Cardiac Cycle Mechanics Lecture

Overview: Mechanical Layer of the Cardiac Cycle

• Think of the cardiac cycle as a multilayered story.
  • Electrical layer (previously covered): timing established by the SA node, atrial depolarization → ventricular depolarization.
  • NEW mechanical layer (this video): how contraction/relaxation changes chamber volumes & pressures to create pump action.
• Sequence (big picture):
  • Atria contract → Ventricles contract → Brief pause (all chambers relaxed) → Next cycle.

Fundamental Physical Principles

Pous​euille’s* Law (simplified)

• Blood (any fluid) flows only from higher pressure to lower pressure.
• If $\Delta P = 0$ (no pressure gradient) ⇒ no flow.
• Applies to liquids and gases; same logic explains airflow in lungs.
• Immediate diagnostic insight:
  • If blood exits a ventricle into its artery → $P<em>{ventricle} > P</em>{artery}$ at that moment.
  • If flow stops → pressures have equalized or reversed.

Boyle’s Law (simplified, inverse relation)

• For a closed container of fluid/gas: $P \propto \frac{1}{V}$ or $P<em>1V</em>1 = P<em>2V</em>2$.
  • ↓ Volume (contraction) → ↑ Pressure.
  • ↑ Volume (relaxation/expansion) → ↓ Pressure.
• Heart applies Boyle’s Law every beat.

Blood-Pressure Concept Refresher

• Blood pressure = force exerted by blood against its container wall.
  • Arteries, veins, capillaries, atria, ventricles all have their own pressures.
• Visual analogy: Two people pushing opposite sides of a door = competing pressures across a valve.

Valves & Flow Direction

• Atrioventricular (AV) valves: tricuspid (R), bicuspid/mitral (L).
• Semilunar (SL) valves: pulmonary (R), aortic (L).
• Valves open/close passively in response to the direction of $\Delta P$:
  • $P<em>{upstream} > P</em>{downstream}$ → valve swings open.
  • $P<em>{upstream} < P</em>{downstream}$ → valve closes to prevent regurgitation.

Phases & Sub-Phases of the Cardiac Cycle

(Ventricular activity emphasized because ventricles propel blood through pulmonary & systemic circuits.)

1. Ventricular Systole (Contraction)

• A. Isovolumetric Contraction
  • Event sequence:
  • Ventricles begin to contract.
  • AV valves slam shut (↑ ventricular pressure exceeds atrial pressure) → first heart sound (S₁).
  • SL valves still closed (ventricular pressure < arterial pressure).
  • Result: No change in ventricular volume (iso-volumetric) yet pressure rises steeply.
• B. Ventricular Ejection
  • Continual contraction → $P<em>{ventricles}$ finally > $P</em>{arteries}$.
  • SL valves open; blood exits to aorta & pulmonary trunk.
  • Flow rate proportional to size of $\Delta P$.

2. Ventricular Diastole (Relaxation)

• A. Isovolumetric Relaxation
  • Ventricles start to relax → volume ↑, pressure ↓ rapidly.
  • When $P<em>{ventricles}$ just falls below $P</em>{arteries}$ → SL valves snap shut → second heart sound (S₂).
  • AV valves not open yet (ventricular pressure still > atrial).
  • Result: Fixed residual volume for a brief moment.
• B. Ventricular Filling
  • Passive Filling (≈80% of ventricular EDV)
  • Ventricles fully relaxed; $P<em>{ventricles} < P</em>{atria}$.
  • AV valves open → continuous column of flow from veins → atria → ventricles.
  • Entire heart is relaxed (pause between beats).
  • Active Filling (Atrial Systole; ≈20% "atrial kick")
  • SA node fires → atria contract.
  • Atrial contraction ↓ atrial volume → ↑ atrial pressure.
  • Extra bolus of blood driven into ventricles, topping them to End-Diastolic Volume (EDV).

Detailed Walk-Through (Starting at Ventricular Systole)

• Pre-contraction (end of ventricular filling):
  • AV valves open, SL valves closed, ventricles at maximal volume (EDV).
• Isovolumetric Contraction:
  • Instantaneous rise in $P_{vent}$ closes AV valves.
  • No inflow/outflow; pressure skyrockets.
• Ventricular Ejection:
  • $P<em>{vent}$ surpasses $P</em>{artery}$ → SL valves open.
  • Stroke Volume (SV) ejected; remainder = End-Systolic Volume (ESV).
• Isovolumetric Relaxation:
  • Ventricular muscle relaxes; volume still fixed at ESV.
  • SL valves close as $P<em>{vent} < P</em>{artery}$; AV valves still shut.
• Passive Filling:
  • $P<em>{vent}$ falls below $P</em>{atria}$ → AV valves reopen.
  • Blood flows from veins through atria into ventricles.
• Active Filling (Atrial Systole):
  • Atria contract, boosting ventricular volume by ~20%.
  • Ventricles now primed; cycle repeats.

Analogies, Examples, & Broader Relevance

• Diffusion comparison: Concentration gradient drives solute flow just as pressure gradient drives fluid flow.
• Respiratory system: Boyle’s Law governs thoracic volume changes → air pressure gradients for breathing.
• Clinical tie-in:
  • Valve defects (e.g., regurgitation) disrupt the expected pressure gradients & audible heart sounds.
  • Measuring pressures (cardiac catheterization) can confirm valve competence & phase timing.

Practical / Ethical / Philosophical Notes

• Understanding pressure-flow relations is foundational for:
  • Safe administration of IV fluids & blood products.
  • Interpreting blood pressure readings & murmurs.
  • Designing mechanical circulatory supports (e.g., LVADs) that must replicate physiologic gradients.
• Ethical duty: Clinicians must respect these principles to avoid iatrogenic hemodynamic disturbances.

Key Numerical Relationships & Terminology

• $\text{Stroke Volume} = EDV - ESV$
• Typical values (healthy adult): $EDV \approx 120\,mL$, $ESV \approx 50\,mL$ → $SV \approx 70\,mL$ (not given in transcript but foundational).
• Heart sounds:
  • $S_1$ = AV valve closure (marks onset of isovolumetric contraction).
  • $S_2$ = SL valve closure (marks onset of isovolumetric relaxation).

Core Takeaways

• Pressure gradients and valve states dictate every movement of blood.
• Boyle’s Law + Pous​euille’s Law = mechanical explanation for the electrical timing you already learned.
• Sub-phasing (isovolumetric vs. filling/ejection) provides diagnostic precision when analyzing heart sounds, pressure curves, or imaging.