O2 Sensor Function as Relates to Ventilators?

Dear sir/madam

i want to know adout how its o2 sensor work, because we are using 14 types of ventilators here. iam Practically open the o2 sensor, inside, that means bottom of o2 sensor, some chemicals are stored securely and top of the o2 sensor, single multilayer circuit are available. 4 resistors are fixed under the multilayer circuit . nothing connection between chemicals and circuits. 3 pins available on top of the multilayer board .what type of reaction happening between chemicals and circuits