Cardiac Cycle & Heart Sounds – Study Notes

Cardiac Cycle: General Overview

• One complete heartbeat = series of coordinated contractions (systole) and relaxations (diastole) of atria and ventricles.
• Blood flows continuously; an arbitrary starting point is chosen: heart in total relaxation (mid-to-late diastole).
• Three principal mechanical phases repeat cyclically:
  • Ventricular filling (diastolic phase 1)
  • Isovolumetric contraction ➜ Ventricular ejection (systolic phase 2)
  • Isovolumetric relaxation (early diastolic phase 3)
• Electrical precedes mechanical:
  • $P$ wave ➜ atrial depolarization/contraction
  • $QRS$ complex ➜ ventricular depolarization/contraction; atria repolarize simultaneously
  • $T$ wave ➜ ventricular repolarization/relaxation

Phase 1 – Ventricular Filling (Mid-to-Late Diastole)

• Pressures everywhere are low; AV (tricuspid & mitral) valves are open.
• Blood returning from systemic & pulmonary veins flows passively through atria into ventricles.
• Semilunar (SL) valves (aortic & pulmonary) remain closed.
• Passive filling supplies > 80 % of ventricular end-diastolic volume.
• Near end of diastole:
  • Atrial systole (triggered by $P$ wave) forces the remaining ~20 % of blood into ventricles.
  • Ventricular chambers now hold their peak volume, the End Diastolic Volume (EDV).
  • Slight, brief rise in atrial pressure accompanies this “atrial kick.”
• Atria then enter diastole for the rest of the cycle.

Phase 2 – Isovolumetric Contraction → Ventricular Ejection (Systole)

• Begins with ventricular depolarization ($QRS$ complex).
• Ventricular muscle contracts, intraventricular pressure rises sharply.
• AV valves snap shut, generating the first heart sound (“lub”).
• For a split-second, all valves are closed = isovolumetric contraction; chamber volume fixed while pressure builds.
• When ventricular pressure exceeds arterial pressures (≈ $120\,\text{mm Hg}$ in aorta; variable in pulmonary trunk):
  • SL valves are forced open.
  • Blood is ejected rapidly into the aorta & pulmonary trunk = ventricular ejection phase.
  • Aortic pressure peaks near $120\,\text{mm Hg}$ under resting conditions.

Phase 3 – Isovolumetric Relaxation (Early Diastole)

• Follows peak of $T$ wave; ventricles relax.
• Remaining blood (the End Systolic Volume (ESV)) is no longer compressed; ventricular pressure plummets.
• Arterial blood momentarily flows backward, filling SL cusps → SL valves snap shut (“dup”), producing a brief pressure rebound in the aorta called the dicrotic notch.
• All valves are again closed; ventricles are isolated ➜ isovolumetric relaxation period.
• Meanwhile atria, having filled during ventricular systole, now possess higher pressure:
  • When intra-atrial pressure slightly exceeds ventricular pressure, AV valves reopen ➜ Phase 1 begins anew.

Timing & Durations (Resting Heart Rate ≈ 75 bpm)

• Total cardiac cycle length ≈ $0.8\,\text{s}$.
  • Atrial systole: $0.1\,\text{s}$
  • Ventricular systole: $0.3\,\text{s}$
  • Quiescent (total relaxation) period: $0.4\,\text{s}$

Pressure-Flow Principles & Valve Dynamics

• Blood flow is entirely pressure-driven; always moves down a pressure gradient.
• Alternating myocardial contraction/relaxation continually reshapes these gradients.
• Valves open passively when pressure downstream is lower than upstream; close when the reverse occurs, ensuring unidirectional flow.

Right vs. Left Heart: Pressure Differences, Volume Equality

• Right ventricle pumps against a low-pressure pulmonary circuit
  • Pulmonary artery pressures ≈ $24\,/\,10\,\text{mm Hg}$ (systolic/diastolic).
• Left ventricle faces high-pressure systemic circuit
  • Aortic pressures ≈ $120\,/\,80\,\text{mm Hg}$.
• Despite pressure disparity, both sides eject identical stroke volume each beat to maintain circuit balance.

Heart Sounds (Phonocardiography Basics)

• Two normal sounds per cycle, best heard with a stethoscope:
  1. S₁ – "lub"
  • Closure of AV valves at onset of ventricular systole.
  • Louder, longer, more resonant.
  1. S₂ – "dup"
  • Closure of SL valves at onset of ventricular diastole.
  • Short, sharp.
• Pattern: lub-dup, pause, lub-dup, pause…; pause equals relaxation interval.

Key Quantitative & Derived Relationships

• End Diastolic Volume (EDV): volume of blood in each ventricle after atrial contraction, before systole.
• End Systolic Volume (ESV): residual blood post-ejection.
• Stroke Volume (SV) (noted though not in transcript, vital for comprehension): $SV = EDV - ESV$.
• Cardiac Output (CO) (contextual link): $CO = SV \times HR$.

Clinical & Educational Connections

• Dicrotic notch visible on arterial pressure tracings; useful for timing aortic valve closure.
• Abnormal timing, intensity, or addition of extra sounds (gallops, murmurs) assist in diagnosing valve pathology, ventricular dysfunction, or septal defects.
• Interactive tutorials (Mastering A&P → Study Area → Interactive Physiology) recommended for visualization of dynamic pressure-volume changes.