The Cardiovascular System: Cardiac Muscle and Cardiac Cycle

Cardiac Muscle Structure and Properties

The heart is composed of specialized muscle cells called cardiomyocytes, which possess unique structural and functional characteristics essential for cardiac function.

1. Striated, short, and branched cells: Cardiac muscle cells are striated like skeletal muscle but are shorter, branched, and typically have one nucleus.

2. Intercalated discs: These are found at the end of the cardiomyocytes. These increase surface area to increase contact with neighboring cells to facilitate cell-to-cell contact and synchronized contraction.

3. Desmosomes: Prevent cells from pulling apart during contraction.

4. Gap junctions: Allow ions to flow directly between cells, enabling rapid and coordinated electrical signaling.

Energy Requirements of Cardiac Muscle

Cardiac muscle cells require substantial energy to sustain continuous contractions throughout life.

• ATP production: Cardiomyocytes produce and consume approximately 5 kg of ATP per day, primarily through aerobic respiration.

  • ATP production happens exclusively through aerobic respiration.

  • Resistent to fatigue

• Mitochondria: These cells contain exceptionally large and numerous mitochondria to meet their energy demands.

• Oxygen supply: Adequate oxygen is critical, highlighting the importance of coronary circulation.

Comparison: Cardiac vs. Skeletal Muscle Cells

Cardiac muscle cells differ from skeletal muscle cells in several key aspects, as summarized in the following table:

• Cardiac muscles receive action potentials from their own system/cells. The action potential is slow whereas the skeletal muscle has a quick, spiked action potential. This means that cardiac muscles have a more prolonged contraction to ensure all of the blood is pushed out and emptied into the chambers.

Cardiac Conduction System

Overview of the Cardiac Conduction System

The heart is autorhythmic, meaning it generates its own rhythmic contractions through a specialized conduction system composed of non-contractile cardiomyocytes.

• The heart can continue to beat even after disconnected from the nervous system.

• Heartbeat controlled by specialized cardiomyocytes. Specialized cardiomyocytes are concentrated into TWO masses:

  1. Sinoatrial (SA) node: The primary pacemaker of the heart, located in the right atrium.

  2. Atrioventricular (AV) node: Receives signals from the SA node and delays them to allow proper atrial contraction and ventricular filling.

• AV bundle, bundle branches, and Purkinje fibers: Distribute the electrical signal throughout the ventricles for coordinated contraction.

Steps in Cardiac Conduction

1. SA node (primary pacemaker) depolarizes: Initiates the heartbeat and sets the pace (sinus rhythm, ~75 beats/min).

   • Impulses spread via gap junctions across the atria and to the AV node

2. Signal spreads across atria: Both atria contract.

3. AV node fires: This step delays the signal, allowing atria to finish contracting.

   • The signal is delayed because there are fewer gap junctions.

4. Signal travels down AV bundle: Moves through the interventricular septum.

5. Purkinje fibers distribute signal: Ventricles contract in a coordinated manner.

SA and AV Node Functions

• SA node: Initiates all heartbeats and stimulates atrial contraction.

• AV node: Acts as a resistor, slowing the signal so atria can fully contract and ventricles can fill before contraction.

Nerve Supply to the Heart

Although the heart has a pacemaker, its rate is modulated by the autonomic nervous system.

• Sympathetic fibers: Increase heart rate and force of contraction via cardiac nerves to the SA and AV nodes and myocardium.

• Parasympathetic fibers: Decrease heart rate via the vagus nerves.

Electrocardiogram (ECG or EKG)

Principles of ECG

An electrocardiogram (ECG) records the electrical activity of the heart, reflecting depolarization and repolarization of cardiac muscle cells.

• Non-invasive diagnostic tool: Provides a snapshot of heart function and helps diagnose various heart conditions.

• Major events: P wave, QRS complex, and T wave.

ECG Waves and Their Significance

• P wave: Represents atrial depolarization. About 70% of blood flows passively from atria to ventricles; the final 30% is moved by atrial contraction.

• QRS complex: Represents ventricular depolarization. The largest muscle mass generates the strongest electrical current. Atrial repolarization occurs but is masked.

• T wave: Represents ventricular repolarization. Both ventricles repolarize before the cycle repeats.

The Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle consists of one complete contraction and relaxation of all four heart chambers, divided into two main phases: systole (contraction) and diastole (relaxation).

1. Phase 1: All chambers relaxed, AV valves open, blood passively fills ventricles.

2. Phase 2: SA node fires, atrial systole occurs, P wave produced, last 30% of blood moves to ventricles.

3. Phase 3: AV node fires, QRS wave produced, atria relax, ventricles contract, AV valves shut, semilunar valves closed.

4. Phase 4: Ventricular pressure exceeds aorta/pulmonary trunk, semilunar valves open, ventricular ejection (stroke volume).

5. Phase 5: Ventricles repolarize and relax, T wave produced, all chambers in diastole, semilunar valves close, AV valves reopen, cycle repeats.

Cardiac Output

The purpose of the cardiac cycle is to eject blood into the pulmonary trunk and aorta. Cardiac output (CO) is the volume of blood ejected by each ventricle per minute.

• Stroke volume (SV): Volume ejected per heartbeat (ml/beat).

• Heart rate (HR): Beats per minute.

• Formula:

Summary Table: Cardiac Cycle Phases

Example: During exercise, stroke volume increases, and as much as 90% of ventricular blood is expelled, compared to just over half at rest.

Additional info: The cardiac cycle and conduction system are fundamental to understanding heart physiology and are directly relevant to Anatomy & Physiology college courses.