Sputum is respiratory fluid, and when inflammatory lesions occur in the respiratory tract the composition of the fluid changes and sputum is formed. The color of sputum varies in nature, and sputum in the lungs is considered to be caused by lung infection. Once there is sputum in the lungs, you will feel sputum in your throat but have difficulty coughing it up, and the sputum is yellow and sticky. Normally, when you breathe, you feel a whirring in your lungs, and when you wake up in the morning, you cough up phlegm. The doctor can hear wet rales and phlegm sounds on auscultation, or take a film to see what is going on in the lungs to determine if there is a discharge in the lungs.