Anatomical Basis of The Cardiac Cycle
The heart is a four-chambered organ consisting of right and left halves. Two of the chambers, the left and right atria, are entry-points into the heart, while the other two chambers, the left and right ventricles, are responsible for contractions that send the blood through the circulation. The circulation is split into the pulmonary and systemic circulation. The role of the right ventricle is to pump deoxygenated blood to the lungs through the pulmonary trunk and pulmonary arteries. The role of the left ventricle is to pump newly oxygenated blood to the body through the aorta.
Importantly, the right and left ventricles contract simultaneously, and so in consideration of the cardiac cycle the events that are occurring on one side of the heart are equivalent to the events occurring on the other side of the heart. However, the ventricles contract shortly after the atria. The sinoatrial node sends out electrical waves of excitation to both atria, and it is prevented from flowing into the ventricles by strands of non-conducting fibrous tissue situated laterally from the tricuspid/bicuspid valves to the septum. These waves of excitation travel towards the septum and into the atrioventricular node, where they are held for roughly 0.1 seconds. They are then discharged down the bundle of His, then down the Purkinje tissue, which are both situated inside the septum. The waves flow down towards the apex of the heart and are then discharged into the ventricles, causing them to contract (ventricular systole) This creates the well known beat of the heart.
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