Cardiac Cycle Research Paper

Describe the Cardiac Cycle

The Cardiac cycle is a description of the activities that take place in the period from the start of one heart the next. The conducting system autonomically controls the heart beat impulses it is also called the cardiac conduction system or the nodal system. It is made up of six main components the sinoatrial node (SA-Node), internodal paths, atrioventricular node (AV-Node), atrioventricular bundle (AV bundle) or bundle of His, bundle branches and Purkinje fibres. Contractile cells make up about ninety nine percent of the cardiac muscle tissue on the walls of the aria and ventricles of the heart. The conducting cycle begins at the (SA-Node) when the Vagus nerve stimulates an action potential in the sinoatrial node.
This leads to the stimulation of the contractile cells which are stimulated autonomically by Purkinje fibres.

The sinoatrial node (SA-Node) is made up of pacemaker cells and therefore is know as the hearts natural pace maker functioning to maintain a normal heart rate.
Stimuli such as exercise, stimulant drugs, or hormones can change the rate of the SA-Node. The SA-Node is attached to the upper posterior wall of the right atrium near the superior vena cava. The SA-Node has specialized muscle cells which generate spontaneous action potentials which takes about fifty milliseconds to spread to the atria-ventricular node, or AV node. These electrical impulses are generated at the SA-Node one hundred to one hundred and ten per minute. After fifty milliseconds when the action potential moves through the atria surface via the internodal pathways to reach the AV node. During the transition of the action potential moving through the internodal path conducting cells stimulate the contractile cells of the
atria.

The AV node is found in the inferior-posterior region of the right atrium. After the impulse has left the internodal path the impulse starts to slow down this is because the AV node cells have a smaller diameter compared to the conductive cells in the internodal path. For this reason about one hundred milliseconds elapse at the AV node before the impulse is allowed to enter the AV bundles. At this time the impulse is delayed at the AV node so that there is enough time for a complete atria depolarization and contraction before the ventricular contraction. At this point about one hundred and fifty milliseconds has passed since SA node depolarization. After atria contraction has taken place the impulse then travels through the AV bundles. The AV bundle extends down the interventricular septum and is separated into the right and left bundle branches. However the size of the left bundle branch is significantly larger than the right bundle this is because the area of the left ventricle is larger than the right. Both these bundle branches curve at the apex of the heart then the impulse is conducted by. Purkinje fibres are very fast conducting fibres which conduct the impulse at speeds of about 4m/sec in the ventricles. From here it takes about seventy five milliseconds in order to depolarize the ventricles.

A single cardiac cycle can be separated into two separate basic phases called the systolic phase and diastole phase. The phase of contraction is systole and the relaxation phase is called diastole. The components involved in a cardiac cycle are as follows: atrial systole, atrial diastole, ventricular systole, and ventricular diastole. During the start of a cardiac cycle all the chambers are in a relaxed state and the Ventricles are approximately 70 percent full from the previous contraction. The cardiac cycle begins with the P wave of the electrocardiogram (ECG); this represents the depolarization of the atria. Because at the start of the cardiac cycle the pressure in the atria are higher compared to the pressure in the ventricles the atrioventricular valves (AV valves) open while the semi-lunar valves close (to prevent back flow of blood) allowing a small amount of blood flows into the adjacent chamber of the relaxed ventricle. This blood passed to the ventricle increases the blood volume of the ventricle by about 10 percent. In a resting adult the atrial systole phase takes about 100msec. The filling of the Ventricles over the atrial systole period varies directly with the venous return to the atria via the superior vena cava and pulmonary veins. For example the more blood pumped into the atria means during atria contraction more blood will be pumped into the ventricles. After this the atria go into atrial diastole (which extent is about 700 msec) phase dropping the atria pressure across the AV valves. At this point both ventricles contain maximum volumes of blood. This volume is termed the end-diastolic volume (EDV); the typical EDV in a resting adult is about 130 ml.

The next stage in the cardiac cycle is ventricular systole. This stage is represented by the QRS complex in the ECG produced by the depolarization of the ventricles. The first action that occurs during ventricular systole is the closing of all valves to prevent back flow and since now the pressure in the ventricles exceeds the pressure in the atria. Once the valves have been closed individual fibers of the ventricles contract, building up the pressure in the ventricles causing the shape of the ventricle chambers to change to a more circular looking shape. This is known as isovolumetric contraction. This stage is necessary so that ventricle pressure is high enough to force blood through the pulmonary and aortic trunk at great pressures. Ventricular systole can last for about 270 msec in a resting adult.

The next step in the ventricular systole phase is where the aortic valve and pulmonary valve to the pulmonary circuit open and allowing blood is rapidly ejected into the pulmonary circuit and systemic circuit.
The amount of blood ejected from the ventricles is called the Stroke Volume (SV) this volume is usually between 70 to 80 ml. This movement is created by the relative pressure differences between the semi-lunar valves and the high pressure level that was generated in the ventricles. This is when both the ventricles contract to deliver the blood into the pulmonary and systemic circulation by the shortening of muscle cells. After a duration of about 150 msec when ventricle contraction reaches a peak the ventricular pressure starts decreasing quickly. During this time the semi-lunar valves are shut due to blood from the aorta and pulmonary trunk flowing back into the ventricles. As the pressure in the semi-lunar valves increase they are closed and the amount of blood left in the ventricles at this point is about 50 ml. This volume is called the end-systole volume (ESV). Also during all this time the relaxed atria have been slowly filling passively through venous return from the lungs therefore increasing there
pressure.

Ventricle diastole takes place for the next 530 msec of the heart beat up until the end of the atrial systole phase. In the ventricle diastole stage the pressure in the ventricles is still greater than the pressure in the atria this ensures that blood can not flow into the ventricles. During this time isovolumetric relaxation is occurring and the AV valves are forced open due to the ventricular pressure rising above the atrial pressure. This leads to excess blood flowing back into the ventricles causing the semi-lunar valves to close. So far until this point the filling of the atria has been occurring from the systemic veins for about 270 msec. Also during this time of atria and ventricular diastole the ventricles have been passively filled this is the primary method of filling the ventricles to 75 percent of their capacity before atrial systole can reoccur. The whole cardiac cycle during a normal heart rate of 75 bpm takes a total time of 800 msec to complete, but when the heart rate is increased the time for a complete cardiac cycle is lowered.