Normal atmospheric pressure in mmhg
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Exposure to 100% oxygen at 2 - 3 atm for less than 5 hours can greatly increase the PaO 2 (~ 2000 mm Hg) and increase tissue PO 2 without toxic side effects (or minimal side effects). If given for prolonged periods, supplemental oxygen at 1 atm of pressure can have severe side-effects, particularly in infants.
Normal atmospheric pressure of air is 150 mm Hg for oxygen (around 610 mm Hg for nitrogen). This equals 760 mm Hg in the atmosphere. In the alveoli, however, pressure is lower because 30% is being used by tissues, leaving 70% of 150 mm Hg oxygen in the alveoli. That means around 100-105 mm Hg is the partial pressure of oxygen in alveoli.