The cardiac cycle describes the alternating sequence of myocardial contraction and relaxation that allows the delivery of blood into the systemic and pulmonary circulations.
The cardiac cycle can be divided into ventricular systole and ventricular diastole. Ventricular systole is initiated by an electrical signal passing through the myocardium, which leads to a synchronised contraction and the expulsion of blood into the great vessels. During ventricular diastole the myocardium relaxes and the atrio-ventricular valves open, this permits filling of the ventricles.
The schematic diagram above illustrates the electro-mechanical relationship that governs the beating of the heart.
The depolarisation of the ventricular myocardium induces intracellular changes resulting in contraction of the muscle. The contraction increases pressure within the ventricle. Once the pressure in the ventricle exceeds the pressure in the atria, the atrio-ventricular valves close.
The first phase of ventricular systole is known as isovolumetric contraction. During this phase, the pressure in the ventricles increases sharply, but is not yet high enough to open the aortic and pulmonary valves. Therefore, no blood leaves the ventricles. As illustrated by the ventricular volume trace on the diagram, the blood volume in the ventricles remains constant.
The second phase of systole is the ejection phase. As the pressure in the ventricles increases further, the aortic and pulmonary valves are forced opened, and blood is pumped into the systemic and pulmonary vessels.
Having expelled much of the blood, the ventricles begin to relax. The relaxation of the myocardium is triggered by the repolarisation of the cell membrane, which can be seen on the ECG as a T wave. The aortic and pulmonary valves close as the pressure in the ventricles decreases. This marks the end of ventricular systole.
Not all the blood is expelled during systole, the blood that remains in the ventricles is known as the End Systolic Volume (ESV). The volume of blood in the ventricles before contraction is known as End Diastolic Volume (EDV). The difference between these two figures gives the Stroke Volume (SV), which is the volume of blood pumped out per heartbeat.
Diastole begins with a period of isovolumetric relaxation. In this phase, the outflow of blood is prevented by the closure of the aortic and pulmonary valves, and the influx of blood is prevented by the closed atrio-ventricular valves.
Once the ventricles have relaxed even further, the atrio-ventricular valves open and blood can flow into the ventricles. The period of time where blood is flowing into the ventricles is known as the filling period.
The depolarisation of the atrial muscle is seen on the ECG trace as a 'P wave'. The resulting contraction increases atrial pressure and empties the atrial chambers into the ventricles - this constitutes active filling of the ventricles. At resting heart rates, the atrial contraction only contributes 10% to the filling of the ventricle.
Immediately after atrial systole, the atria begin to fill again with blood, this occurs throughout the cardiac cycle. The filling of the atria is a passive process, dictated by the rate of venous return.
After isoventricular relaxation, the atrio-ventricular valves re-open and the ventricles begin to fill with blood from the atria. This passive filling of the ventricles is governed by pressure gradients.
At resting heart rates, passive filling is responsible for 90% of ventricular filling. During exercise, diastole shortens to facilitate an increased heart rate. This means that there is less time for passive filling of the ventricles. Atrial systole compensates for this by contracting with greater force.
The atrial pressure changes displayed on the diagram are transmitted backwards to the jugular veins as there is no intervening valve. On clinical examination one can observe abnormally high atrial pressures as a 'raised JVP'.
The transfer of blood around the body is determined by pressure gradients. Pressure gradients are created by the synchronised contraction of the myocardium, which in turn is coordinated by the cardiac pacemaker and conducting system.
The table below gives the approximate cardiac pressure in each of the cardiac chambers.
The Cardiac Cycle diagram: http://en.wikipedia.org/wiki/Wiggers_diagram (accessed on 16th July 2012)
http://www.cvphysiology.com/Heart%20Disease/HD002.htm (accessed on 16th July 2012)
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