how are the lungs designed in human beings to maximise the area for exchange of gases

Respiration in human beings takes place through a special system called as respiratory system.This involves nostrils, nasal cavity, larynx, bronchi, bronchioles ending in alveoli inside the lungs. Lungs are designed in such a way that they are lined by a thin membrane to facilitate the exchange of gases. 
Bronchi and bronchioles are the parts of respiratory system located in the lungs. 

--Primary bronchi are the structures which arise by the bifurcation of the wind pipe before entering the lungs. Primary bronchi divide and give rise to secondary bronchi. Bronchioles are the small tubular extensions of repeatedly dividing secondary bronchi inside the lung.

--Bronchi end in bronchioles. Each bronchi divides at least 20 times to form smaller bronchioles    Bronchioles end in small tubular structures called as alveolar tubes.Bronchi are made up of fibrocartilaginous layer. Bronchi serve as tubes of air conductors. Bronchi are larger in diameter  

--Bronchioles branch into many alveolar tubes. Bronchioles are lined by smooth muscles. Bronchioles are air conductors and others are respiratory gas exchangers. Bronchioles are smaller in diameter.
--Alveoli are richly supplied by blood vessels which help in exchange of gases on the alveolar membrane.