Cardiac Physiology and Blood Components

Prevention of Tetanus in the Heart

• Skeletal muscle can undergo tetanus (sustained contraction) ➜ useful for posture or holding a dumb-bell.

• Complete tetanus = sustained contraction without any relaxation.

• In myocardium, tetanus would be catastrophic:

  • Heart could not relax/fill with blood.

  • Heart could not pump blood into systemic circulation.

• Classic clinical image: “lock-jaw” in tetanus poisoning (botulinum toxin) ≠ desired for heart.

Electrical Activity of Myocardial Contractile Cells (MCCs)

• Action-potential tracing has three key regions:

  • Rapid depolarization due to $\text{Na}^+$ influx.

  • Plateau phase (unique) caused by slow “dribble” of $\text{Ca}^{2+}$ influx that halts early repolarization.

  • Final repolarization via massive $\text{K}^+$ efflux.

• Plateau delays full repolarization ➜ prolongs refractory period.

• Mechanical twitch (blue) vs. electrical activity (red):

  • Because of plateau, electrical & mechanical events end at ~same time.

  • Cell cannot fire a second action potential until relaxation has begun ➜ myocardium cannot tetanize.

Comparison to Skeletal Muscle

• Skeletal muscle AP is brief; ends before peak of contraction.

• Multiple APs can arrive before relaxation ➜ temporal summation & tetanus.

• Cardiac plateau prevents this; safety mechanism for rhythmic pump.

Electrical Activity of Myocardial Autorhythmic Cells (MACs)

• Cells of SA node, AV node, etc. show continuous depolarization-repolarization cycles for life.

• No true resting membrane potential (graph oscillates between $\approx -60\,\text{mV}$ and $+20\,\text{mV}$).

• Rate in complete autonomic absence ≈ $80!\text{–}!100\;\text{times/min}$.

Pacemaker Potential (PP)

• Slow depolarizing slope from $-60\,\text{mV} \rightarrow -40\,\text{mV}$ (threshold).

• Carried by funny current channels ($I_f$):

  • Permit both $\text{Na}^+$ influx & $\text{K}^+$ efflux.

  • Net inward current because electrochemical drive for $\text{Na}^+$ > $\text{K}^+$.

• At threshold ( $-40\,\text{mV}$ ) ➜ voltage-gated $\text{Ca}^{2+}$ channels open ➜ rapid upstroke to $+20\,\text{mV}$.

• Repolarization: $\text{K}^+$ channels open ➜ return to $-60\,\text{mV}$ and re-open $I_f$ channels.

• Cycle repeats indefinitely (* “forever” unless pathology).

Autonomic Modulation of MACs

Parasympathetic (Vagal) Influence

• Neurotransmitter: Acetylcholine ➜ muscarinic (M2) receptors (G-protein-coupled).

• Effects:

  • ↑ Permeability to $\text{K}^+$.

  • ↓ Permeability to $\text{Ca}^{2+}$.

• Graphical outcome:

  • PP slope less steep; longer time to reach $-40\,\text{mV}$.

  • Peak depolarization rises more slowly.

• Functional result: intrinsic rate slowed to $60!\text{–}!80\;\text{times/min}$ ("vagal tone").

Sympathetic Influence

• Neurotransmitters: Norepinephrine / Epinephrine ➜ β₁-adrenergic receptors.

• Effects:

  • ↑ Permeability to $\text{Na}^+$ (steeper PP).

  • ↑ Permeability to $\text{Ca}^{2+}$ (faster upstroke).

• Graph output: more peaks in same time span.

• Functional result: rate > $100\;\text{times/min}$ (tachycardic shift).

Summary of Rates

• No autonomic input: $80!\text{–}!100\;\text{times/min}$.

• Dominant parasympathetic tone: $60!\text{–}!80\;\text{times/min}$.

• Dominant sympathetic drive: >100\;\text{times/min}.

Autonomic Effects on Contractile Myocardium

• Sympathetic β₁ stimulation on MCCs ➜ ↑ $\text{Ca}^{2+}$ influx ➜ stronger ventricular force.

• No direct parasympathetic innervation to MCCs (force is not decreased directly; rate is slowed via MACs).

Blood Composition (≈ $5!\text{–}!6\;\text{L}$ total)

Centrifuged Layers

1. Hematocrit (bottom) – $\approx45\%$ of volume.

   • Red blood cells (RBCs; erythrocytes).

   • Function: transport $\text{O}<em>2$ to tissues & $\text{CO}</em>2$ from tissues.

   • Carry hemoglobin; anucleate, organelle-poor ➜ “bags of Hb.”

2. Buffy coat – <1\% of volume.

   • Leukocytes (white blood cells): cellular immune defense.

   • Platelets (thrombocytes): fragments essential for hemostasis/clotting.

3. Plasma (top) – $\approx55\%$ of volume.

   • Mostly water plus dissolved constituents:

     • Ions: $\text{Na}^+$, $\text{K}^+$, $\text{Ca}^{2+}$, $\text{Cl}^-$, $\text{Mg}^{2+}$.

     • Nutrients: glucose, fatty acids, amino acids.

     • Gases: $\text{O}<em>2$, $\text{CO}</em>2$.

     • Waste: urea, creatinine, etc.

   • When clotting proteins removed ➜ called serum.

• Collectively, erythrocytes, leukocytes, & platelets are “formed elements;” only leukocytes are true, nucleated cells.

These notes capture every major & minor point, numerical value, mechanism, and clinical/physiological implication from the video transcript for thorough exam preparation.