact that living beings developed a use of oxygen only slowly shows that they are not adapted to breathing pure oxygen.

Why was (and is) oxygen originally toxic?

Its toxicity is easy to understand. It is a strong oxidizing agent and that means that it oxidizes or "burns", i.e. destroys other substances. So you have to protect yourself against this. Oxygen, for example, still oxidizes hemoglobin in the red blood cells (and of course other substances in the blood). As a result, they could soon no longer be transporting oxygen. But there are enzymes that reverse this oxidation (methemoglobin reductase). If you now breathe pure oxygen, the oxidation of hemoglobin and of course other proteins is promoted to such an extent that they cannot be repaired quickly enough. So you can only breathe pure oxygen for a limited time if you don't want to damage yourself.

A very important indication or proof that the development took place in this form is to be seen in the fact that "biological oxidations" are generally not oxidations with the help of oxygen, but that these oxidations proceed in such a way that a substance that is to be oxidized, two hydrogen atoms are removed, creating a double bond which then reacts with water. After the reaction, the substance then has one more oxygen atom.

Ozone, which is the triatomic form of oxygen, is even more toxic than oxygen. Since this compound is much more unstable, it is much more reactive and therefore highly aggressive oxidizing. In principle, the same damage occurs as with "normal" oxygen, but much faster (ozone warnings on sunny, hot summer days).

On the other hand, we are grateful for this that this substance is present in greater quantities at great heights, because there it absorbs the UV light that could otherwise trigger mutations in our body.

Some quantitative information on oxygen and its toxicity

Under normal conditions, atmospheric air contains 21% by volume of oxygen. A partial pressure of 213 mbar corresponds to this volume fraction. The organism can only adapt to an increased or decreased partial pressure of oxygen to a limited extent. If the oxygen partial pressure drops, then oxygen deficiency (hypoxia) occurs. If the oxygen partial pressure is increased, the oxygen acts as a poison from around 0.5 bar with a time and concentration effect. Oxygen may therefore only be breathed within certain limits under increased partial pressure. If the limit ranges are exceeded, cell metabolic disorders occur, which can lead to the most serious clinical pictures and death. A slow decrease in normal oxygen tension can be compensated to a certain extent by adaptive reactions. By deepening and accelerating breathing, by increasing the hemoglobin content of the red blood cells, by increasing the number of blood cells and the total amount of blood, an adjustment to the decreased O takes place2-Partial pressure. Such processes take place when, for example, people on long high mountain expeditions gradually get used to staying at ever greater heights over days and weeks. The adaptation limit for permanent human residence is at an altitude of around 5500 m.

Will the O2-Partial pressure reduced quickly, e.g. B. during high-altitude flights, balloon rides or ascents with cable cars, the so-called altitude sickness sets in with a large individual fluctuation range at altitudes above 3000 m. The symptoms of altitude sickness, initially a feeling of great well-being (euphoria), concentration and coordination disorders, later headaches, nausea, vomiting, unconsciousness and seizures, suggest that the cells of the brain that are particularly sensitive to lack of oxygen are most severely affected.

Under a partial pressure of 0.5 bar, oxygen can be breathed indefinitely without damage.

If the partial pressure of the oxygen is increased to 0.8 bar, you can breathe for 42 hours without harmful consequences. Breathing pure oxygen under normal atmospheric pressure is safe for 24 hours. If pure oxygen is breathed for longer than 24 hours under these conditions, throat irritation, pain behind the sternum and shortness of breath occur. After 40 hours of pure oxygen breathing at 1 bar, bronchitis, pneumonia and pulmonary edema (fluid accumulation in the alveoli) occur. The clinical picture can take on life-threatening forms.


Created by: Helmut Hupfeld. SBOM with CC BY-SA license provided by: Mario Hupfeld, Konstanz.