The respiratory system allows oxygen to enter the body and carbon dioxide to exit the body. It includes the nose, nose hair, lungs, windpipe and mouth. The nose and mouth allow air to enter where it then travels through the windpipe into the lungs. The lungs supply oxygen to the blood and remove carbon dioxide, although diseases, allergies or infections can sometimes impair lung function. Key elements of the respiratory system include the nose for smelling and breathing, lungs for oxygenating blood, mouth for eating and the windpipe for carrying air to the lungs.