The present work presents a mathematical model and a control system based on it, for a Lung Ventilator Prototype entirely developed in our University. This prototype has two inlet lines, one for air flow and one for oxygen sensed by differential pressure sensors and controlled by two proportional valves; and an outlet line controlled by an on-off valve. At the end there is a passive lung simulator with a pressure sensor. In this paper is explained the no linear Mathematical model that simulates with 1.0 % accuracy: delay, time rise and stable value of the oxygen and air flow in the range of 0 to 90L/min and the pressure at the entrance of the lung. A control system to achieve the desired flow with a 5% precision with 100ms time response mix of air and oxygen flow in the range of 5 to 80L/min for a Lung Ventilator Prototype developed in our university. The proposed system is modification of the classical Proportional Integrative (PI) algorithm control. These results will be useful for futures ventilations modes implementation like CMV or SIMV in this third prototype.