Final answer:
The contraction of the ventricles that pumps blood into the aorta and pulmonary arteries is called ventricular systole, occurring after the electrical impulse has traveled from the SA node, through the AV node, and down the Purkinje fibers to cause the ventricles to contract.
Step-by-step explanation:
The contraction of the ventricles, which pumps blood into the aorta and pulmonary arteries, is called ventricular systole. During this phase of the cardiac cycle, the heart muscles contract and blood pressure rises, initially closing the tricuspid and mitral valves to prevent blood from flowing back into the atria. This is the phase where the ventricles propel blood into the circulatory system. The process begins with an electrical impulse originating from the SA node, which causes the atria to contract. The impulse then pauses at the AV node before spreading to the walls of the ventricles, and eventually, the Purkinje fibers conduct the impulse through the ventricular myocardium. This orchestrated contraction effectively pumps blood into the systemic and pulmonary circulation.