Respiration: ventilation

The replacement of alveolar gases requires continual respiratory movements. Inspiration is active and expiration is normally passive. The lungs and chest wall recoil in opposite directions; the lungs inwards and the chest wall outwards. The recoil forces are due to the elastic properties of cartilage and muscles (chest wall) and to surface tension (lungs). The intrapleural pressure is slightly subatmospheric at functional residual capacity and becomes more negative after inspiration. During inspiration, intrapleural pressure is more negative than indicated by compliance forces alone, and during expiration, less negative. These differences reflect the extra forces required to move air through airways that have a resistance to airflow and to distort the tissues of the lung. They represent total pulmonary resistance. Work done during inspiration is required to overcome compliance and resistance forces. During expiration, the work released by the recoil of the lungs and chest wall is normally greater than that required to overcome pulmonary resistance, so expiration is passive. However, during airway constriction or rapid breathing, the forces required to produce airflow are increased and expiration becomes active. There is a gradient of intrapleural pressure from the apex to the base of the lung in an upright individual, the pressure at the apex being more subatmospheric. This means that the apical alveoli are on different regions of the compliance line. Since the compliance line of the lung is slightly curved, the fall in intrapleural pressure during inspiration causes the alveoli at the apex of the lung to be expanded less than those at the base.