🫀 Cardiac Cycle — Quick & Easy Notes

What is the Cardiac Cycle?

One complete heartbeat = one cardiac cycle (~430 ms) It includes systole (contraction) + diastole (relaxation)

🔑 Key Terms

⚙️ 3 Main Phases (Simple)

1️⃣ Atrial Systole & Diastole

• Atria contract → push remaining 20–30% blood into ventricles
• Called "atrial kick"
• ECG: P wave

2️⃣ Ventricular Systole

• Ventricles contract → blood pumped to lungs & body
• Two stages:
  • Isovolumetric contraction (valves closed, pressure builds)
  • Ventricular ejection (pressure opens semilunar valves, blood exits)
• ECG: QRS complex

3️⃣ Ventricular Diastole

• Ventricles relax → semilunar valves close (dicrotic notch)
• Blood flows back from atria into ventricles
• Two stages:
  • Isovolumetric relaxation
  • Ventricular filling
• ECG: T wave

🔄 Pressures & Flow (Simple Rule)

Blood always flows from HIGH pressure → LOW pressure

• Atria pressure > Ventricle pressure → blood flows in ✅
• Ventricle pressure > Aorta/Pulmonary pressure → blood flows out ✅

⭐ IMPORTANCE (Must Know!)

The cardiac cycle is the foundation of all cardiovascular function.

• Without proper coordination of systole & diastole, organs don't get blood
• Understanding it explains heart sounds (lub = AV valves closing, dub = semilunar valves closing)
• It's the basis for reading ECGs, understanding heart failure, valve diseases, and cardiac output
• Stroke volume & EDV are critical in conditions like heart failure, where EDV rises but pumping efficiency drops

📊 ECG Summary

P wave     → Atrial depolarization (atria contract)
QRS complex → Ventricular depolarization (ventricles contract)
T wave     → Ventricular repolarization (ventricles relax)

"P pumps atria, QRS pumps ventricles, T takes a rest" 🧠

Cardiac Cycle

Anatomy & Physiology Notes

What is it? 🤔

• Squeeze = Systole (pumping blood out)
• Relax = Diastole (filling with blood)

Pressure & Flow — the golden rule 📌

Blood moves from high pressure → low pressure (just like water flows downhill)

The 3 Phases — step by step 📝

Phase 1 — Atrial Systole

• Both atria contract
• Push the last 20–30% of blood into the ventricles
• This is called the "atrial kick"
• AV valves (tricuspid & mitral) are OPEN
• Semilunar valves are CLOSED

📌 ECG shows → P wave

Phase 2 — Ventricular Systole

• Ventricles start squeezing
• ALL valves are CLOSED
• Pressure builds up inside
• No blood is moving yet

• Pressure gets high enough
• Semilunar valves OPEN
• Blood shoots out to lungs & body 🫁❤️
• Volume in ventricle before ejection = EDV (~130 mL)
• Volume left after ejection = ESV (~60 mL)
• Amount pumped = Stroke Volume = 70 mL

📌 ECG shows → QRS complex

Phase 3 — Ventricular Diastole

• Ventricles stop squeezing
• Semilunar valves CLOSE ← causes "dub" sound
• ALL valves closed again
• Pressure drops

• AV valves OPEN
• Blood flows in from atria (70–80% fills passively)
• Atria still relaxed = atrial diastole

📌 ECG shows → T wave

Heart Sounds in Simple Words 🔊

Quick Numbers to Remember 🔢

• EDV = 130 mL
• ESV = 60 mL
• Stroke Volume = 70 mL
• Atrial kick fills = 20–30%
• Passive filling = 70–80%

⭐ Why is this Important?

• Every drug, disease, or treatment affecting the heart links back to this cycle
• Heart failure = cycle goes wrong → body doesn't get enough blood
• ECG is just this cycle drawn as waves
• Valve diseases = the open/close timing is disturbed
• Doctors use EDV, ESV, Stroke Volume to check if heart is working well

If you understand the cardiac cycle, you understand the heart.

Super Short Summary ✍️

Atria squeeze (P wave)
    ↓
Ventricles squeeze — build pressure (QRS)
    ↓
Blood pumped out
    ↓
Ventricles relax (T wave)
    ↓
Heart fills again
    ↓
REPEAT ♾️

Cardiac Cycle 🫀

What is Cardiac Cycle?

⭐ Both atria and ventricles undergo systole and diastole — blood must be carefully regulated to pump efficiently to the body

Systole vs Diastole

• Systole = chambers fill with blood
• Diastole = chambers contract, pumping blood out

⭐ It is essential these are coordinated so blood is pumped efficiently

Pressures and Flow

⭐ When blood flows into atria → pressure rises → blood moves into ventricles → atrial systole raises pressure further → pumps blood into ventricles

Phases

Atrial Systole

• Atrial muscles contract from superior portion toward septum
• Pumps blood through AV valves (tricuspid & mitral)
• Atrial kick = fills remaining 20–30% of ventricles
• Lasts ~100 ms

⭐ Atrial contraction contributes the remaining 20–30% of ventricular filling

Ventricular Systole

• Follows QRS complex on ECG
• Lasts ~270 ms

• All valves closed
• Pressure builds — no blood moves yet

⭐ Blood cannot be ejected until pressure in ventricles exceeds pressure in pulmonary trunk and aorta

• Semilunar valves open
• Blood pumped to lungs & body
• EDV = ~130 mL → ESV = ~60 mL
• Stroke volume = 70–80 mL

⭐ Stroke volume = amount of blood pumped per beat (EDV − ESV)

Ventricular Diastole

• Follows T wave on ECG
• Ventricles relax → semilunar valves close (dicrotic notch)

• All valves closed
• Pressure drops inside ventricle

⭐ No change in volume during this phase — valves are all shut

• AV valves open
• Blood flows from atria → ventricles
• 70–80% fills passively

⭐ Both ventricles pump the same amount of blood — ensuring balanced circulation

ECG Connection

⭐ The cardiac cycle and ECG are directly linked — every wave = a mechanical event in the cycle

Heart Sounds

⭐ Heart sounds are detected by auscultation and reflect valve activity during the cycle

Key Numbers

Cardiac Cycle — 10 Marks Answer

Definition

Systole and Diastole

• Systole — The period during which the heart contracts and pumps blood out
• Diastole — The period during which the heart relaxes and chambers fill with blood
• Both atria and ventricles undergo systole and diastole
• It is essential that these components are carefully regulated and coordinated to ensure blood is pumped efficiently to the body

Pressures and Flow

• As blood flows into the atria from veins, pressure rises
• This causes blood to move into the ventricles
• During atrial systole, pressure rises further, pumping blood into the ventricles
• During ventricular systole, pressure exceeds that of the pulmonary trunk and aorta, ejecting blood

Phases of the Cardiac Cycle

Phase 1 — Atrial Systole

• Atria contract from the superior portion toward the atrioventricular septum
• Blood is pumped through the tricuspid and mitral (bicuspid) valves into the ventricles
• This is called the "atrial kick" — contributing the remaining 20–30% of ventricular filling
• The remaining 70–80% had already filled passively before this phase
• Atrial systole lasts approximately 100 ms
• ECG correlation → P wave (atrial depolarization)

Phase 2 — Ventricular Systole

A. Isovolumetric Contraction

• Ventricular muscles begin to contract
• All valves are closed — no blood is moving
• Pressure builds rapidly inside the ventricles
• Blood cannot be ejected until ventricular pressure exceeds the pressure in the pulmonary trunk and aorta

B. Ventricular Ejection

• Pressure overcomes resistance → semilunar valves open
• Blood is pumped:
  • Right ventricle → pulmonary trunk → lungs
  • Left ventricle → aorta → body
• Volume before ejection = End Diastolic Volume (EDV) ≈ 130 mL
• Volume remaining after ejection = End Systolic Volume (ESV) ≈ 60 mL
• Stroke Volume = EDV − ESV = ~70 mL

Phase 3 — Ventricular Diastole

A. Isovolumetric Relaxation

• Ventricular muscles relax
• Semilunar valves close → produces the "dub" heart sound
• All valves are closed — volume in ventricle does not change
• Pressure drops inside the ventricle

B. Ventricular Filling

• When ventricular pressure drops below atrial pressure → AV valves open
• Blood flows passively from atria into ventricles (70–80% of filling)
• Atria are in diastole during this phase
• AV valves (tricuspid and mitral) are open; semilunar valves remain closed
• The cardiac cycle is now complete and begins again

Heart Sounds

Heart sounds are detected by auscultation and directly reflect valve activity within the cycle

ECG Correlation

Key Values

Conclusion