Where lungs form sputum in man?

Sputum is a thick fluid produced in the respiratory tract and expelled by coughing. It is not produced in the lungs, but rather in the bronchial tubes and trachea. When there is inflammation or infection in the respiratory tract, the body produces mucus to trap and remove foreign particles and microorganisms. This mucus, along with other substances such as dead cells and bacteria, forms sputum.