The cardiac cycle

Systole:

• Contraction

Diastole:

• Relaxation

Relaxation of the heart (diastole):

• Blood returns to the atria through the pulmonary vein and the vena cava
• As the atria fill the pressure in them rises
• When this pressure exceeds that in the ventricles, the atrioventricular valves open allowing the blood to pass into the ventricles
• The passage of blood is aided by gravity
• The muscular walls of both the atria and ventricles are relaxed at this stage
• The relaxation of the ventricle wall causes them to recoil and reduces the pressure within the ventricle
• This causes the pressure to be lower than that in the aorta and the pulmonary artery, and so the semi-lunar valves in the aorta and the pulmonary artery close

Contraction of the atria (systole):

• The contraction of the atrial walls along with the recoil of the relaxed ventricle walls forces the remaining blood into the ventricles from the atria
• Throughout this stage the muscle of the ventricle walls remains relaxed

Contraction of the ventricles (systole):

• After a short delay to allow the ventricles to fill with blood their…