Download 3_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 3 phases:
1. Atrial systole
• Both atria contract
• Pressure in atria exceeds
that in the ventricles
• A-V valves open
• Semilunar valves close
• Blood is forced from the atria into the
ventricles
• Once full the ventricles start to contract
and the AV valves snap shut
2. Ventricular Systole
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Atria relax
Ventricles contract (starting from apex)
A-V valves and semilunar valves closed
Pressure increases forcing blood out of the heart
by opening the semilunar valves
3. Diastole
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•
•
•
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Atria and ventricles relax
Elastic recoil returns ventricles to original size
A-V and semilunar valves are closed
Blood flows passively into the atria from the veins.
The A-V valves open and blood starts to enter the
ventricles
Cardiac Cycle Animation
AV valves
Flaps of tissue
arranged in a cup
shape They fill
with blood as the
ventricles
contract
ensuring blood
flows out of the
heart
Semilunar
valves
At the base of major arteries
preventing back flow of blood
as the ventricles relax
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.
Electrocardiograms (ECG)
• Place electrodes on the skin over opposite
sides of the heart and record the electrical
potentials generated
P is the wave
of excitation
sweeping over
the atrial walls
T is the
recovery
of the
ventricle
walls
QRS complex is the wave of
excitation speading through the
ventricular walls