Give reasons - lungs always contain residual volume of air.

1. Residual volume is the amount of gas remaining in the lungs of the and of a maximal exhalation.The lungs always contain a residual volume of air so that during breathing cycle there is sufficient time for oxygen to be absorbed and for the carbon dioxide to be released. Residual volume is important because it prevents the lungs from collapsing.

2. Harmful substances and germs present in the inhaled air get trapped in the hairs and mucus present in nostrils. In this way, they help in the filtration of inhaled air.

After that, if some germs get escaped from filtration, their movement in the respiratory system is stopped by mucus and cilia present in the air passages. Mucus is secreted by the cells of the trachea and bronchial tubes which keep the air passages moist, clean and stop the entry of germs in lungs.