The present invention comprises a portable ventilator that uses a small, low-inertia, high-speed, high-efficiency Roots-type blower. Roots-type blowers are known for high-efficiency and small size. However, they are inherently noisy. In the past, they have not been suited for use in mechanical ventilators because their excessive noise is disruptive to patients, who often require round-the-clock breathing assistance. The ventilator of the present invention overcomes the noise problems of prior art Roots-type blowers through the combined use of novel noise reducing pressure compensating orifices in the Roots blower housing (as described in co-pending patent application Ser. No. 10/985,528 filed Nov. 10, 2004, the specification and drawings of which are incorporated by reference herein) and multiple baffling chambers within the ventilator's housing (as described in co-pending patent application Ser. No. 11/088,316 filed Mar. 23, 2005, the specification and drawings of which are incorporated by reference herein). The use in the present invention of a Roots-type compressor, together with specially configured flow control and power systems, reduces the size and power consumption of the ventilator as a whole. Embodiments of the invention provide full ventilator functionality, including the capability of operating in both volume and pressure control modes, in small, truly portable units that for the first time provide real mobility to patients. In one embodiment, the ventilator is a portable, self-contained ventilator that approximates the size of a laptop computer while providing several hours of battery powered, full-service breathing assistance.