School of Voice Production



Physics of Breathing

Boyle's Law


If gas is kept at a constant temperature, the pressure and volume are in inverse proportion or have a constant product. If air is confined at atmospheric pressure in an airtight container, there will be equal amounts of pressure acting on the outer and inner walls of the container and the differential pressure will be zero. Decreasing the volume of the container will increase the pressure inside with respect to the outside, whereas an increase in the volume of the container will decrease the pressure in the container with respect to the outside. In man, the lungs lie enclosed in the airtight container of the chest cavity (thoracic cavity) and communicate with the outside through the windpipe (trachea) and mouth (oral cavity). The structure of the chest cavity is such that its volume can be made to increase or decrease. Increase in pressure of the chest cavity creates negative pressure in the lungs with respect to the outside (inspiration). Consequently, air will rush into the lungs until the inside and outside pressures are equalised. Decrease in the size (volume) of the chest cavity causes air to rush out of the lungs in the same way (expiration).

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