The Universal F-circuit is a rebreathing circuit that can be used on small animals up to 200 lbs. Rebreathing circuits allow the animal to rebreathe exhaled and excess vaporized anesthetic agent which have circulated through a CO2 absorber system thus reducing the amount of anesthetic agent used. The unique single limb circuit 1) replaces the need for a traditional adult and pediatric circuit, 2) reduces the clutter around the animal and 3) takes the weight off the end of the endotracheal tube. The tube within a tube design allows heat and moisture to be retained in the circuit, which helps to maintain body temperature and recover the patient more quickly.