Q.Describe the events in cardiac cycle. Explain "double circulation".

The cardiac cycle begins with all four chambers in joint diastole, during which blood flows passively from the veins through the open tricuspid and bicuspid valves into the ventricles. The SA node then fires, triggering atrial systole, which pushes in an additional roughly 30 per cent of blood; the impulse then passes via the AV node and AV bundle to the ventricles, triggering ventricular systole, during which rising ventricular pressure shuts the tricuspid/bicuspid valves and then forces open the semilunar valves, ejecting blood into the pulmonary artery and aorta. The ventricles then relax (ventricular diastole), the semilunar valves close, and falling ventricular pressure lets the tricuspid/bicuspid valves reopen, returning all chambers to joint diastole as the cycle repeats roughly 72 times a minute. Double circulation means this pumping drives two separate loops: pulmonary circulation, in which the right ventricle sends deoxygenated blood to the lungs to be oxygenated and returned to the left atrium, and systemic circulation, in which the left ventricl …

The cardiac cycle. At the outset, all four chambers of the heart are in joint diastole: the tricuspid and bicuspid valves are open, and blood arriving from the pulmonary veins and vena cava flows passively through the atria into the ventricles, while the semilunar valves stay closed. The SA node then fires an action potential, triggering atrial systole — both atria contract together, pushing an additional roughly 30 per cent of blood into the ventricles.

The impulse is then carried onward by the AV node and the AV bundle to the ventricles, which contract (ventricular systole) while the atria relax. As ventricular pressure rises, it first shuts the tricuspid and bicuspid valves (preventing backflow into the atria), and once pressure climbs high enough, it forces open the semilunar valves guarding the pulmonary artery and aorta, ejecting blood out of the heart. The ventricles then relax (ventricular diastole); falling pressure closes the semilunar valves, and once pressure drops further, the tricuspid and bicuspid valves reopen, letting blood flow in again, returning all chambers to joint diastole. This entire sequence repeats roughly 72 times a minute, with each ventricle ejecting about 70 mL of blood (the stroke volume) per cycle, giving a cardiac output of roughly 5000 mL per minute. …

Method: Structuring a Multi-Phase Physiological Process Description

Use this approach for "describe the events in..." questions covering a repeating multi-step physiological cycle, especially when paired with a second concept to explain.

Steps

Step 1: Fix the starting point and the phase order

Pick one consistent starting phase for the cycle (here, joint diastole) and list the phases strictly in their real temporal order -- atrial systole, ventricular systole, ventricular diastole, back to joint diastole. Never describe phases out of sequence, since the cycle's logic depends on order.

Step 2: For each phase, state what triggers it, what happens, and what it enables next

For every phase, cover three things: (i) what electrical/pressure event triggers this phase,

(ii) which valves open/close and which chambers move blood,

(iii) how it sets up the next phase -- e.g., rising ventricular pressure shuts the AV valves, then opens the semilunar valves.

Step 3: Treat "explain double circulation" as a second, separate structure …