Cardiac Cycle and Transport

• Cardiac cycle


  The series of events that take place in one heart beat, including muscle contraction and relaxation
• Systole


  The contraction of the heart
• Diastole


  The relaxation of the heart
• One cardiac cycle is followed by another in a continuous process
• There is no gap between cycles where blood stops flowing
• Volume and pressure changes

  1. Contraction of the heart muscle causes a decrease in volume in the corresponding chamber of the heart, which then increases again when the muscle relaxes

  2. Volume changes lead to corresponding pressure changes

  3. When volume decreases, pressure increases

  4. When volume increases, pressure decreases
• Valves opening and closing

  1. Valves open when the pressure of blood behind them is greater than the pressure in front of them

  2. They close when the pressure of blood in front of them is greater than the pressure behind them
• Valves are an important mechanism to stop blood flowing backwards
• Atrial systole


  1. The walls of the atria contract

  2. Atrial volume decreases

  3. Atrial pressure increases

  4. The pressure in the atria rises above that in the ventricles, forcing the atrioventricular (AV) valves open

  5. Blood is forced into the ventricles

  6. There is a slight increase in ventricular pressure and chamber volume as the ventricles receive the blood from the atria
• Ventricular systole


  1. The walls of the ventricles contract

  2. Ventricular volume decreases

  3. Ventricular pressure increases

  4. The pressure in the ventricles rises above that in the atria, forcing the AV valves to close, preventing backflow of blood

  5. The pressure in the ventricles rises above that in the aorta and pulmonary artery, forcing the semilunar (SL) valves open so blood is forced into the arteries and out of the heart
• Diastole


  1. The ventricles and atria are both relaxed

  2. The pressure in the ventricles drops below that in the aorta and pulmonary artery, forcing the SL valves to close

  3. The atria continue to fill with blood

  4. Blood returns to the heart via the vena cava and pulmonary vein

  5. Pressure in the atria rises above that in the ventricles, forcing the AV valves open

  6. Blood flows passively into the ventricles without need of atrial systole
• Valves during the cardiac cycle

  • Atrioventricular valves
  • Semilunar valves
• Cardiac output (CO)


  The volume of blood that is pumped by the heart (the left and right ventricle) per unit of time
• An average adult has a cardiac output of roughly 4.7 litres of blood per minute when at rest
• Individuals who are fitter often have higher cardiac outputs due to having thicker and stronger ventricular muscles in their hearts
• Cardiac output increases when an individual is exercising so that the blood supply can match the increased metabolic demands of the cells
• Heart rate


  The number of times a heart beats per minute
• Stroke volume


  The volume of blood pumped out of the left ventricle during one cardiac cycle
• Calculating cardiac output


  Cardiac output = heart rate x stroke volume