Cardiac cycle

• What is the pacemaker of the heart?

  In the wall of the right atrium, region of specialised fibres called sinoatrial node is the pacemaker.
• Myogenic definition


  Hearts ability to initiate its own contraction
• Why do ventricles do not start contracting until atria stops?

  Due to presence of tissue of base of atria that is unable to conduct wave of excitation (septum)
• How do ventricles contract?

  1. Electrical wave eventually reaches the atrioventricular node ( in between two atria passing excitation to ventricles)
  2. Down to bundle of his to apex of the heart
  3. Bundle of his branches into purkyne fibres carrying wave upwards
  4. Causing ventricles to contract
• Why do ventricles contract at apex?

  To force most blood possible upwards out of the aorta and pulmonary artery.
• 3 stages of cardiac cycle CARDIAC DIASTOLE STEP 1

  1. Atria and ventricles relax, elastic recoil of heart lowers pressure in heart chambers and blood returns to heart from vena cava (body) and pulmonary vein (from lungs) and fills atria. Pressure increases in atria, atrioventricular valves then open letting blood flow to ventricle. Relaxed atria and ventricle = semi lunar valves closed. Muscular walls of V relaxed so recoil and reduce pressure, lower pressure than aorta and Pul Artery so SL valve close.
• 3 stages of cardiac cycle ATRIAL SYSTOLE STEP 2

  Contraction of atrial walls and recoil of relaxed ventricle wall force remaining blood to ventricles
• 3 stages of cardiac cycle VENTRICULAR SYSTOLE STEP 3

  Contraction of ventricles: after filling with blood, walls contract simultaneously increasing blood pressure, forcing shut atrioventricular valves preventing back flow to atria. Closed atrioventricular valve increase pressure and once exceeds pressure in aorta and pulmonary artery, blood forced from ventricles to blood,
• Ventricles have thick muscle walls, so they can contract forcefully creating high pressure necessary to pump blood around body