<prompt> Aerial blood gas of a 5-year-old child done at sees level gives the following results: pH 7.41, PaO2 100 mgHg, and PaCO2 40 mm Hg. The child is being ventilated with 80 oxygen. What is the A-a PO2?570.4 mm Hg520.4 mm Hg470.4 mm Hg420 mm Hg  <response>: The alveolar gas equation is a formula used to approximate the paial pressure of oxygen in the alveolus PAO2:PAO2PB-PH2OFiO2-PaCO2Rwhere PB is the barometric pressure, PH2O is the water vapor pressure usually 47mmHg, FiO2 is the fractional concentration of inspired oxygen, and R is the gas exchange ratio. Ref: guyton and hall textbook of medical physiology 12 edition page number:331,332,333 