Oxygen is extremely important to all the cells of our body. Without oxygen molecules (O2) we will begin to die within three to four minutes, unless our bodies are much cooler than normal, such as, when immersed in cold water. Yet individual oxygen atoms (O) and ozone molecules (O3) within our bodies are classified as free radicals, very dangerous poisons.
Oxygen molecules (O2) are indispensable as a component in producing sufficient energy resources for cellular function; they increase the amount of energy made available to our cells from glucose eightfold. Without O2, our cells through glycolysis produce four adenosine triphosphate (ATP) molecules and two molecules of pyruvic acid, better known as lactic acid. ATP is the "battery" molecule of our cells, they supply the energy for all of the cellular reactions requiring an energy input. The lactic acid lowers the pH within the cell, which can damage the cell, possibly irreversibly, if the cell is deprived of sufficient O2 for too long. With O2, our cells can increase ATP production to 32 molecules per glucose molecule, by breaking down the lactic acid.
But even with sufficient O2, the danger of cellular harm is not over. Oxygen atoms (O) are very rare in the air, but may be released within the cell during that part of the ATP manufacturing process that catabolizes (breaks down) the lactic acid, called the Krebs cycle. Oxygen atoms are extremely reactive, an oxidant, one of the free radicals that can cause severe damage to cells. One of the reasons that anti-oxidants are so important in our diets.
Ozone (O3) is the variety of oxygen that protects us from ultraviolet rays when it is in the upper atmosphere; it forms a layer there that has been making the news for at least the last 30 years because of the holes in it. While very beneficial in the upper atmosphere, ozone is toxic to organisms, so at ground level it's a poison. Ozone is highly reactive just like oxygen atoms, but more common in ground level air, particularly around power plants and factories. It is O3's very reactivity that makes it the shield from ultraviolet rays that it is in the upper atmosphere, and the reason some of our pollutants are thinning and eating holes in it.
Our blood is made up of erythrocytes (red blood cells), leukocytes (white blood cells) and plasma. Plasma is everything besides the blood cells; it is composed of fluids, dissolved gases, nutrients, carbohydrates, proteins, fats and hormones. Oxygen is only relevant to plasma as a potential corrosive substance. Oxygen molecules (O2) are mildly corrosive to blood but mostly bond with hemoglobin, so cause little damage; oxygen atoms and ozone (O3) are very corrosive and extremely damaging if they get into the bloodstream. The oxygen atoms that escape during the Krebs cycle will cause their damage within the cell, so blood plasma is most threatened by external sources of ozone entering through the lungs.
Oxygen is essential to our cells because our cells evolved to expect it. Anaerobic bacteria, for example, are single cell organisms that do not require oxygen. Oxygen is used in the Krebs cycle to greatly increase the amount of ATP that can be produced from a molecule of glucose. But in glycolysis, the first four molecules of ATP are produced without oxygen. Because we evolved with plenty of oxygen available, the oxygen content of air is about 21 percent and all but an extremely small fraction of that is O2, our cells evolved to need the increased amount of ATP available to them through cellular respiration, "breathing" O2 into the cell and carbon dioxide (CO2) out.
They also retain the damaging, acidic derivatives of glucose, the lactic acids, produced at the same time as the first four ATP molecules. When we exercise heavily and our muscles start to burn, it is because of the build up of lactic acids in the muscle cells. These damaging, derivatives of glucose remain within the cell rather than being expelled, until sufficient oxygen has become available to process them through the rest of the energy production cycle, the part requiring oxygen called the Krebs cycle.
Adenosine triphosphate acts like a battery for the cells by storing energy in a chemical bond. Every chemical reaction that requires an energy input within the cell needs one or more molecules of ATP to provide that energy. The "tri" part means it has three molecules of phosphate daisy-chained to the molecule of adenosine. The last chain is a high energy chain that provides energy when it is broken. The ATP becomes adenosine diphosphate (ADP), a single phosphate molecule and energy. The ATP is produced by using the solar energy trapped in glucose by a plant photosynthesizing, to combine ADP and a phosphate molecule.
In nerve and muscle cells particularly, there are very many reactions that require an energy input. The Krebs cycle occurs in a cell organelle called a mitochondrian. The cardiac (heart) muscle cells have the largest number of mitochondria per cell. Despite being about 1/200 of the body mass, 1/20 or five percent of the oxygenated blood leaving the heart through the aorta splits off almost immediately into the cardiac artery to supply oxygen to the cardiac muscle.