In humans, blood passes from the ventricles to pulmonary artery and aorta due to
Increase in ventricular pressure
Increase in the pressure within ventricles during systole gets passed on to the walls of the great vessels (aorta & pulmonary artery). Blood pressure is just the pressure exerted by the contraction of heart on the walls of arteries. Blood can flow through the vessels and maintain unidirectionality due to this action.