What happens in the cardiac cycle?

The cardiac cycle is driven by the myocytes in the heart, which are a special type of muscle tissue which are able to contract without input from the nervous system. This starts with a tissue called the Sino-Atrial node, which acts as the pacemaker of the heart. This sends out an electrical impulse, which causes the atria to contract and push blood into the ventricles. This is called Atrial systole. The impulse travels to a group of fibres called the Bundle of His, which causes an important delay in the transmission of the electrical signal further. This delay of about 1ms prevents the atria and ventricles contracting at the same time, which keeps the blood flowing in one direction. 

This signal then passes to the Atrio-ventricular node, which relays down a bundle of cells called purkinje fibres. These pass the electrical signal through the ventricles so that the contraction goes from the base to the top, ensuring that the blood is pushed in the right direction. At this time, the Atrio-Ventricular valves shut, making a characteristic "lup" sound. This is Ventricular Systole. As the blood enters the aorta, the ventricles and atria both relax, and the semi lunar valves in the ventricles shut with a "dup" sound, to prevent the backflow of blood into the heart. This final stage of the cycle is diastole, where the cardiac muscles relax and blood flows passively into the atria, ready to begin the cycle again.