Is it true that during exercise, the oxygen concentration in the blood increases to twice that of resting levels

Yes because once exercising your heart beats faster pumping more blood, so your blood increases along the way twice of “resting” levels when you exercise you’re not at rest so

Musculoskeletal or gastrointestinal

This gel or liquid can be placed under the tongue of an unconscious person. this is important because you should never pour anything down the throat of an unconscious person-- s/he could choke.