Find the volume occupied by 18.0 g of helium gas at 29.0 ∘c and 1.33 atm of pressure.

We can use the ideal gas law equation to find the volume occupied by the gas
PV = nRT
where 
P - pressure - 1.33 atm x 101 325 Pa/atm = 134 762 Pa
V - volume 
n - number of moles - 18.0 g / 4 g/mol = 4.5 mol 
R - universal gas constant - 8.314 Jmol⁻¹K⁻¹
T - temperature in Kelvin - 29 °C + 273 = 302 K
substituting these values in the equation 
134 762 Pa x V = 4.5 mol x 8.314 Jmol⁻¹K⁻¹ x 302 K 
V = 83.8 L
volume occupied by He is 83.8 L 

They carry out the exchange of gases as we breathe ( oxygen and carbon dioxide).