Conway (1985) described the geometry of the Mapleson classification and the behaviour of the breathing systems under conditions of spontaneous and controlled respiration. The Mapleson A (Magill) system (Figure 48) has the fresh gas inlet remote from the subject while the expiratory valve is near the subject. Dead space is shown as the shaded area. This makes the system particularly economic to use during spontaneous respiration.

Figure 49 shows the hypothetical pressure and flow inside the Mapleson A system during expiration. Flow during expiration, which is usually a passive process, is maximal at the beginning of expiration, and it falls off exponentially. Pressure inside the system rises exponentially but at a faster rate than the expiratory flow falls because of accumulation of fresh gas in the system; this rise in pressure is cut off when the expiratory valve opens at 5–6 kPa and this pressure is then maintained until end-expiration. Areas under the flow curve give expired volumes (see the chapter on flow and volume measurement); dead space gas (VD), which is identical in composition to fresh gas is expired first until point A on the graph. When the expiratory valve opens at point B, alveolar gas (VA) beyond that point is vented out. Furthermore, alveolar gas that was deposited inside the system between points A and B (start of alveolar gas expiration and opening of expiratory valve) is after the opening of expiratory valve being pushed out by the fresh gas inflow (VF).