Cardiac Cycle

Cardiac Cycle The cardiac cycle is coordinated by specialized tissues that initiate and distribute electrical (cardiac) impulses (fig. 1-34). The contractions of the heart are stimulated and maintained by the sinoatrial (SA) node, commonly called the pacemaker of the heart. The SA node is an elongated mass of specialized muscle tissue located in the upper part of the right atrium. The SA node sets off cardiac impulses, causing both atria to contract simultaneously. The normal heart rate, or number of contractions, is about 70 to 80 beats per minute. This same cardiac impulse continues to travel to another group of specialized tissue called the atrioventricular (AV) node. The AV node is located in the floor of the right atrium near the septum that separates the atria. The cardiac impulse to the AV node is slowed down by junctional fibers. The junctional fibers conduct the cardiac impulse to the AV node; however, these fibers are very small in diameter, causing the impulse to be delayed. This slow arrival of the impulse to the AV node allows time for the atria to empty and the ventricles to fill with blood. Once the cardiac impulse reaches the far side of the AV node, it quickly passes through a group of large fibers which make up the AV bundle (also called the bundle of His). The AV bundle starts at the upper part of the interventricular septum and divides into right and left branches. About halfway down the interventricular septum, the right and left branches terminate into Purkinje fibers. The Purkinje fibers spread from the interventricular septum into the papillary muscles, which project inward from the ventricular walls. As the cardiac impulse passes through the Purkinje fibers, these fibers in turn stimulate the cardiac muscle of the ventricles. This stimulation of the cardiac muscles causes the walls of the ventricles to contract with a twisting motion. This action squeezes the blood out of the ventricular chambers and forces it into the arteries. This is the conclusion of one cardiac cycle. Blood Pressure Blood pressure is the pressure the blood exerts on the walls of the arteries. The highest pressure is called systolic pressure, because it is caused when the heart is in systole, or contraction. A certain amount of blood pressure is maintained in the arteries even when the heart is relaxed. This pressure is the diastolic pressure, because it is present during diastole, or relaxation of the heart. The difference between systolic and diastolic pressure is known as pulse pressure. Normal blood pressure can vary considerably with an individual's age, weight, and general condition. For young adults, the systolic pressure is normally between 120 and 150 mm of mercury, and the diastolic pressure is normally between 70 and 90 mm of mercury. On average, women have lower blood pressure than men. 1-27 HM3F0134 INTERATRIAL SEPTUM S-A NODE AV NODE AV BUNDLE LEFT BUNDLE BRANCH PURKINJE FIBERS INTERVENTRICULAR SEPTUM Figure 1-34.—Cardiac cycle.