The Cardiac Cycle and Cardiac Muscle Tissue: Structure, Function, and Electrical Conducting System

The Cardiac Cycle

Overview of the Cardiac Cycle

The cardiac cycle refers to the sequence of events that occur during one complete heartbeat, including both contraction and relaxation phases of the heart chambers. It ensures the effective pumping of blood throughout the body.

• Heartbeat: A single contraction of the heart, involving all four chambers acting in two pairs (atria first, then ventricles).

• Cardiac Cycle: The period from the beginning of one heartbeat to the beginning of the next, including a rest period after each beat.

• Phases:

  • Systole: Contraction phase.

  • Diastole: Relaxation phase.

• Normal Rate: 70-75 beats per minute; each cycle takes about 0.8 seconds to complete.

Blood Pressure and Flow in the Cardiac Cycle

Blood pressure inside the heart chambers fluctuates during the cardiac cycle, driving blood flow from areas of high to low pressure. This flow is regulated by the timing of contractions and the action of one-way valves.

• Pressure falls during diastole and rises during systole.

• Valves ensure unidirectional blood flow.

Phases of the Cardiac Cycle

The cardiac cycle consists of several distinct phases, each characterized by specific mechanical and electrical events.

1. Passive Filling: All chambers are relaxed; blood flows passively into atria and ventricles.

2. Atrial Systole Begins: Atria contract, AV (atrioventricular) valves (tricuspid and bicuspid) are open, allowing blood to flow into ventricles.

3. Atrial Systole Ends: AV valves close.

4. Ventricular Systole Begins: Ventricles contract, pressure rises, but not enough to open semilunar (SL) valves; isovolumetric contraction occurs.

5. Ventricular Ejection: Pressure forces SL valves open; blood is ejected into pulmonary and aortic trunks. Only about 60% of end-diastolic volume (EDV) is ejected (stroke volume).

6. SL Valves Close: Blood ejection causes a drop in pressure; SL valves close to prevent backflow. Ventricles still contain about 40% of EDV (end-systolic volume, ESV).

7. Ventricular Diastole Begins: Isovolumetric relaxation; all valves are closed.

8. Ventricular Diastole Ending: Atrial pressure exceeds ventricular pressure; AV valves open, and passive filling resumes.

Heart Sounds

Heart sounds are produced by the closing of valves and the movement of blood within the heart. Four heart sounds are recognized, with the first two being most prominent.

• S1 – "Lubb": Sound of AV valve closing.

• S2 – "Dupp": Sound of SL valves closing.

• S3: Sound of blood flowing into ventricles.

• S4: Sound of atrial contraction.

Cardiac Muscle Tissue

Structure and Characteristics

Cardiac muscle tissue is specialized for continuous rhythmic contraction and is structurally distinct from skeletal muscle.

• Cardiac muscle cells have sarcomeres and striations like skeletal muscle, but myofibrils are more branched and variable in diameter.

• Cells are smaller, shorter, and can be branched.

• Each cell has a central nucleus and more/larger mitochondria for high energy demand.

• Specialized intercellular connections include intercalated discs and gap junctions.

Intercalated Discs

Intercalated discs are unique junctions between cardiac muscle cells that facilitate synchronized contraction.

• Connect the end of one cell to another.

• Prevent cells from separating during contraction.

• Visible under a microscope.

Gap Junctions

Gap junctions are small pores within intercalated discs that allow direct electrical communication between cells.

• Form channels through the membranes of adjacent cells.

• Enable rapid transmission of action potentials, creating a functional syncytium (cells contract as a unit).

The Conducting System of the Heart

Automaticity and Cardiac Muscle Cell Types

The heart can contract independently of nervous input due to its intrinsic conducting system. Two main types of cardiac muscle cells are involved:

• Pacemaker Cells (Autorhythmic Cells):

  • Generate action potentials that stimulate contraction.

  • Have a threshold but no true resting potential; instead, they exhibit a "prepotential" or "pacemaker potential" due to leaky Na+ channels.

  • Depolarization occurs rapidly via Ca2+ channels once threshold is reached.

• Contractile Cells:

  • Do not generate action potentials spontaneously.

  • Action potentials travel from neighboring cells, causing voltage-gated Na+ channels to open (depolarization).

  • Slow-moving Ca2+ and K+ channels create a long refractory period, preventing tetanus.

Components of the Conducting System

The conducting system coordinates the sequence of cardiac muscle contraction, ensuring efficient blood flow.

• Sinoatrial (SA) Node: Located in the posterior wall of the right atrium near the superior vena cava; initiates each heartbeat.

• Internodal Pathways: Conduct impulses from the SA node to the AV node.

• Atrioventricular (AV) Node: Located in the floor of the right atrium; receives impulses from the SA node and delays the signal by about 100 ms, allowing atria to contract before ventricles.

• AV Bundle (Bundle of His): Located in the interventricular septum; transmits impulses from the AV node to the ventricles via two branches.

• Purkinje Fibers: Large diameter conducting cells that rapidly propagate action potentials, triggering ventricular systole and conducting impulses to papillary muscles.

Electrocardiogram (ECG)

An electrocardiogram is a recording of the electrical events occurring within the conducting system during the cardiac cycle. It is used clinically to assess heart function and diagnose arrhythmias.

Key Terms and Formulas

• End-Diastolic Volume (EDV): The volume of blood in a ventricle at the end of diastole.

• End-Systolic Volume (ESV): The volume of blood remaining in a ventricle after systole.

• Stroke Volume (SV): The amount of blood ejected by a ventricle during systole. Formula: $SV = EDV - ESV$

• Cardiac Output (CO): The volume of blood pumped by the heart per minute. Formula: $CO = SV \times HR$ where $HR$ is heart rate.

Summary Table: Cardiac Cycle Phases

Phase	Event	Valves Status	Blood Flow
Passive Filling	All chambers relaxed	AV open, SL closed	Blood flows into atria and ventricles
Atrial Systole	Atria contract	AV open, SL closed	Blood pushed into ventricles
Ventricular Systole (Isovolumetric)	Ventricles contract, pressure rises	All valves closed	No blood movement
Ventricular Ejection	SL valves open	AV closed, SL open	Blood ejected into arteries
Ventricular Diastole (Isovolumetric)	Ventricles relax	All valves closed	No blood movement

Additional info: Academic context and terminology have been expanded for clarity and completeness. Images referenced in the original notes have been described in text for accessibility.