Download Cardiac_Cycle

Describe the cardiac cycle, with reference to
the action of the valves in the heart.
• The cardiac cycle is the sequence of events in one
heartbeat.
• It has three phases.
Diastole
• The atria and ventricles relax and blood flows into the
heart from the veins.
• Blood flows into the atria, through the AV valves and
into the ventricles.
– Atrial diastole - atria relax (0.7s)
– Ventricular diastole - ventricles relax (0.5s)
Atrial systole
• Both atria contract pushing blood into the ventricles
(0.1s).
• Once the ventricles are full they start to contract and
the AV valves shut.
Ventricular Systole
• All 4 valves are closed for short time.
• Walls of ventricles contract (starting from apex),
increasing pressure and forcing blood out of the heart,
opening the semilunar valves. (0.3s). Cardiac Cycle Animation
AV valves
Semilunar
valves
Heart sounds
• Sound is made by the valves closing.
• First sound = lub – made by AV valve closing
as ventricles start to contract.
• Second sound = dub – semilunar valves closing
as ventricles start to relax.
Stroke Volume and Cardiac Output
• Stroke volume is the volume of blood pumped
by the heart in one cardiac cycle.
• Typically about 80cm3.
• Stroke volume increases during exercise.
• Cardiac Output (CO) is the volume of blood
pumped in one minute.
CO = stroke volume x heart rate.
• expressed in litres of blood per minute.
Factors Affecting Heart Rate
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Adrenaline.
Movement of limbs (stretch receptors)
Levels of respiratory gases in the blood
Blood pressure – if it gets too high a safety
mechanism prevents heart rate increasing
further.