4-24)give reasons for the following

a)Nostrils are lined with mucus

Harmful substances and germs present in 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 trachea and broncheal tubes which keep the air passages moist, clean and stop the entry of germs in lungs.