The lungs are the largest and main organs of the respiratory system. Every person has one right and one left lung. They are located on both sides of the mediastinum in the thorax. The right lung is situated on the right side of the heart and the mediastinum, while the left lung is on the left side.

Inferior to the right lung below the diaphragm lies the liver. It causes the right dome of the diaphragm to be higher, and therefore, the right lung is shorter than the left lung. The right lung also appears wider than the left, as the left lung has a more prominent cardiac impression created by the heart. These anatomical differences cause the right lung to weigh more, and it also has a greater capacity.

The main function of the lungs is to provide the external and internal respiration processes. 

• During external respiration, the lungs fill up with air inhaled from the environment. The gas exchange happens at the smallest structural and functional units of the respiratory system - alveoli. It occurs between the pulmonary capillaries containing carbon dioxide and air in the alveoli rich in oxygen. Once the exchange has occurred, the air now rich in carbon dioxide is released back into the external environment by exhalation. This entire process is also known as breathing. 
• During internal respiration, the gas exchange happens at the level of the body capillaries between blood that contains oxygen and body cells rich in carbon dioxide.