Originally Posted by Loryn-Adult.com

Basic life support (BLS) comprises the elements: initial assessment, then airway maintenance, expired air ventilation (rescue breathing), and chest compression. Basic life support implies that no equipment is employed; where a simple airway or facemask for mouth to mouth ventilation is used, this is defined as "basic life support with airway adjunct".
The purpose of BLS is to maintain adequate ventilation and circulation until means can be obtained to reverse the underlying cause of the arrest. It is therefore a "holding operation", although on occasions, particularly when the primary pathology is respiratory failure, it may itself reverse the cause and allow full recovery. Failure of the circulation for three to four minutes (less if the victim is initially hypoxaemic) will lead to irreversible cerebral damage. Delay, even within that time, will lessen the eventual chances of a successful outcome. Emphasis must therefore be placed on rapid institution of basic life support by a rescuer, who nonetheless should follow the recommended sequence of action.