The correct use of sophisticated devices by the anaesthetist for life support and the monitoring of vital functions require the understanding of several basic laws of physics. By way of example, we examine two extremely important topics: the gas laws, which are fundamental for understanding the physiology of respiratory exchange and inhalation anaesthesia, and fluid dynamics, which is a principle common to cardiovascular and respiratory function. For a more specific analysis of the physical applications, the reader is invited to refer to specialist articles (echocardiography, aortic and thoracic acoustic imaging, neuro-muscular monitoring, infrared spectroscopy, etc.).