A cycle can be defined by a series of repeated steps that produce an end product which is the same as the start product. There are many cycles in biology including cycles that occur inside organisms such as the cardiac cycle, calvin cycle and the krebs cycle. There are also biological cycles that occur around us in the outside world such as the carbon cycle and the nitrogen cycle.
I have decided to focus my essay on the biological cycles that occur inside organisms. The cardiac cycle is vital in all organisms with a heart, to pump blood round the body. There are phases of the cardiac cycle; systole (contraction) and diastole (relaxation). The heart consists of 4 chambers, 2 being the atria at the top of the heart and the other 2 being the ventricles at the bottom of the heart. Systole occurs separately in the atria and ventricles and diastole occurs simultaneously in all 4 chambers. The cardiac cycle is controlled by electrical waves that spread throughout the heart. The blood flows into the heart straight into the atria via the vena cava and pulmonary vein. A wave of electrical activity is firstly spread from the Sino-atrial node which spreads across both atria, this causes the atria to contract which forces blood down into the ventricles through the atrioventricular valves.
The atrioventricular septum prevents the wave crossing the ventricles. The wave passes to the atrioventricular node where it is passed down the septum down specialised fibres known as the bundle of His. This occurs after a short delay to allow all the blood to flow from the atria to the ventricles. This wave passes down the bundle of His to the Apex of the heart where the Ventricles contract upward, pumping blood out of the ventricles into the pulmonary artery and aorta through the semilunar valves. Here the blood is then passed round the body where it then returns to the heart and the process repeats.