The Cardiac Cycle

• The contraction of the heart is called systole, while the relaxation of the heart is called diastole.
• One systole and diastole makes a heartbeat and lasts around 0.8 seconds in humans. This is the cardiac cycle.
• Atrial systole is the period when the atria are contracting and ventricular systole is when the ventricles are contracting. Systole reduces the volume of the heart chambers and increases the pressure of the blood within that chamber. When the pressure within a chamber/vessel exceeds that in the next chamber/vessel the valves are forced open and the blood moves through. During ventricular systole, blood is forced out of the pulmonary artery (to the lungs) and aorta (to the rest of the body). During systole, the the atrioventricular valves are closed and the semilunar valves are open.
• During diastole, the heart is relaxing. The atrioventricular valves are open and the semilunar valves are closed.
• During systole and diastole, heart valves open and close as a result of pressure changes.
• Contraction of the muscles in the wall of the heart reduces the volume of the heart chambers and increases the pressure of the blood within that chamber.
• When the pressure within a chamber/vessel exceeds that in the next chamber/vessel the valves are forced open and the blood moves through.
• When the muscles in the wall of the heart relax they recoil which increases the volume of the chamber/vessel and decreases the pressure so that the valves close.
• One cardiac cycle is followed by another in a continuous process.
• There is no gap between cycles where blood stops flowing.
• The Cardiac Cycle - Stage 1:

  • Atria contract pushing blood into the ventricles
  • Ventricles relax
• The Cardiac Cycle - Stage 2:

  • Atria relax
  • Ventricles contract pushing blood out of the heart
• The Cardiac Cycle - Stage 3:

  • Atria relax
  • Ventricles relax
  • Blood flows into the heart
• When describing the pressure changes in the cardiac cycle:

  The three graphs represent pressure in the left atrium, the aorta and the left ventricle. The points at which the lines intersect indicate when valves open and close .

  
• A - both the left atrium and left ventricle are relaxed.

  
• A to B - atrial systole. The left atrium contracts and empties blood into the left ventricle

  
• B - the start of of ventricular systole. The AV valve shuts, ventricular pressure increases and atrial pressure drops as the atrium expands.

  
• C - pressure in the ventricle exceeds that in the aorta.  The aortic valve opens and blood enters the aorta

  
• D - ventricular diastole.  The left ventricle empties as the muscles relax, and pressure falls below that of the aorta.  The aortic valve closes and the AV valve opens.

  
• E - There is a short period of time during which the left ventricle expands.  This increases the internal volume of the left ventricle which decreases the pressure.

  
• Cardiac output (CO) is the term used to describe 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. This is so that the blood supply can match the increased metabolic demands of the cells.
• The CO of an individual can be calculated using their heart rate and stroke volume.
• Heart rate is the number of times a heart beats per minute. This can also be described as the number of cardiac cycles per minute.
• Stroke volume is the volume of blood pumped out of the left ventricle during one cardiac cycle.
• cardiac output=heart rate x stroke volume