Alveoli is a thin, irregular walled and vascularized bag-like structure. It is main site for diffusion of gases. Terminal bronchioles give rise to alveoli. Lungs without alveoli is a dead lung as y play a major role in respiration.

Partial pressure of oxygen in atmospheric air is 159 mm Hg and in alveoli is 104 mm Hg. Partial pressure of carbon dioxide in atmospheric air is 0.3 mm Hg and in alveoli is 40 mm Hg.

Emphysema is a chronic disorder which is marked of damage of alveolar walls. This leads to inadequate supply of oxygen to body parts. The most common cause found so far is cigarette smoking.

Pnuemocyte type-I help in gaseous exchange whereas Pnuemocyte type-II secretes lecithin, which act as surfactant and prevents collapsing of alveoli by reducing surface tension.

Larynx is known as soundbox of our body system. It is made of cartilage and helps in production of sound, hence name soundbox. The larynx is guarded by epiglottis. Also, larynx helps in regulation of our pitch and volume.

Nearly 70% carbon dioxide is transported by plasma as sodium bicarbonate. Approximately, 5-7% of carbon dioxide is transported in dissolves form in plasma. About 20-25% carbon dioxide react with amine group of haemoglobin and form carbamino-haemoglobin.

At time of inspiration diaphgram contracts leading to increase in thoracic volume in anterior posterior axis. Whereas movement of sternum causes increment in antero-ventral direction.

Diffusing capacity depends on solubility of gases, thickness of respiratory membrane, partial pressure difference. These all factors play an important role in exchange of gases through process of simple diffusion.

Normal expiration is a passive process as during expiration, contraction (in any muscle) does not take place. Thus, re is no expenditure of energy. Relaxation in diaphgram and external intercoastal muscles is found.

Trachea is a straight tube extending up to anterior part of thoracic cavity. It is also known as windpipe. The trachea connects soundbox to bronchi. Also, it provides passage for air.