Pneumonia is considered a dangerous disease because,
(i) It affects the respiratory system and causes shortness of breath. In severe cases it affects the oxygen, carbon dioxide exchange in the alveoli. This may lead to hypoxia (decrease of oxygen). Finally death also can happen.
(ii) Pneumonia causes cough and spread of infectious organism in air. This will lead to infection of other people nearby. Thus it will spread easily and affect many.

Body makes long term adjustments when living in high altitude because in high altitudes the O₂ content is low in atmosphere and by increasing more production of RBCs the adjustments of respiration are brought.

(i) Entry of air with high pressure may collapse the tracheal tree.
(ii) The smooth muscles which contract and relax to adjust the diameter of air passage with a typical resistance against airways under normal pressure.

(i) A thin elastic flap called epiglottis is found in the upper respiratory tract.
(ii) It prevents the food from entering into the larynx.
(iii) It avoids choking of food.

Conditions which create the problems in O₂ transport:
(i) Poor ventilation.
(ii) Less quantity of haemoglobin.
(iii) Anaemia
(iv) Histotoxic conditions of tissue fluid.
(v) Inability for the transport of O₂ into cell due to cellular disorder.

(i) The alveolar regions of lungs are the centres of gaseous exchange for the association of Hb and O₂ to form HbO₂ and for the dissociation of HbCO₂ into Hb and CO₂.
(ii) But visceral regions of gaseous exchange constitute tissue fluids where dissociation of HbO₂ into Hb and O₂ and association of Hb and CO₂ into HbCO₂ occurs.

External nostrils → Nasal cavity → Internal nostrils → Nasopharynx → Larynx → Trachea → Bronchi → Bronchioles → Hilum → Lungs → Bronchioles → Alveoli