Cardiac cycle

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The cardiac cycle is the repetitive sequence of events showing the way in which the heart beats and how blood fills during a diastole phase and leaves the heart during the systole phase at a given time in the cycle.

Sequential events of the heart relative to blood movement

Start

  1. Late systole- Both chambers are relaxed, and both atria are emptying whilst the ventricvles are filling passively.
  2. Atrial Systole- Atria contracts forcing blood to actively enter into ventricles.
  3. End- Diastole volume (EDV) - End of ventricular relaxation. The volume of blood in the ventricle will be at its maximum.
  4. Isovolumic ventricular contraction- 1st phase of ventricular contraction where AV valves are pushed closed but pressure is not quite high enough to open the semilunar valve. At this point, the pressure in the ventricle is equal to either the aortic vessel or pulmonary artery hence 'iso' in isovolumic.
  5. Ventricular ejection- Point at which ventricular pressure exceeds that of arteries. Semi lunar valves open and blood is forced out the heart and around the body.
  6. Isovolumic ventricular relaxation- As the ventricles begin to relax after contraction, semilunar valves close and prevent backflow of blood.

Back to start

Note: End-systolic volume is the volume of blood that remains in the ventricle after contraction. This can be used to calculate what's called a stroke volume. The volume of blood that leaves the heart per beat.

⇒ EDV-ESV = Stroke volume

Furthermore, the stroke volume can be used to calculate the ejection factor.This is how effciently the heart ejects blood per beat and works by comparing the maximum input of blood into the ventricle to the amount of blood the leaves per beat.

⇒ Ejection Facrtor= SV/ EDV

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