A ventilator comprising: (a) a housing having an interior and an exterior ; (b) an inlet port extending from the exterior to the interior of the housing; (c) a flow generator disposed within the housing and being structured to geerate a flow of gas; (d) an outlet port adapted to discharge the flow of gas from the housing; (e) a patient circuit including a patient interface and a passive exhalation device , wherein the passive exhalation device is adapted to provide a flow of exhalation gas from the patient circuit to an ambient atmosphere, wherein the patient circuit is in fluid communication with the outlet port and is structured so as to deliver a predetermined inhalation volume of the flow of gas to an airway of a patient during an inspiratory phase of a ventilatory cycle; (f) a controller operatively coupled to the flow generator, wherein the controller is adapted to selectively control the inhalation volume of the flow of gas delivered to the patient, taking into account the exhaust gas flow lost during inspiration from the passive exhalation device, to provide volume control ventilation therapy; (g) a plurality of probes disposed in the housing; and (h) an interchangeable porting block (102, 102', 102") adapted to couple to the housing , wherein the interchangeable porting block (102, 102', 102") has an arrangement of passageways (104, 106, 108, 110, 112, 114, 104', 106', 108', 110', 112', 114', 104", 106", 108", 110", 112", 114") that are structured to selectively cooperate with the plurality of probes in a predetermined manner to configure the ventilator to operate in a predetermined mode. (a) a housing having an interior and an exterior ; (b) an inlet port extending from the exterior to the interior of the housing; (c) a flow generator disposed within the housing and being structured to geerate a flow of gas; (d) an outlet port adapted to discharge the flow of gas from the housing; (e) a patient circuit including a patient interface and a passive exhalation device , wherein the passive exhalation device is adapted to provide a flow of exhalation gas from the patient circuit to an ambient atmosphere, wherein the patient circuit is in fluid communication with the outlet port and is structured so as to deliver a predetermined inhalation volume of the flow of gas to an airway of a patient during an inspiratory phase of a ventilatory cycle; (f) a controller operatively coupled to the flow generator, wherein the controller is adapted to selectively control the inhalation volume of the flow of gas delivered to the patient, taking into account the exhaust gas flow lost during inspiration from the passive exhalation device, to provide volume control ventilation therapy; (g) a plurality of probes disposed in the housing; and (h) an interchangeable porting block (102, 102', 102") adapted to couple to the housing , wherein the interchangeable porting block (102, 102', 102") has an arrangement of passageways (104, 106, 108, 110, 112, 114, 104', 106', 108', 110', 112', 114', 104", 106", 108", 110", 112", 114") that are structured to selectively cooperate with the plurality of probes in a predetermined manner to configure the ventilator to operate in a predetermined mode. <EOS>