Question

1) how  lungs play an important role in respiration ?

Answer 1

as they contain bronchi and alveoli which purify air and remove unwanted gases

Answer 2

Lungs are a  part of respiratory system and center for purification of air filled in our body. 
The air that circulates is O$_{2}$ and the other that is given out is CO$_{2}$.
The blood circulates in our body with the help of a pump called as "Heart".
The heart takes in the deoxygenated blood from the body and sends it to lungs through pulmonary artery. 
The pulmonary artery is further transformed to capillaries( this is not a magic but a fact)
We imagine lungs like a pair of balloons but this is not true, rather lungs are like a bunch of grapes. The branches are like bronchus and the grapes i.e. alveoli are hanging upon bronchiole. (for further knowledge see the image below)
The alveoli are wrapped up with cappilaries and this helps in exchange of gas.
The freshly inhaled O$_{2}$ is exchanged with the waste gas i.e. CO$_{2}$ , and exhaled out. 
Now the blood is oxygenated and returns to heart through pulmonary vein.
and is supplied to the body through aorta, for metabolic activities of the body.
At an average an adult person repeats this cycle for about 15-17 times a minute.