oxygen
Meaning of oxygen: Carrying it is one job and delivering it is another, which is why a normal saturation reading can sit above starving tissue.
Definition of oxygen
Oxygen is a colorless, odorless gas making up about a fifth of the air, and nearly every animal and plant cell needs it to release energy from food. Breathed in, it crosses from the air sacs of the lungs into the blood, binds to hemoglobin inside red cells, and is carried to every tissue in the body.
How does oxygen get from the air to a cell?
Air drawn into the lungs reaches millions of thin walled sacs wrapped in capillaries. Oxygen crosses that barrier by simple diffusion, moving from the higher pressure in the air sac to the lower pressure in the blood, and nothing has to pump it. Once in the blood, almost all of it binds to hemoglobin inside red cells, with only a small fraction dissolved in the plasma. Hemoglobin then releases its cargo where conditions call for it, since working tissue is warmer, more acidic and richer in carbon dioxide, and all three loosen its grip. Inside the cell, oxygen sits at the end of the chain of reactions that extracts energy from food. Without it that chain stalls within minutes, which is why heart muscle and brain tissue fail first.
What does an oxygen reading actually measure?
A pulse oximeter clipped to a finger shines light through the tissue and calculates the percentage of hemoglobin currently carrying oxygen, the figure reported by an oxygen saturation test. That percentage is not the same as the amount of oxygen reaching tissue. Delivery also depends on how much hemoglobin is present and on how well blood is flowing, so someone severely anemic can be fully saturated and still short of oxygen where it counts. Cold hands, poor circulation, movement, nail varnish and darker skin pigmentation all affect the accuracy of the reading. Carbon monoxide poisoning is the most deceptive case, because the meter counts bound carbon monoxide as though it were oxygen and returns a reassuring number.
When is extra oxygen given?
Supplemental oxygen therapy is used when the blood is carrying less than the body needs, whether from lung disease, heart failure, severe infection or blood loss. It is handled as a drug rather than as fresh air: the amount is chosen deliberately, too much is harmful in specific conditions, prolonged high concentrations injure lung tissue directly, and the gas makes any nearby fire burn fiercely.
| Delivery | What it provides |
|---|---|
| Nasal tubing | A small added amount mixed with room air, while eating and talking continue |
| Face mask | A moderate amount across a wider range of need |
| Mask with a reservoir bag | A high concentration, generally for short term use |
| High flow nasal system | Warmed, humidified gas at rates matching or exceeding the effort of breathing |
| Ventilator | A controlled concentration delivered with mechanical support of breathing |
Used in a sentence
Supplemental oxygen was provided after the patient's oxygen saturation was found to be lower than expected.
Good to know
Oxygen saturation reflects the proportion of hemoglobin carrying oxygen, which is related to but distinct from how much oxygen is actually being delivered to tissues, since delivery also depends on blood flow and total hemoglobin present.
Word origin
Where “oxygen” comes from
Coined from Greek oxys, meaning sharp or acid, and -gen, from gennan, meaning to produce, reflecting an early, later disproven belief that oxygen was essential to the formation of all acids.