An anesthesia machine
mixes the anesthetic gas, such as halothane or isoflurane, with oxygen and delivers it to the animal. This combination is breathed in by the animal through the endotracheal tube, which has been placed in the trachea (windpipe). The anesthetic gas moves down the branches of the lungs until it reaches the microscopic alveolar sacs
. The gas transfers from the alveolar sacs to the bloodstream. It then travels in the blood to receptors in the brain where it causes the state of anesthesia.
As the amount of anesthetic is decreased, either to lessen the anesthetic depth during surgery or to waken the animal after surgery, the process reverses. The gas travels from the receptor site, into the bloodstream, passes into the alveolar sacs, and is breathed out of the lungs when the patient exhales.
The depth of anesthesia can be changed quickly during surgery as needed by increasing or decreasing the percent of anesthetic gas that mixes with the oxygen.