7.5 The cardiac cycle

• The cardiac cycle describes when the heart undergoes a sequence of events that is repeated in humans around 70 times each minute when at rest
• Two phases to the beating heart

  • Systole
  • Diastole
• Systole is the contraction which occurs separately in the ventricles and the atria - described in two stages
• Diastole is the relaxation which takes place simultaneously in all chambers of the heart - one phase
• The cardiac cycle:

  1. Relaxation of the heart (diastole)
  2. Contraction of atria (atrial systole)
  3. Contraction of ventricles (ventricular systole)
• Relaxation of the heart (diastole)

  1. Blood returns to the atria through the pulmonary vein (from lungs) and the vena cava (from body)
  2. As the atria fill, the pressure rises
  3. When pressure exceeds that in the ventricles, the atrioventricular valves open allowing blood to pass into the ventricles - muscular walls of the atria and ventricles are relaxed
  4. This relaxation causes them to recoil and reduces the pressure within the ventricle - causing the pressure to be lower than that in the aorta and the pulmonary artery
  5. The semi-lunar valves in the aorta and the pulmonary artery close
• 2. Contraction of the atria (atrial systole)

  The contraction of the atrial walls, along with the recoil of the relaxed ventricle walls, forces the remaining blood from the atria into the ventricles

  Throughout this stage, the muscle of the ventricle walls remains relaxed
• 3. Contraction of the ventricles (ventricular systole)

  1. Ventricle walls contract simultaneously
  2. The blood pressure increases, forcing the atrioventricular valves to close, preventing backflow of blood into the atria
  3. Pressure in the ventricles increase further - once exceeding that in the aorta and pulmonary artery, blood is forced from the ventricles into these vessels
  4. The ventricles have thick muscular walls which mean they contract forcefully -creates the high pressure to pump blood around the body
• 3. Contraction of the ventricles (ventricular systole)

  • Thick wall of left ventricle has to pump blood to the extremities of the body
  • Thinner wall of right ventricle pumps blood to lungs