I am having a doubt regarding, partial pressure in the lungs and in blood vessels as well.so ,there is Partial pressure of oxygen in the inspired air will be 100mm of Hg and the venous blood had 40mm of Hg,after gaseous exchange the partial pressure of oxygen in blood will 100mm Hg that is equilibrium to Alveolar partial pressure and where as coming to CO2 partial pressure of CO2 in inspired air is 0 in alveoli ,where as venous blood partial pressure of CO2 is 46 mm Hg . how come the equilibrium is 40 mm Hg in PCo2