Anaes · Applied cardiovascular & respiratory physiology

Lung volumes & capacities

Also known as Tidal volume · Functional residual capacity · Closing capacity · Vital capacity · Dead space · Spirometry

The lung holds characteristic volumes of air at each phase of breathing, and the way those volumes change with position, anaesthesia and disease explains both gas exchange and atelectasis. The framework rests on five exam-critical ideas: the four lung volumes (tidal volume, inspiratory reserve, expiratory reserve, residual volume) combine into four capacities (inspiratory capacity, functional residual capacity, vital capacity, total lung capacity) (standard teaching; volume/capacity numbers are textbook furniture, not in any retrieved abstract); the functional residual capacity is the oxygen reservoir that buffers against hypoxaemia during apnoea (recumbency lowers FRC and promotes dependent airway closure [1]); closing capacity rises with age and, when it exceeds functional residual capacity, small airways close during tidal breathing causing atelectasis and shunt (dependent-closure mechanism [1]; propofol sedation lets closing volume exceed FRC [6]); dead space (anatomical, alveolar and physiological) is ventilated but non-exchanging volume (standard teaching; no retrieved abstract studies the Bohr equation); and anaesthesia and the supine position lower FRC, bringing closing capacity closer and predisposing to atelectasis [1]. Built on the patient-positioning review (Harris 2026), the high-flow nasal preoxygenation simulation study (Bouvet 2026), the China pulmonary-function-testing pathways consensus with Delphi 14-question scope (CACP Task Force 2026), the Global Lung Initiative 2022 reference-equation applicability study in Jordanian adolescents (Al-Qerem 2026), the university-student spirometry study (Tabbah 2026), and the paediatric non-invasive-ventilation distribution study (Chidini 2025).