There is more oxygen in the air.
Inhaled air is richer in oxygen than exhaled air.
For seawater about 10ppb.
Inhaled air contains more oxygen than exhaled air because the cells have not yet used that oxyginated air.
Oxygen consternation is the most important for patient. Oxygen concentrate provides oxygen to the patients who require more oxygen than normal air
Air only has about 20% oxygen, almost all of the rest is nitrogen. An oxygen tank has 100% oxygen and is used in some cases for breathing sicknesses. These patients don't need 100% oxygen, but just a bit more that is mixed with normal air when they breath in.
Seawater is more dense than air. That is why the air rests on top of the seawater and not the other way around.
In modern submarines, the electrolyser separates oxygen from hydrogen in seawater to produce breathable oxygen. There is also a scrubber that removes excess CO2 from the air.
Strangely enough, oceans are oxygen neutral in transferring to / from the air. Oxygen is liberated from water by plankton, and consumed by the various animal cells. There is some minor oxygen exchange with rainfall, wave breaking near shores / shoals, and some carried into the ocean by rivers, etc. Oceans *do*absorb large quantities of carbon dioxide directly from the air... which really hurts the coral reefs.
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Fish and marine life are able to live because oxygen is soluble in water, including salt water/seawater.
Seawater and fresh water contains oxygen that fish absorb using their gill. So the substance is oxygen.
Inhaled air is richer in oxygen than exhaled air.
For seawater about 10ppb.