The respiratory system functions to provide oxygen to the blood and remove carbon dioxide. Air enters and exits through the nose, which contains nasal cavities for smell and mucus-secreting cells. The mucosa moistens and warms inhaled air to match body temperature before reaching the lungs. Smoking tobacco is an addictive disease characterized by nicotine dependence and can cause diseases like dry cough, bronchitis, pneumonia, and esophagitis by depositing tar in the lungs. Young people are particularly at risk of smoking addiction due to high smoking rates starting in early teens.