What is systole? The sinoatrial artery acts as the natural pacemaker that sends an impulse. This impulse causes contraction of the atria and forces blood into the ventricles. This impulse then transfers to the atrioventricular node and His-Purkinje network. This transfer of the impulse causes ventricular contraction. The ventricular contraction pumps blood to the aorta and pulmonary artery. Then, the ventricles relax. This relaxation of the ventricles causes the heart chambers to fill with blood. The arterial blood pressure is at its maximum during contraction. The pressure is at its minimum during relaxation.