(1) The lung consists of many alveoli, surrounded by abundant capillaries and elastic fibers. The walls of alveoli and capillaries are very thin, with only one layer of epithelial cells. These characteristics are conducive to gas exchange, so the lung is the main place for gas exchange.

(2) According to the analysis, the respiratory system consists of respiratory tract and lung.

(3) According to the analysis, the respiratory tract includes: ① nasal cavity, ② pharynx, ③ larynx, ④ trachea and ⑤ bronchus.

(4) According to the analysis, pharynx is both a respiratory channel and a digestive channel. The air and food inhaled by the nose will pass through the pharynx, then the air will enter the trachea and the food will enter the esophagus. When swallowing, epiglottis will cover the trachea and apnea will occur. After swallowing, epiglottis will quickly return to its original position and breathe as usual. Joking while eating, if it is too late to cover epiglottis when swallowing, food will enter the trachea and cause severe cough.

So the answer is: (1) gas exchange; (2) respiratory tract; Lung; (3)①②③④⑤; (4) pharynx; trachea