Drying expiratory limb with tailored temperature profile and multi-lumen configuration

A drying expiratory limb of a breathing circuit, the drying expiratory limb comprising:

• a wall having a first end and a second end separated by an expiratory limb length, the wall defining a space within and wherein at least a part of said wall comprises a breathable material configured to allow transmission of water vapor but substantially prevent transmission of liquid water;
• a first opening in the first end of the wall, the first opening configured to receive a gas at a first temperature and a first relative humidity;
• and
• a second opening in the second end of the wall configured to allow the gas to exit the drying expiratory limb, the gas having a second temperature that is lower than the first temperature and a second relative humidity upon exiting the drying expiratory limb,
• wherein the drying expiratory limb is configured such that a temperature of the gas has a temperature drop along the drying expiratory limb length and the first relative humidity and the second relative humidity are approximately between about 90% and about 99%.