Mechanical Events of the Cardiac Cycle – Key Vocabulary

Cardiac Cycle as a Repeating Loop

• The heart cycle can be described from any point because it is continuous; in this video the description begins late in diastole (the pause between beats).
  • All four chambers (atria + ventricles) are relaxed ➜ pressures are low.
  • Blood flows passively from veins → atria → ventricles because \Delta P = P{veins}-P{ventricles} > 0 (no ATP required by myocardium).

Electrical Triggers & Their Mechanical Outcomes

• SA-node firing
  • Initiates atrial depolarization (P-wave on ECG, shown later).
  • Leads to atrial systole (active filling).
• AV-node delay (~0.1 s)
  • Ensures atria finish contracting before ventricles start.
  • Fibrous skeleton electrically insulates atria from ventricles.
• Impulse travels through Bundle of His → bundle branches → Purkinje fibers ➜ ventricular depolarization and contraction.

Phase-by-Phase Mechanical Events

1. Passive Ventricular Filling (late diastole)

• P{ventricles} < P{atria} ➜ AV valves open.
• Gradual rise in both atrial & ventricular pressures as volume increases.
• Ventricular volume tracing: gentle upward slope.

2. Active Filling (atrial systole)

• Atria contract, expend ATP ➜ final “atrial kick.”
• Sharp bump in both pressures and ventricular volume.
• Contributes a minority of total ventricular filling (majority is passive).

3. Isovolumetric Ventricular Contraction (first half of ventricular systole)

• Ventricular myocardium begins to contract ➜ rapid pressure rise.
• Valve status:
  • AV valves snap shut when P{ventricles} > P{atria} (no further filling).
  • Semilunar valves still closed because P{ventricles} < P{artery}.
• All four valves closed ⇒ ventricular volume constant (horizontal line on V-volume graph).
• Produces S₁ ("lub") heart sound (turbulent back-flow through narrowing AV valves).

4. Ventricular Ejection (second half of ventricular systole)

• Continues contraction ➜ P{ventricles} > P{artery}.
• Semilunar valves open → blood exits to aorta/pulmonary artery.
• Ventricular pressure slightly > arterial throughout ejection.
• Ventricular volume falls steeply; end-systolic volume remains (cannot eject 100 %).

5. Isovolumetric Relaxation (early ventricular diastole)

• Ventricular muscle begins relaxing ➜ pressure drops fast.
• Semilunar valves close instantly when P{ventricles} < P{artery}.
• AV valves still closed because P{ventricles} > P{atria}.
• Volume constant again (horizontal segment) ➜ defines isovolumetric relaxation.
• Generates S₂ ("dub") as semilunar valves shut; sound limited to early part of phase.

6. Return to Passive Filling

• Complete relaxation ⇒ P{ventricles} < P{atria}.
• AV valves reopen; cycle restarts with passive filling.

Interpreting Pressure–Volume & Pressure–Time Graphs

• Top panel: Pressures (mm Hg)
  • Green = left ventricular
  • Purple = left atrial
  • Blue = aortic (arterial)
• Bottom panel: Ventricular volume (mL)
• Key observations:
  • Passive filling: green < purple; both slopes up.
  • Atrial systole: small spike in both pressures & volume.
  • Isovolumetric contraction: green rises steeply; no change in volume.
  • Ejection: green just above blue; volume declines.
  • Isovolumetric relaxation: green falls below blue; volume plateau.
  • Aortic pressure rises during ejection then decays (to be addressed in vascular unit).

Valve Dynamics Summarized

Origin of Normal Heart Sounds

• Mechanism: Turbulent blood flow through a narrowing orifice under high pressure → tissue vibrations → sound waves.
• Materials: Valves = fibrous connective tissue (flexible, not rigid); sound is NOT valves “slamming.”
• Timing
  • S₁: during closure of AV valves (early isovolumetric contraction).
  • S₂: during closure of semilunar valves (early isovolumetric relaxation).
• Sounds fade quickly once valves fully shut and turbulence stops.

Abnormal Sounds: Heart Murmurs

• Defined as a sound outside S₁–S₂ windows.
• Always indicate turbulent flow through an inappropriate narrow opening.
• Common anatomical causes:
  • Valvular stenosis (doesn’t open fully) or insufficiency/regurgitation (doesn’t close fully).
  • Congenital septal defects (interatrial or interventricular holes).
• Clinical importance: location in cycle pinpoints defect (systolic vs diastolic murmurs).

Practical / Clinical Connections

• Understanding pressure relationships guides stethoscope auscultation, Doppler studies, and interpretation of pressure–volume loops used in cardiology.
• Quantities such as stroke volume = EDV - ESV and ejection fraction = \frac{SV}{EDV} derive from these mechanical events.
• AV-node delay and fibrous skeleton are crucial; damage (e.g., ischemia, fibrosis) can lead to atrioventricular block and disrupt the precise sequence, reducing cardiac efficiency.
• Recognition of murmurs assists early detection of congenital heart disease, valvular degeneration, rheumatic fever sequelae, etc.
• Energy-saving design: majority of ventricular filling is passive, minimizing myocardial oxygen demand.

Key Numerical/Formula References

• Typical left-side pressures (rest):
  • P_{atria}^{max} \approx 10\,\text{mmHg}
  • P_{ventricle}^{systole} \approx 120\,\text{mmHg}
  • P_{aorta}^{rest} \approx 80\,\text{mmHg} diastolic → 120\,\text{mmHg} systolic.
• Normal end-diastolic volume (EDV): \sim 120\,\text{mL}; end-systolic volume (ESV): \sim 50\,\text{mL}.
• Stroke volume: SV = 70\,\text{mL} (illustrative average).
• Flow principle: \text{Flow} = \dfrac{\Delta P}{R} (Ohm’s law analogy for fluids).

Conceptual Take-Aways

• The heart functions as a pressure pump; valve status is entirely dictated by instantaneous pressure gradients.
• Iso-volumetric phases are "all-valves-closed" intervals critical for pressure transitions.
• Atrial contribution is small but significant for maximal ventricular preload, especially when heart rate is high or in certain pathologies.
• Heart sounds are hemodynamic, not mechanical “clicks;” any extra sound reflects pathology.
• Graphical literacy (pressure-time, pressure-volume) is essential for higher-level cardiovascular physiology and clinical practice.