Question

Why we says that our body take only oxygen bt we all know there are many number of gases present in the atmosphere?

Answer 1

1We say that because we only need the oxygen for metabolism, creating the energy by burning food.

There are two factors we need to control. One is the partial pressure of the oxygen and the other is the effect of the other gas we mix it with.

(Partial pressure is the important term, it is the contribution of a gas to the total pressure that matters) .

All gases pass in to your bloodstream, but most of the oxygen under normal circumstances is intercepted in the lungs and bound to hemoglobin blood cells.

The role of nitrogen is to reduce the partial pressure of oxygen, too much oxygen will cause tissue damage, called oxygen poisoning. In fact 21% oxygen is more than we need, we can go down to much less, in some computer rooms you can breathe at 15% oxygen but a match will not ignite.

Too high a partial pressure for the diluting gas can cause narcosis, the companion gas to oxygen can be any gas with low lipid solubility. Too high a partial pressure of the companion gas and it will clog up cell membranes causing life threatening narcosis. Alternatives to nitrogen are helium, argon, and even hydrogen for deep sea diving.

Not to be confused with decompression sickness, that is when bubbles of nitrogen form, caused by breathing higher than normal nitrogen partial pressure. That in turn is because the oxygen ratio is lowered to 10% by increasing the companion gas, to compensate for the pressure increase when diving.

Some reasonably common gases are entirely unsuitable to breathe of course. Carbon monoxide gas is poisonous not so much because if it's effect in tissues, but because it competes with oxygen for hemoglobin sites, and forms a stable carboxyheamoglobin complex. You don't get oxygen. A gas like hydrogen sulphide is heavy and can build up, it is smelly but it deadens your sense of smell. It also blocks off oxygen but it does it at the cellular level.

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