Normal air is composed of about 21% oxygen, 78% nitrogen, 0.04% carbon dioxide gas and smaller percentages of other gases and water vapour. This means that the oxygen gas constitutes 21% of the air pressure. The partial pressure of oxygen (pO2), i.e. the pressure O2 creates alone is 21% of 760 mm Hg, i.e. 159mm Hg. Similarly, the partial pressure of nitrogen (pN2) is 597mm Hg, and the partial pressure of carbon dioxide gas (pCO2) is 0.3 mmHg.