Why do we need oxygen?
Oxygen is vital for life-sustaining aerobic respiration in humans and animals and is arguably the most important ‘nutrient’ for survival. When deprived of food and water, we can probably survive for a few days, or even up to a few weeks. However, without oxygen, a human being can only live for a few minutes. Oxygen is a nutrient closely related to production of energy through a process that occurs inside the cell mitochondria. Mitochondria are tiny power generators inside your cells that convert the glucose and fats from our food into adenosine triphosphate (ATP) - the molecule that supplies energy to all active cell processes. Within the mitochondrial inner membrane, oxygen acts as the terminal electron acceptor at the end of a series of biochemical processes (the oxidative electron transport chain) which results in the synthesis of ATP. By-products of the oxidative electron transport chain are water and carbon dioxide. In other words, in the presence of oxygen, glucose from the food you eat is ‘burned’ in the mitochondria to produce lots of energy (ATP), with water and carbon dioxide as harmless by-products. You can consider this as a form of ‘clean’ energy process, without producing toxic wastes.
Normal ways that oxygen gets to your cells
The air we breathe contains 21% oxygen. With each breath, the air travels into small airspaces in the lungs called alveoli, where oxygen gets transferred across a thin membrane to the blood carried in tiny blood vessels that wrap around the alveoli.
How is oxygen carried in blood?
Oxygen is not very soluble in plasma (water part of blood) when the air pressure is normal (at sea level). As a result, the amount of oxygen carried in the plasma as ‘dissolved oxygen’ is normally very low. Most of the oxygen is therefore carried inside the red blood cells, bound to the red blood pigment called hemoglobin. If your lungs are functioning normally, all the hemoglobin in your blood cells are saturated with oxygen, while the plasma has very little oxygen dissolved in it. Only when the atmospheric pressure increases can there be more oxygen dissolved in plasma.
Delivering oxygen to the cells
The oxygen carried in the red blood cells travel all over the body and is supplied to tissue cells. In order for a cell to receive ‘oxygen’ from the hemoglobin, it has to give up carbon dioxide, a gas that is produced when glucose is ‘burned for energy’ by the oxidative electron transport chain mentioned earlier. This is because the carbon dioxide needs to displace oxygen on the hemoglobin, somewhat like kicking someone off a seat so that you can take his seat.. The actual mechanism of this exchange process acts through changes in acidity produced by carbon dioxide and is known as the Bohr Effect.
How does oxygen exist inside your cells?
Oxygen must first be dissolved in water before it can enter into our body cells. This is because our cells have very high water content, about 60%, and all the cellular activities that require oxygen for energy production occur in this watery medium. Oxygen must be transported through the watery interior of the cell into the watery interior of mitochondria before the oxygen can be used to make energy (ATP). As you have probably figured out by now, dissolved oxygen is very important for cell utilization.