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Chapter 5.2 The Cardiac Cycle Notes
The heart undergoes the cardiac cycle, which occurs around 70 times each minute,
when at rest. There are two phases to the beating of the heart, contraction (systole)
and relaxation (diastole). Contraction occurs separately in the ventricles and the atria,
so is described in two stages. Relaxation is treated as a single phase. The direction of
blood flow through the heart is maintained by pressure changes and the action of
Relaxation of the Heart (diastole):
Blood returns to the atria by the pulmonary vein (from the lungs) and the vena cava
(from the body). As the atria fill, their pressure increases, pushing open the
atrioventricular valves, allowing the blood to pass into the ventricles. All the muscular
walls are relaxed at this stage. The relaxation of the ventricle wall reduces the
ventricular pressure, which means that the pressure is lower than that of the aorta
and the pulmonary artery and so the semi-lunar valves in the aorta and the pulmonary
artery close, which causes a heart beat.
Contraction of the Atria (Atrial Systole):
The muscles of the atrial walls contract, forcing the remaining blood they contain
into the ventricles. The blood has only to be pushed a very short distance and
therefore the atria's muscular walls are very thin. During this stage, the muscles are
Contraction of the Ventricles (Ventricular Systole):
After the ventricles fill with blood, their walls contract simultaneously. This increases
the blood pressure within them, forcing the atrioventricular valves shut and
preventing backflow of blood into the atria. With the valves closed, pressure rises
further, forcing open the semi-lunar valves and pushing blood into the pulmonary
artery and aorta.
Valves in the Control of Blood Flow:
Blood flow is maintained by pressure created by the heart. Valves in the
cardiovascular system so that they open whenever the difference in blood pressure
either side of them favours the movement of blood in the required direction. When
pressure differences are reversed, the valves are designed to close.
Atrioventricular valves: between atria and ventricles. These prevent the
backflow of blood when contraction of the ventricles means that ventricular
pressure exceeds atrial pressure. Closure of these valves ensures that blood
moves within the ventricles to the aorta and pulmonary artery, not the atria.
Semi-lunar valves: are found in the aorta and pulmonary artery. They prevent
the backflow of blood into the ventricles when the recoil action of the elastic
walls of these vessels creates a greater pressure in the vessels than in the
Pocket valves: are found in veins, and occur throughout the venous system.
They ensure that when veins are squeezed, blood flows back into the heart
rather than away from it.
Each valve is made from a number of flaps of tough, flexible, fibrous tissue. They are
cusp-shaped. When pressure is greater on the convex side, they move apart to let
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When pressure is greater on the concave side, blood collects,
pushing the valves together, this prevents the passage of blood.
The volume of blood pumped by one ventricle in one minute. It is dependant on two
factors, the heart rate and the stroke volume.
Cardiac output = heart rate x stroke volume
How is the Cardiac Cycle Controlled?
The cardiac muscles are myogenic, so do not need stimulation from nervous