cardiac cycle

• the heart

  A) vena cava

  B) aorta

  C) pulmonary

  D) pulmonary veins

  E) atrium.

  F) AV valve

  G) ventricle

  H) semilunar

  I) ventricle

  J) vena cava

  K) semilunar

  L) right AV

  M) atrium

  N) pulmonary veins

  
• The cardiac cycle is a series of events that take place in one heartbeat. 
• The contraction of the heart is called systole.
• Relaxation of the heart is called diastole. 
• 1. Contraction of the heart muscles causes a decrease in volume in the corresponding chamber of the heart, which increases again when the muscles relax. 
  2. Volume changes lead to corresponding pressure changes. 
  3. When volume decreases pressure increases. 
  4. When volume increases, pressure decreases. 

   
• Throughout the cardiac cycle, heart valves open and close as a result of pressure changes in regions of the heart. 
• Valves open when the pressure of the blood behind them is greater than the pressure in front of them. 
• Valves close when the pressure in front of them is greater then the pressure behind them. 
• Valves stop blood flowing backwards. 
• Atrial systole is the walls of the atria contracting. This means atrial volume decreases and atrial pressure to increase. 

  The pressure in the atria rises above that in the ventricles opening the atrioventricular valves. 

  Blood is forced into the ventricles with a slight increase in ventricular pressure as they receive blood from the atria. 

  The ventricles are relaxed at this point, meaning ventricular diastole corresponds with atrial systole. 
• Ventricular systole is when the walls of the ventricles contract. Ventricular volume decreases so pressure increases. 

  The pressure in the ventricles rises above that in the atria, closing the AV valves closed, preventing backflow. 

  Pressure in the ventricles rises above that in the aorta and pulmonary artery, forcing the semilunar valves open, so blood leaves the heart. 

  During ventricular systole the atria relax, atrial diastole corresponds with ventricular systole. Blood flow to the heart continues and relaxed atria fill with blood again. 
• Diastole:

  1. Ventricles and atria are both relaxed
  2. Pressure in the ventricles drops below that in the aorta and pulmonary artery, closing the SL valves 
  3. The atria fill with blood 
  4. Blood returns to the heart via vena cava and pulmonary vein 
  5. Pressure in the atria rises above that in the ventricles, opening the AV valves 
  6. Blood flows passively into the ventricles without atrial systole 
  7. The cycle begins again with atrial systole. 
• Cardiac output = stroke volume x heart rate