Oxygen in air and water: In air we breathe, in water we drown. Why?

So many bad answers to such a simple question.

Water in rivers, lakes, and seas contains a little oxygen gas (O2) dissolved in the water. The membranes in gills can extract that dissolved oxygen and provide it to the organism. But mostly because of the brain, which uses a lot of energy, the human metabolic rate is far too high to be sustained by the amount of dissolved oxygen gills could extract.

The oxygen in the water molecules is bound to the hydrogen by very strong covalent chemical bonds. Even if an animal could use metabolic energy to break the bonds, the best it could do is break even when the body later uses the oxygen in its metabolic processes to provide energy to the body. So we let sunlight do that bond-breaking work for us, and we use plants as the agent and photosynthesis as the process. But there it's carbon doixide that's broken to release oxygen.

In air, the oxygen and other gases are simply mixed together. Water is not a mixture of oxygen and hydrogen gas, the oxygen is chemically bonded to the hydrogen. It takes a fair amount of energy to break the bond, which our lungs can't do. And liquid water is a lot denser than air, so our lungs and muscles we need to breathe can't handle moving the water.

There is some dissolved oxygen gas in water (which is what fish use), but even if our lungs could handle being filled with water, the oxygen content is much lower than in air.

No. It has a lot to do with water being a liquid and our lungs being unable to extract the oxygen from water where they are ideal for extracting oxygen from a gas. You are right about hydrogen being lighter than nitrogen but you forget that water also contains oxygen which is heavier than nitrogen.

Its because in order to take in the air from the water we would have to filter the water out like fish do and keep the oxygen but our bodies don't have a filter system so we drown instead.

wateris NOT 33% O2

there is some dissolved O2 in water but the O@ concentration is about 20% in air, much higher and lungs of all land animals have adapted to taking in the air mix and capturing the O2 in our blood cells, carrying it around the body and relaeasing some into the body cells which live using it . thecells ecrecte CO2 which goes backin theblood and is lungs and is exhaled.

it is the build up of CO2 in the blood which triggers the desperate need to breathe

not the lack of O2

You left out the simple fact that water is a liquid, obviously we drown in water since when we open our mouths in water the water floods in and fills our lungs and our lungs do not have the strength to blow it out.

It was a good thought, but never leave out the common since, I've made similar mistakes.

Both air and water have different chemical formulas (there is a different structure and amount of atoms making up each molecule of air and water) and even with just the slightest change in the chemical formula, it will completely change the substance.

Our lungs can only use oxygen in it's gaseous form as O2. Water is well... a liquid,

I'm no scientists but my guess would be that our lungs can't get the oxygen out of the water? They evolved for filtering oxygen out of air not out of water... dunno if you were looking for a different/better answer but tha's my opinion...

Because we don't have gills...