Atmospheric Gas Composition The earth's atmosphere is a gaseous mixture consisting of about, 78% molecular nitrogen (N2) 20.9 % molecular oxygen (O2) 1.0 % argon (Ar) 0.03% carbon dioxide (CO2) Other gases also detectable, but only in trace amounts The total pressure (i.e., barometric pressure; PB) exerted by a mixture of gases is equal to the sum of the individual partial pressures of the gases comprising the mixture (i.e. the pressure of each gas in proportion to its concentration), independent of other gases (Dalton's Law). Each gas exerts its own partial pressure denoted as P(gas). For the earth's atmosphere, at a PB = 760 mmHg (1 atmosphere) DRY AIR FULLY SATURATED AT 37oC Partial Pressure of N2 (pN2) = 592.8 mmHg Partial Pressure of N2 (pN2) = 556.1 mmHg Partial Pressure of O2 (pO2) = 158.8 mmHg Partial Pressure of O2 (pO2) = 149.0 mmHg Partial Pressure of CO2 (pCO2) = 0.23 mmHg Partial Pressure of CO2 (pCO2) = 0.21 mmHg Partial Pressure of other gases = 8.17 mmHg Partial Pressure of other gases = 7.69 mmHg Partial Pressure of H2O (pH2O) = 0 mmHg Partial Pressure of H2O (pH2O) = 47 mmHg Total 760 mmHg Total 760 mmHg This principle applies in the breathing process as both oxygen and carbon dioxide can diffuse in opposite directions in the same place, down their respective pressure gradient, i.e. from an area of higher pressure to an area of lower pressure.