Mechanical Physiology of the Heart: The Cardiac Cycle

Mechanical Physiology of the Heart

Introduction to Mechanical Physiology

Mechanical physiology describes the processes by which blood fills the cardiac chambers and is pumped out of them. This involves coordinated contraction of cardiac muscle cells, pressure changes within the heart, and the function of heart valves to ensure unidirectional blood flow.

• Cardiac muscle contraction: Cardiac muscle cells contract as a unit, producing a coordinated heartbeat. The spiral arrangement of muscle cells creates a 'wringing' action during contraction.

• Pressure changes: Contractions generate pressure gradients that drive blood through the heart, with valves preventing backflow.

• Cardiac cycle: The sequence of events within the heart from one heartbeat to the next.

Pressure Changes, Blood Flow, and Valve Function

Pressure Gradients and Blood Flow

Blood flows in response to pressure gradients. As ventricles contract and relax, pressure in the chambers changes, causing blood to push on valves and open or close them.

• When ventricles contract:

  • Pressure rises above that in the atria and great vessels (pulmonary trunk and aorta).

  • Both AV valves are forced shut by blood pushing against them.

  • Both semilunar valves are forced open by outgoing blood.

• When ventricles relax:

  • Pressure falls below that in the atria and great vessels.

  • Higher pressure in atria forces AV valves open, allowing blood to drain into relaxed ventricles.

  • Higher pressure in pulmonary trunk and aorta pushes semilunar valves closed.

Heart Sounds and Auscultation

Heart sounds are detected using a stethoscope and are produced by vibrations of the heart and blood vessel walls as valves close.

• S1 ('lub'): Occurs when AV valves close; longer and louder, but lower in frequency.

• S2 ('dub'): Occurs when semilunar valves close; shorter and higher in frequency.

• Sounds are not due to valves 'slamming shut' but to vibrations.

Heart Murmurs and Extra Heart Sounds

Heart murmurs are abnormal sounds caused by turbulent blood flow, often due to defective valves or septal defects. Extra heart sounds (S3, S4) may indicate pathology but can also be heard in healthy hearts.

• Heart murmur: Audible sound due to turbulent flow, often from defective valves.

• S3: Occurs as blood begins to flow into ventricles after S2; due to recoil of ventricular walls.

• S4: Heard when most blood has finished draining into ventricles, just before S1; due to blood being forced into stiff or enlarged ventricles.

The Cardiac Cycle

Phases of the Cardiac Cycle

Each cardiac cycle consists of one period of relaxation (diastole) and one period of contraction (systole) for each heart chamber. The cycle is divided into four main phases: filling, contraction, ejection, and relaxation.

• Filling phase: Blood drains from atria into ventricles.

  • Ventricular pressures are lower than atrial and great vessel pressures.

  • Semilunar valves are closed, preventing backflow.

  • 80% of atrial blood volume drains passively; remaining 20% is ejected during atrial systole.

  • End-diastolic volume (EDV): Volume of blood in ventricles at end of filling (~120 mL).

• Isovolumetric contraction:

  • Ventricles begin to contract; pressure rises rapidly.

  • AV valves close (S1 sound); semilunar valves remain closed.

  • Ventricular volume does not change.

• Ventricular ejection phase:

  • Pressure in ventricles exceeds that in great vessels; semilunar valves open.

  • Rapid outflow of blood; about 70 mL ejected, 50 mL remains (end-systolic volume, ESV).

• Isovolumetric relaxation:

  • Ventricular diastole begins; pressure declines.

  • Semilunar valves snap shut (S2 sound); AV valves remain closed.

  • Ventricular volume remains constant briefly.

Pressure Changes During the Cardiac Cycle

Pressure changes in the left and right ventricles and great vessels are critical for valve function and blood flow. These changes are illustrated in the Wigger's diagram and pressure graphs.

• Maximum pressure in left ventricle is higher than in right ventricle.

• Pressure changes coordinate with valve opening/closing and phases of the cardiac cycle.

Wigger's Diagram

The Wigger's diagram integrates electrical and mechanical events in the heart, showing relationships between ECG, heart sounds, pressure changes, and volume changes during the cardiac cycle.

• Phases: Atrial systole, isovolumetric contraction, ventricular ejection, isovolumetric relaxation, ventricular filling.

• Key events: Valve status, pressure gradients, volume changes, heart sounds, and ECG waves.

Key Terms and Formulas

• End-diastolic volume (EDV): Volume of blood in ventricle at end of diastole (~120 mL).

• End-systolic volume (ESV): Volume of blood in ventricle at end of systole (~50 mL).

• Stroke volume (SV): Amount of blood ejected per beat. $SV = EDV - ESV$

• Cardiac output (CO): Volume of blood pumped by each ventricle per minute. $CO = SV \times HR$

Table: Heart Sounds and Their Clinical Significance

Sound	Cause	Clinical Significance
S1 ('lub')	Closure of AV valves	Start of ventricular systole
S2 ('dub')	Closure of semilunar valves	Start of ventricular diastole
S3	Recoil of ventricular walls	May indicate pathology or be normal
S4	Blood forced into stiff ventricle	May indicate pathology
Heart murmur	Turbulent blood flow	Often due to valve defects

Example: Application of Cardiac Cycle Concepts

During exercise, heart rate increases, shortening the duration of each cardiac cycle. This increases cardiac output ($CO$), allowing more oxygenated blood to reach tissues.

Additional info: The Wigger's diagram is a foundational tool for understanding the timing and coordination of cardiac events, and is essential for interpreting clinical data such as ECGs and heart sounds.