Final answer:
The timing of the phases of the cardiac cycle depend on the heart rate, which is controlled by the SA node and the conduction system of the heart.
Step-by-step explanation:
The timing of the phases of the cardiac cycle are based upon the heart rate, which is expressed in beats per minute. The heart rate is determined by the pace at which the SA node, the pacemaker of the heart, generates electrical impulses. Without nervous or endocrine control, the SA node initiates an impulse approximately 80-100 times per minute.
As the impulse spreads through the conduction system, the rates progressively slow down. The AV node would generate a heart rate of 40-60 beats per minute, the atrioventricular bundle would fire at a rate of 30-40 impulses per minute, the bundle branches at 20-30 impulses per minute, and the Purkinje fibers at 15-20 impulses per minute.