1. An artificial respiration ventilator for controlling the volume of continuous air flow passing into a patient's lungs comprising: respiratory circuit means for conducting a continuous air flow from a source to a patient's lungs during inhalation and exhalation phases of a respiratory cycle; sensing means for sensing a generation of positive and negative airway pressures in the respiratory circuit means caused by inhalation and exhalation efforts; means for generating a plurality of threshold signals corresponding to predetermined positive and negative airway pressure threshold value threshold detection means, responsive to said sensing means, for ascertaining when said sensed positive and negative airway pressures exceed said threshold values; means for generating a plurality of reference input signals corresponding to a plurality of air volume flow rate values; switching means, responsive to said threshold detection means, for choosing one from a plurality of reference input signals when one of the plurality of threshold values is exceeded; and air volume flow rate adjusting means, responsive to the output of said switching means, for continuously and instantaneously adjusting the air volume flow rate through the respiratory circuit means in accordance with the reference input value chosen by said switching means such that the continuous air flow through substantially inversely proportioned to the sensed positive and negative airway pressures.