Feynman Note — Explaining the Cardiac Cycle to a Child — Notes

The Concept

The cardiac cycle is the complete sequence of events that happen in one heartbeat — from when the heart fills with blood to when it pumps it out, all the way to the next filling.

Explaining It Simply

Imagine your heart is a house with FOUR rooms arranged in two pairs. On the right side: a hallway (right atrium) connected to a big room (right ventricle). On the left side: another hallway (left atrium) and a larger, stronger room (left ventricle).

The heart has a job: pump blood in a circle, nonstop, your whole life.

Here is one complete heartbeat, broken into three steps:

Step 1 — Atrial Systole (0.1 seconds): The hallways squeeze. The two hallways (atria) squeeze gently, pushing any remaining blood down through doors (AV valves: tricuspid on right, mitral on left) into the big rooms (ventricles). The doors that lead to the arteries (semilunar valves) stay shut.

Step 2 — Ventricular Systole (0.3 seconds): The big rooms squeeze hard. First, all doors close momentarily (isovolumetric contraction — pressure builds). Then, when the big rooms are squeezed hard enough, they push open the exit doors (semilunar valves) and blast blood into the arteries. The right room sends blood to the lungs (pulmonary artery); the left room sends blood to the whole body (aorta).

Step 3 — Joint Diastole (0.4 seconds): Everything relaxes and refills. All rooms relax. The exit doors (semilunar valves) snap shut to prevent blood flowing back. The hallway doors (AV valves) open again. Blood flows in from veins → atria → passively into ventricles. The house is ready for the next beat.

Total time: 0.8 seconds. Heart rate: 75 beats per minute.

The Alarm System (Conducting System)

The heart has its own electrical alarm clock — the SAN — that fires 70-75 times per minute. It sends an electrical signal (like a text message) from the atria down to the ventricles through the AVN (which pauses 0.1 s — a polite delay so the atria can finish their work) and then through cables (Bundle of His + Purkinje fibres) to all ventricular muscle cells at once.

Why This Is Hard to Get Wrong

Once you understand WHY the valves open and close (pressure differences), you can reconstruct the cardiac cycle from scratch. AV valves open when atrial pressure > ventricular pressure (filling). AV valves close when ventricular pressure > atrial pressure (systole starts). Semilunar valves open when ventricular pressure > arterial pressure (ejection). Semilunar valves close when arterial pressure > ventricular pressure (diastole).