Abstract
Clinical application of anatomical and physiological knowledge of respiratory system improves patient's safety during anaesthesia. It also optimises patient's ventilatory condition and airway patency. Such knowledge has influence on airway management, lung isolation during anaesthesia, management of cases with respiratory disorders, respiratory endoluminal procedures and optimising ventilator strategies in the perioperative period. Understanding of ventilation, perfusion and their relation with each other is important for understanding respiratory physiology. Ventilation to perfusion ratio alters with anaesthesia, body position and with one-lung anaesthesia. Hypoxic pulmonary vasoconstriction, an important safety mechanism, is inhibited by majority of the anaesthetic drugs. Ventilation perfusion mismatch leads to reduced arterial oxygen concentration mainly because of early closure of airway, thus leading to decreased ventilation and atelectasis during anaesthesia. Various anaesthetic drugs alter neuronal control of the breathing and bronchomotor tone.