the cardiac cycle

• (1) Atrial systole -

  • ventricles are relaxed and atria contract,
  • this decreases their volume and increases the pressure which forces AV valves to open full
  • then blood is pushed into the ventricles
• (2) Ventricular systole -

  • Atria relax, ventricles contract which decreases the volume and increases pressure forcing AV valve close.
  • Once pressure has increased enough SL valves open and blood is pumped out the arteries
• (3) Diastole -

  • atria and ventricles are both relaxed
  • pressure in the arteries is now higher than the ventricles so the SL valves shut.
  • Blood enters through the veins into the atria
  • increases pressure of the atria until the pressure exceeds the ventricle pressure and AV valves open slightly, and blood flows to the ventricles
• cardiac cycle diagram:

  A) atrial systole

  B) ventricular systole

  C) diastole

  D) semi-lenuar valve closes

  E) semi-lunar valve opens

  F) AV valve closes

  G) AV valve opens

  
• what causes the lub-dub sound?

  the closing of the atrioventricular valve followed by the closing of them semi-lunar valve
• The AV valve closes because the pressure in the ventricle becomes higher than the atrial pressure because its contracting
• The SL valve opens because the pressure in the ventricles has become higher than the pressure in the arteries because they are contracting
• The SL valves close because pressure in the ventricles drop as they are relaxing and pressure in the aorta is now higher
• The AV valves opens as pressure in the atria is increasing, as blood flows in and is higher than ventricular pressure as its relaxed
• cardiac output = stroke volume x heart rate