The diaphragm at this time moves downward enlarging the chest cavity. Reduced air pressure in the lungs causes air to enter the lungs. Exhaling reverses theses steps. Image from W.H. Freeman and Sinauer Associates, used by permission. The condition of the airways and the pressure difference between the lungs and atmosphere are important factors in the flow of air in and out of lungs.
Many diseases affect the condition of the airways.
Asthma narrows the airways by causing an allergy-induced spasms of surrounding muscles or by clogging the airways with mucus . Bronchitis is an inflammatory response that reduces airflow and is caused by long-term exposure to irritants such as cigarette smoke, air pollutants, or allergens . Cystic fibrosis is a genetic defect that causes excessive mucus production that clogs the airways. The differences between oxygen and carbon dioxide concentrations are measured by partial pressures. The greater the difference in partial pressure the greater the rate of diffusion.