Describe the events in the cardiac cycle. Explain 'double circulation'.

(N/A) The cardiac cycle consists of the sequential events in the heart which are cyclically repeated. It involves the following phases:
$1$. Atrial Systole: The $SA$ node generates an action potential,stimulating both atria to undergo simultaneous contraction. This increases the flow of blood into the ventricles by about $30\%$.
$2$. Ventricular Systole: The action potential reaches the ventricles via the $AV$ node and Bundle of His,causing ventricular contraction. This leads to the closure of $AV$ valves (producing the first heart sound,'$lub$') and the opening of semilunar valves,forcing blood into the aorta and pulmonary artery.
$3$. Joint Diastole: Both atria and ventricles are in a relaxed state. The semilunar valves close (producing the second heart sound,'$dub$'),and the $AV$ valves open,allowing blood to flow from the atria into the ventricles.
Double circulation refers to the pathway where blood passes through the heart twice in one complete cycle:
$1$. Pulmonary Circulation: Deoxygenated blood is pumped from the right ventricle to the lungs for oxygenation and returned to the left atrium.
$2$. Systemic Circulation: Oxygenated blood is pumped from the left ventricle to all body tissues and returned as deoxygenated blood to the right atrium.