I am a high school student and I am a little confused in the concept of breathing:
My confusion is that, when we talk of exchange of gases we say that gases always diffuse from their high "PARTIAL PRESSURE" to their "LOW PARTIAL PRESSURE" and we know that partial pressure of oxygen in our alveoli is about 104mmHg and in our blood vessels it is about 95mmHg ,so we can see that this statement sounds true.
But partial pressure of oxygen in atmospheric air is about 159mmHg and in alveoli it is about 104mmHg then why the air that we exhale also have about 16% of oxygen in it? I think this statement is not correct oxygen don't have brain it will always move from the higher PRESSURE OF TOTAL MIXTURE TOWARDS THE LOWER PRESSURE OF TOTAL MIXTURE, e.g assume a balloon filled with different gases in it and total pressure of gases inside the balloon in much higher than the surrounding, but balloon has very less amount of Oxygen in it as compared to other gases so, its partial pressure is less then the surrounding so of course when you let it go all the air from balloon rushes out even oxygen, and its seems logical as oxygen doesn't know how is it different from other gases , how only oxygen can left inside the balloon?
I think beside this In exchange of gases we would talk in terms of partial pressure because we have to talk only about oxygen and carbon-dioxide ,not all gases, so just to represent their concentrations we talk in terms of partial pressures? But saying this statement that gases always diffuse from their high partial pressure to their low partial pressure this is wrong ,please correct me If I am going wrong somewhere and please give a logical explanation to it.