Fisiología Cardíaca: Ciclo Cardíaco (Períodos, Presiones, Volúmenes) | Ruidos Cardíacos S1-S2

Main ideas / concepts

• Cardiac cycle definition: The cardiac cycle is a synchronized sequence of electrical and mechanical events that begins at the start of one heartbeat and ends at the start of the next.
• Electrical control: Governed by the heart’s electrical/cardio-electrical system, mainly the sinoatrial (SA) node.
• Two major phases:
  • Systole = contraction phase
  • Diastole = relaxation phase
• Cycle duration depends on heart rate (HR):
  • Cardiac cycle duration ≈ 1 / HR (in minutes), converted to seconds for typical values.
  • Example: HR ≈ 75 beats/min → cycle duration ≈ 0.8 seconds
  • Typical split (at ~75 bpm):
    • Systole: ~0.2–0.3 s
    • Diastole: ~0.5 s
  • If HR increases (e.g., 80–90 bpm or more), the whole cycle shortens, and both systole and diastole become briefer.

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Five phases / distinct periods (with what happens)

1. Passive rapid filling (ventricular filling) — diastole

1. Blood flows from atria into ventricles directly.
2. ~80% of total incoming blood occurs this way.
3. Atrioventricular (AV) valves open
4. Aortic and pulmonic valves closed
5. Remaining ~20% is handled by the next filling mechanism (described as assisted recirculation).

2. Active rapid filling (ventricular filling) — diastole

• Adds additional blood due to atrial contraction.
• Additional ~20% enters ventricles.
• At the end of this phase:
  • Ventricular volume ≈ 120 mL
  • Named end-diastolic volume (EDV)
• Example pressures:
  • Left atrium: ~7–8 mmHg
  • Right atrium: ~4–6 mmHg
  • Ventricles start low: ~2–3 mmHg
  • End of this step: ventricles rise to ~5–7 mmHg
• AV valves remain open; aortic/pulmonic valves still closed at this moment.

3. Isovolumetric (isometric) contraction — early systole

• Ventricular muscle contracts without changing ventricular volume (no ejection yet).
• Blood volume stays about ~120 mL.
• Pressures rise sharply:
  • Right ventricle: ~8 mmHg
  • Left ventricle: ~~80 mmHg
• Valve logic (key threshold idea):
  • Pulmonic valve opens when RV pressure is high enough (threshold: ~8 mmHg vs pulmonary artery pressure ~8 mmHg).
  • Aortic valve opens when LV pressure exceeds aortic pressure (aorta given as ~80 mmHg).
• AV valves close
• Aortic and pulmonic valves begin opening

4. Ejection phase — systole

• Blood is ejected:
  • LV → aorta
  • RV → pulmonary artery
• Pressures generated (example values):
  • RV: ~25 mmHg
  • LV: ~120 mmHg
• Concepts (as described):
  • Afterload: arterial pressure the ventricle must overcome (e.g., aorta ~80 mmHg; pulmonic ~8 mmHg)
  • Preload: pressure/force present in the ventricle leading into ejection (described using the generated systolic pressure context)
• Blood pressure connection (as stated):
  • Diastolic BP: ~80 mmHg (aortic pressure)
  • Systolic BP: ~120 mmHg (ventricular pressure during ejection)
• Stroke volume / ejection fraction (defined conceptually via numbers given):
  • From EDV ~120 mL, about ~60% is ejected each beat.
  • 60% of 120 mL ≈ 72 mL
  • This ejected amount is stroke volume
  • Serves as an index of ventricular function
• Sub-phases of ejection:
  • Rapid ejection: ~70% of the ejected 72 mL → ~43 mL
  • Slow (continued) ejection: remaining ~30% → ~29 mL
• Heart sounds during this period:
  • First heart sound (S1): produced by closure of AV valves
• Valve behavior:
  • In normal conditions, AV valves remain closed, while aortic/pulmonic valves are open during ejection.

5. Volumetric relaxation / ventricular diastole — end systole into diastole

• End-systolic volume (ESV): the part not ejected stays in ventricles.
  • If EDV ~120 mL and ~60% ejected → remaining ~40%
  • ~48 mL (typical range stated: 40–50 mL)
• Ventricular pressures drop rapidly back to ~2–3 mmHg.
• Valve transitions:
  • After contraction stops:
    • AV valves open
    • aortic/pulmonic valves close

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Heart sound events (S1 and S2) tied to valve closures

• S1 (first heart sound): closure of AV valves (occurs during transition into ejection).
• S2 (second heart sound): closure of aortic and pulmonic valves (occurs when ventricular contraction ends and arterial pressure forces backflow).

Mechanism described for S2:

• During ejection, arterial pressure is high.
• When contraction ceases, ventricular pressure no longer sustains forward flow.
• Blood tends to flow back → aortic/pulmonic valves close → S2.

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Coronary perfusion / nourishment concept

• During diastole, when aortic and pulmonic valves close, blood is directed into sinuses of Valsalva.
• This filling supports coronary arteries, which nourish the heart.
• Overall conclusion stated: the heart is irrigated/nourished in diastole.

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Cardiac output (CO) as a final integration

• The cycle repeats continuously.
• Stroke volume (ejected per beat) plus repetition over time determines output:
  • If ~72 mL/beat and beats occur for 60 seconds, then:
  • Cardiac output = volume ejected into the aorta from the left ventricle per minute.

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Speakers / sources featured

• No specific speakers are mentioned in the subtitles.
• The content is presented as an educational narration about physiology (no named external authors or institutions).