Summary OCR A/AS-Level Biology 3.2.5 Cardiac Cycle

The Cardiac Cycle
Atrial Systole
 Muscles of the atria contract
 Pressure inside atria increases
 Semi-lunar valve in the vena cava and pulmonary vein close
 Tricuspid and bicuspid valves open, allowing blood to flow into the ventricles
 Blood flows from high to low pressure (atria to ventricles)
 Last ~0.1 seconds



Ventricular Systole
 Muscles of the ventricles contract
 Pressure inside ventricles increases
 Tricuspid and Bicuspid valves close
 Semi-lunar valves open, allowing blood to flow into the aorta and pulmonary artery
 Blood flows from high to low pressure (ventricles to blood vessels)
 Last ~0.3 seconds



Diastole
 Muscles of the ventricles relax
 Pressure inside the ventricles decreases
 Semilunar valves close
 All heart muscles relax
 Blood flows from high to low pressure (blood vessels to atria)
 Pressure remains low inside atria and ventricles