Respiration stops in the last stage of expiration, in forced expiration b/c of:

Ans. is 'c' i.e. Dynamic compression of airways. When a forceful expiratory effort is made following changes are seen-In the beginning, when the lung is fully expanded, air flow rate is very high but as the lung volume shrinks (due to continuance of expiration), air flow rate reduces continually.A time, thus comes when despite a maximal expiratory effort, virtually no more air is driven out of the lungs. This volume of lung, at which no air flow from the lung occurs despite effort, is called the closing volume.ReasonA negative intrapleural pressure is required to keep the bronchioles stretched open.As the lung deflates, the intrapleural pressure (Ppl) rises from negative value to 'zero' - this creates a situation when the walls of the bronchioles are no more stretched - bronchodilation ceases - bronchial narrowing - increased air flow resistance - drop of rate of airflow.When the lung volume has sufficiently shrunken, the finest bronchioles are virtually closed - closing volume.