A fluid filled double membranous layer that surrounds the lungs is called pleura, and fluid is pleural fluid in between them. The outer pleural membrane is in close contact with the thoracic lining whereas, the inner pleural membrane is in contact with the lung surface. These collectively reduce friction on lung's surface.

The primary site for the exchange of gases in our body is alveoli. There are 300 millions of alveoli collectively in both the lungs. These alveoli have very thin wall consisting of squamous epithelium. With extensive network of blood capillaries. The presence of blood capillaries in the alveoli, result in easy exchange of gases. Each alveolus is also called as miniature lung.

Emphysema is a chronic disorder of respiratory system, where inflation or abnormal distension of alveolar wall occurs. Cigarette smoking and the inhalation of other smoke or toxic substances over a period of time causes the damaging of septa between the alveoli, and of its elastic tissue is replaced by the connective tissue in lungs. Hence, the respiratory surface decreases, thus causing the emphysema. It causes shortness of breath, production of sputum, chronic bronchitis, etc.

Every 100 mL of oxygenated blood can deliver around 5 mL of $\mathrm{O}_2$ to the tissue under normal physiological conditions.

About $97 \%$ of $\mathrm{O}_2$ is transported by RBCs in the blood. The remaining $3 \%$ of $\mathrm{O}_2$ is carried in a dissolved state through the plasma.