Explain the cardiac cycle — Biology notes

WHAT is the cardiac cycle?

WHY 0.8 s? Because duration = total time ÷ number of events. 60 seconds contains 75 beats, so each beat gets $60/75 = 0.8$ s. This is a derivation, not a fact — change the heart rate and you recompute.

The 0.8 s splits into:

Phase	Duration	What's happening
Atrial systole	~0.1 s	Atria contract, top up ventricles
Ventricular systole	~0.3 s	Ventricles contract, eject blood
Joint diastole	~0.4 s	All chambers relax, passive filling

Check: $0.1 + 0.3 + 0.4 = 0.8$ s ✓

HOW it works — step by step (follow the pressure!)

Phase 1 — Atrial systole (0.1 s)

Atria contract → atrial pressure rises above ventricular pressure.
AV valves (tricuspid + bicuspid/mitral) are already open → last ~20–30% of blood is pushed into ventricles.
Semilunar valves stay shut (aorta/pulmonary pressure is higher than ventricle).

Phase 2 — Ventricular systole (0.3 s)

Isovolumetric contraction: ventricles start squeezing. Ventricular pressure shoots above atrial pressure → AV valves slam shut → this makes the first heart sound "LUB" (S1). All valves briefly shut → volume constant.
Ejection: ventricular pressure exceeds aortic/pulmonary pressure → semilunar valves open → blood ejected into aorta & pulmonary artery.

Phase 3 — Ventricular diastole / Joint diastole (0.4 s)

Ventricles relax → ventricular pressure drops below aortic pressure → aortic blood tries to flow back → semilunar valves snap shut → second heart sound "DUB" (S2).
Isovolumetric relaxation: all valves shut momentarily.
When ventricular pressure falls below atrial pressure → AV valves open → ventricles fill passively (~70–80% of filling is passive, before the next atrial systole).

Formula Cardiac output — derived

WHY: We want to know how much blood the heart pumps per minute.

Stroke volume (SV) = blood ejected per beat = End-Diastolic Volume − End-Systolic Volume. $SV = EDV - ESV = 120 - 50 = 70\ \text{mL}$ Why this step? The ventricle fills to ~120 mL (EDV) and can't empty completely (~50 mL left = ESV). The difference is what actually leaves.
Cardiac output (CO) = volume per beat × beats per minute: $CO = SV \times HR = 70\ \text{mL} \times 75\ \text{min}^{-1} = 5250\ \text{mL/min} \approx 5\ \text{L/min}$ Why this step? "Per minute" total = "per beat" × "beats per minute" — same logic as $0.8$ s above, run in reverse.

Worked examples

Steel-man your mistakes

Flashcards

How long is one cardiac cycle at 75 bpm, and how do you get it?

0.8 s, from

T = 60/75

What causes the first heart sound (S1, "LUB")?

Closing of the AV (tricuspid + bicuspid) valves at the start of ventricular systole.

What causes the second heart sound (S2, "DUB")?

Closing of the semilunar (aortic + pulmonary) valves at the start of ventricular diastole.

State the golden rule for when a valve opens.

It opens when pressure behind it exceeds pressure ahead of it (flow high→low).

Define stroke volume and give the formula.

Blood ejected per beat;

SV = EDV - ESV

(e.g.

120-50=70

mL).

Write the cardiac output formula and a resting value.

CO = SV \times HR = 70 \times 75 \approx 5

L/min.

What fraction of ventricular filling is passive?

~70–80%, during diastole before atrial contraction.

What is isovolumetric contraction?

Brief phase where ventricles contract with ALL valves shut, so pressure rises but volume is constant.

Why does very high heart rate reduce output?

Cycle length shrinks (

T=60/HR

), cutting diastolic filling time → lower EDV → lower SV.

Recall Feynman: explain to a 12-year-old

Imagine the heart as two squishy water balloons stacked on top of two bigger ones, with little swinging doors between them. First the top balloons squeeze a bit to push water into the bottom ones. Then the big bottom balloons squeeze hard — the doors between top and bottom slam shut ("LUB") so water can't go back up, and instead it shoots out through exit doors into the pipes. Then everything relaxes, the exit doors slam shut ("DUB"), and water quietly refills from above. Squeeze, shoot, relax, refill — over and over, about once every 0.8 seconds. Water always flows from the tighter squeeze to the looser space, and every door only opens one way.

Connections

Heart Structure and Chambers — the anatomy that these pressures act on.
Cardiac Valves — AV vs semilunar mechanics.
Heart Sounds and Auscultation — S1/S2 clinical use.
Cardiac Output and Regulation — extends $CO = SV \times HR$ .
Electrical Conduction of the Heart — the SA/AV node signals that time each phase.
Blood Pressure — systolic/diastolic pressures come from this cycle.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, cardiac cycle ka poora funda ek hi baat par tika hai: blood hamesha high pressure se low pressure ki taraf jata hai, aur valves sirf one-way darwaze hain. Jab kisi valve ke peeche ka pressure aage ke pressure se zyada ho jata hai, valve khul jata hai; aur jaise hi blood ulta jaane ki koshish kare, valve turant band ho jata hai. Bas yahi rule yaad rakho aur pura cycle khud predict kar sakte ho — ratna nahi padega.

75 beats per minute par ek cycle $60/75 = 0.8$ second ka hota hai. Isme teen phases: pehle atrial systole (0.1 s) — atria squeeze karke ventricles ko top-up karte hain. Phir ventricular systole (0.3 s) — ventricles zor se squeeze karte hain, AV valves band (yahi "LUB"/S1 sound), aur semilunar valves khulke blood aorta/pulmonary artery me shoot ho jata hai. Phir diastole (0.4 s) — sab relax, semilunar valves band ("DUB"/S2 sound), aur ventricles passive tarike se dobara bhar jate hain.

Do important formulas derive karo, ratna mat: Stroke Volume $= EDV - ESV = 120 - 50 = 70$ mL, kyunki ventricle poora khali nahi hota (50 mL bacha rehta hai). Aur Cardiac Output $= SV \times HR = 70 \times 75 \approx 5$ L/min. Exercise me HR aur SV dono badhte hain, isliye output 15 L/min tak ja sakta hai.

Sabse common galti: log sochte hain valve khulne se sound aata hai — galat! Sound valve ke band hone (snap) se aata hai. Aur doosri galti: atrial contraction se filling hoti hai — nahi, 70-80% filling to passive hoti hai diastole me, atria sirf last thoda top-up karte hain. In do baaton ko theek rakho, exam me confuse nahi hoge.

Test yourself — Circulatory System

Connections