What elements should an external/patient circuit contain?
an inspiratory limb though which fresh gas flows to the patient.
During inspiration and exhalation, the valve closes. The exhalation valve can be inside the vent or in the patient circuit on the exhalation limb.
the patient wye ('Y') joins two limbs together at the patient airway.
Other components include pressure manometer, a humidifier, a heater and a temperature monitoring probe, a bacteria filter on the inspiratory line, and usually on the exhalation side as well, a device to measure exhaled volume, and an apnea monitor. A nebulizer may be included.