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Cardiac Cycle

The Cardiac Cycle
by

Damien Moore

on 15 October 2009

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Transcript of Cardiac Cycle

Cardiac Cycle The cardiac cycle is the cycle of events that occurs as the heart contracts. There are two phases of the cardiac cycle. In the diastole phase, the heart ventricles are relaxed and the heart fills with blood. In the systole phase, the ventricles contract and pump blood into the arteries. Atrial Systole The atria, filled with blood from the vena cavae and pulmonary veins, contract, forcing the blood through the atrioventricular valves into the ventricles Ventricular Systole The ventricles are full of blood. As they contract they force the blood through the semi- lunar valves into the aorta and pulmonary artery. The force of the contraction closes the atrioventricular vales, preventing blood from flowing back into the atria. Diastole As the ventricles relax the drop in blood pressure causes the semi-lunar valves to close. The atria start to fill with blood, and as the pressure increases, blood starts trickling into the ventricles again and the whole process repeats itself Electrical impulses The sinoatrial node (SAN) is the pacemaker of the heart and it controls when the atria and ventricles contract. The SAN triggers a wave of depolarisation across the heart which results in contraction. The waves spread out over the 2 atrial walls so that they contract. There is a band of fibres between the atria and ventricles, which have a high electrical resistance so the waves cannot spread from the atria to the ventricles. There is an area, however, which does conduct in the septum, and the waves can pass from here through the ventricles. This specialised area is called the atrioventricular node (AVN) and will pass on the waves of depolarization after about 0.1s.

It would be disastrous if the ventricles contracted at the same time so that is why there is a short period of delay before the ventricles contract. The AVN passes them on to the Purkinje (also called Purkyne) fibres in the inter-ventricular septum. The excitation is passed to the apex of the heart and then through the ventricle walls. This causes the ventricles to contract from the base upwards ensuring that the blood is forced up and out in the vessels leaving the heart.
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