The Cardiac Cycle

• Overview of the cardiac cycle:

  • Stage One: Diastole - Cardiac muscle is relaxed, blood enters the atria and ventricles
  • Stage Two: Atrial Systole - The atria contract forcing the remaining blood through the valves into the ventricles
  • Stage Three: Ventricular Systole - The ventricles contract, atrioventricular valves close, and semilunar valves open, directing blood to the pulmonary artery and aorta
• The heart muscle is myogenic, stimulating itself to contract without a message from the brain
• Pacemakers in the heart produce small electrical impulses that stimulate muscle contraction
• Impulses from the sinoatrial node (SAN) cause the atria to contract and stimulate the atrioventricular node (AVN)
• The sympathetic nervous system, a branch of the autonomic nervous system, can increase heart rate
• Chemoreceptors detect chemical changes like oxygen concentration, carbon dioxide levels, and blood pH, influencing heart rate
• Baroreceptors in the heart detect changes in blood pressure, triggering responses to regulate blood pressure back to normal levels
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  • Sinoatrial node: known as the natural pacemaker, causes an impulse to travel through the atria, setting the heart's rhythm and rate.
• • Atrioventricular node: detects the impulse traveling through the atria and redirects it to the bundle of His, causing a delay to allow the atria to contract before the ventricles.
• • Bundle of His: a group of fibers in the septum through which the impulse travels to the base of the ventricles
• • Purkinje fibers: act like neurons and are found in the walls of the ventricles, causing the ventricles to contract when the impulse from the bundle of His reaches them