Firefighters Resuscitate A Dog By Mouth-to-snout Insufflation, Wasau, Wisconsin, United States

Normal atmospheric air contains approximately 21% oxygen when created in. After gaseous exchange has taken place in the lungs, with waste products (notably carbon dioxide) moved from the bloodstream to the lungs, the air being exhaled by humans normally contains around 17% oxygen. This means that the human body utilises only around 19% of the oxygen inhaled, leaving over 80% of the oxygen available in the exhalatory breath.
This means that there is more than enough residual oxygen to be used in the lungs of the patient, which then crosses the cell membrane to form oxyhemoglobin.