Speculative scenario where human beings have synthetically engineered their organs to adapt to an oxygen-depleted atmosphere, as deforestation, desertification and population over-growth are contributing to a fall in the long-term loss of oxygen sources. The lowest concentration of oxygen recorded in a populated area is 12%.
In this scenario, lungs are reduced in size and allow inhalation for speech and smell only. The olfactory bulb (an area in the forebrain involved in sensing smell) and nerves compensate for the small lungs. In the urinary system, blood capacity increases in the kidneys. The ureters have increased in size and becomes host to a bacterium – Candidatus Methylomirabilis oxyfera – that produces oxygen from nitrite, a compound produced from nitrate which is found in urine. Carbon dioxide is expelled in the urine.