Describe the cardiac cycle, including the role that valves and pressure differences play in this process.

The cardiac cycle can be split into 2 main parts- diastole (relaxation) and systole (contraction). These can be further split into 4 main phases (atrial systole, atrial diastole, ventricular systole, ventricular diastole). At the beginning of the cycle, both atria and ventricles are in diastole. Blood flows freely into the atria from the respective veins and into the ventricles through the atrioventricular valves. This results in ventricular filling to around 80% of capacity. Atrial systole then occurs where the atria contract and fill the remaining 20% of blood volume in the ventricles. Atrial diastole then causes the atria to relax. At the same time as this, the process of ventricular systole commences. This stage can be further divided into 3 main phases. Firstly, an isovolumetric contraction takes place. This is required as at the beginning of ventricular systole the pressure in the ventricles is higher than that in the atria, thus causing blood to flow back into the atria. This initial contraction causes the closing of the atrioventricular valve, preventing the backflow of blood. Secondly, the ventricular contraction takes place. Now the pressure in the ventricles is higher than that in the arteries that they will expel the blood into, allowing the final phase of ventricular ejection to take place. The semilunar valves open and blood is expelled into the pulmonary artery and aorta. The final phase of the cardiac cycle is ventricular diastole, which can be split into two phases. Firstly as the pressure decreases in the ventricles, blood begins to flow back from the pulmonary artery and aorta. This causes the closing of the semilunar valves, preventing any backflow of blood. this phase is now called isovolumetric relaxation. In the second phase, known as late ventricular diastole, the pressure in the ventricles drops to below that of the atria thus allowing the opening of the atrioventricular valves and the flow of blood between atria and the ventricles. The cycle then repeats.