Cardiac Muscle and Circulation

Cardiac Muscle Types

  • Atrial muscle.
  • Ventricular muscle.
  • Specialized excitatory and conductive muscle fibers.

Cardiac Muscle Action Potential Phases

  • Phase 0 (Depolarization): Fast sodium channels open, membrane potential becomes more positive.
  • Phase 1 (Initial Repolarization): Fast sodium channels close, potassium channels open.
  • Phase 2 (Plateau): Calcium channels open, fast potassium channels close, resulting in a plateau.
  • Phase 3 (Rapid Repolarization): Calcium channels close, slow potassium channels open.
  • Phase 4 (Resting Membrane Potential): Averages about -90 millivolts.

Cardiac Cycle

  • Occurs from the start of one heartbeat to the beginning of the next.
  • Consists of diastole (relaxation/filling) and systole (contraction).

Refractory Period

  • Ventricle refractory period: 0.25 to 0.30 seconds.
  • Atria > Ventricles.

Excitation-Contraction Coupling

  • Action potential leads to muscle contraction.
  • Transverse T tubules trigger calcium ion release, promoting actin and myosin filament sliding.

Heart Rate

  • Increased heart rate decreases cardiac cycle duration.

Cardiac Cycle Periods

  • Rapid Filling: First third of diastole.
  • Isovolumic Contraction: Muscle tension increases with minimal shortening.
  • Ejection: Ventricular pressure rises (Left: >80 mm Hg, Right: >8 mm Hg).
    • Fast ejection (70% of blood).
    • Slow ejection (30% of blood).
  • Isovolumic Relaxation: Intraventricular pressures decrease rapidly.

Preload vs. Afterload

  • Preload: Tension on muscle at the start of contraction.
  • Afterload: Load against which the muscle exerts force.

Frank-Starling Mechanism

  • Greater stretch during filling leads to a greater contraction force.

Ion Effects

  • Potassium: Dilates heart, slows HR, can cause weakness/death.
  • Calcium: Spastic contraction, cardiac weakness.

Temperature

  • Increased temperature increases HR (fever).

Heart Sounds

  • S1: Closure of AV valves (longer, louder, lower pitch).
  • S2: Closure of semilunar valves (shorter).
  • S3: Rapid filling of ventricles (pathological: ventricular gallop).
  • S4: Atrial systole (pathological: atrial gallop).

Sinus (Sinoatrial Node)

  • Located in the right atrium near the superior vena cava.
  • Generates automatic electrical rhythmicity.
  • Self-excitation via sodium and calcium influx.

AV Node

  • Delays cardiac impulse, allowing atria to empty before ventricular contraction.
  • Located in the posterior wall of the right atrium behind the tricuspid valve.

Purkinje Fibers

  • Rapid transmission compared to AV nodal fibers.

Sympathetic Stimulation

  • Increases: sinus nodal discharge, conduction rate, excitability, and force of contraction

Parasympathetic Stimulation

  • Cranial nerve and neurotransmitter responsible

Circulation

  • Systemic Circulation: Greater/peripheral circulation.
  • Pulmonary Circulation: Supplies the lungs.

Functional Parts of Circulation

  • Arteries: Transport blood at high pressure and velocity.
  • Arterioles: Control conduits that regulate blood flow into capillaries.
  • Capillaries: Exchange fluid, nutrients, electrolytes, hormones, etc.
  • Venules: Collect blood from capillaries.
  • Veins: Transport blood back to the heart (major blood reservoir, thin walls, low pressure).

Blood Distribution

  • Systemic circulation: 84%.
  • Heart and lungs: 16%.
  • Veins: 64%.
  • Arteries: 13%.
  • Systemic arterioles and capillaries: 7%.
  • Heart: 7%.
  • Pulmonary vessels: 9%.

Blood Flow Control

  • Locally controlled based on tissue needs.
  • Acute: vasodilation/vasoconstriction (seconds to minutes).
  • Long-term: controlled changes over days/weeks/months.

Cardiac Output

  • Sum of all local tissue flows.

Arterial Pressure Regulation

  • Independent of local blood flow and cardiac output.

Nervous Reflexes

  • Increase heart pumping force.
  • Cause venous reservoir contraction.
  • Induce arteriole constriction to increase arterial pressure.

Lymphatic System

  • Accessory route for fluid flow from interstitial spaces to blood.
  • Transports proteins and large particulate matter.

Blood Flow Functions

  • Delivers oxygen, nutrients (glucose, amino acids, fatty acids).
  • Removes carbon dioxide and hydrogen ions.
  • Maintains ion concentrations.
  • Transports hormones and other substances.

Vasomotor Center

  • Located in the medulla and lower pons.