Atypical air sample in the lungs contains oxygen at 100 mmhg, nitrogen at 573 mmhg, carbon dioxide at 40 mmhg, and water vapor at 47 mmhg. part a why are these pressures called partial pressures

  it  is  called the  partial pressure   because    it  is    pressure  that   is  exerted by  each gas in  a mixture of  gases if it occupy  the same   volume  of  it  own.  the  the partial pressure  of oxygen  in the air sample above is  100 mm hg, that of  carbon dioxide is 40 mmhg, for   water vapor  47 mmhg. and that of  nitrogen  is 573 mmhg