An injection system for the delivery of a gaseous substance to a patient respiratory system is described herein. The injection system includes a control unit and a valve assembly including a valve and a valve actuator allowing partial opening of the valve and controlled by the control unit. The control unit is supplied with gas flow data and controls the valve assembly so that the opening of the valve is a function to the gas flow to thereby enable the control over the concentration of the gaseous substance delivered to the patient.