This figure demonstrates the principle of heat and humidification where inspired gas is warmed and humidified via a heat moisture exchanger and expired gas cooled and dehumidified.
RH: relative humidity.
Modified from: Humidification and the Ventilator Circuit. In: Essentials of Mechanical Ventilation, 3rd ed, Hess DR, Kacmarek RM (Eds), McGraw-Hill Education, 2014.