Cardiovascular Physiology: Heart Function and Electrical Conduction

Measuring Cardiac Performance

Two fundamental equations govern our understanding of hemodynamics:

1. Cardiac Output (CO): This represents the volume of blood pumped by the heart per minute. It is the product of how much blood is pumped per beat and how fast the heart is beating. $CO = SV \times HR$ Where $SV$ is Stroke Volume and $HR$ is Heart Rate.

2. Blood Pressure (BP): This is the force exerted by blood against vessel walls, determined by the output of the heart and the resistance of the vessels. $BP = CO \times SVR$ Where $SVR$ is Systemic Vascular Resistance.

Autonomic Modulation

The Autonomic Nervous System (ANS) acts as the "thermostat" for these values, utilizing different receptors to fine-tune cardiac and vascular activity:

• Sympathetic Nervous System (Fight or Flight):
  • Alpha 1 ($\alpha_1$) receptors: Located in peripheral blood vessels; activation causes vasoconstriction, increasing $SVR$.
  • Beta 1 ($\beta_1$) receptors: Located in the heart; activation increases both heart rate and the strength of contraction.
  • Beta 2 ($\beta_2$) receptors: Located in the lungs and peripheral vessels; activation causes bronchodilation and vasodilation.
• Parasympathetic Nervous System (Rest and Digest):
  • Mediated primarily by the vagus nerve (Cranial Nerve X), this system slows the heart rate and decreases atrioventricular conduction.

💡 Clinical Terminology Tip

When studying pharmacology or pathology, you will encounter these three terms regarding cardiac effects:

• Chronotropy: Refers to Heart Rate (e.g., a positive chronotropic agent increases HR).
• Inotropy: Refers to Contractile Strength (e.g., a positive inotropic agent increases contraction force).
• Dromotropy: Refers to Conduction Speed (e.g., a negative dromotropic agent slows the electrical impulse).

The Heart as an Endocrine Organ

Beyond its mechanical role, the heart participates in hormonal regulation through the secretion of Natriuretic peptides. These hormones act to counter the Renin-Angiotensin-Aldosterone System (RAAS) to reduce blood volume and pressure:

• Atrial Natriuretic Peptide (ANP): Released by atrial muscle cells in response to atrial distention (stretching). It reduces central venous pressure, cardiac output, and blood pressure.
• Brain Natriuretic Peptide (BNP): Secreted primarily by the ventricles in response to excessive stretching of the myocytes.
• C-Type Natriuretic Peptide (CNP): Secreted by the endothelium of blood vessels and serves a vasodilatory function.

Electrical Conduction and Action Potentials

The rhythmic contraction of the heart is driven by specialized cells that generate and conduct electrical impulses. These cells are categorized into two distinct types based on their function.

1. Cardiac Contractile Cells

These cells are responsible for the actual mechanical squeeze of the heart. Their action potential is characterized by specific ion movements:

• Depolarization (Phase 0): Driven by a rapid influx of Sodium ($Na^+$).
• Plateau (Phase 2): A unique phase where Calcium ($Ca^{++}$) enters the cell, facilitating the mechanical contraction.
• Repolarization (Phase 3): Driven by the outward movement of Potassium ($K^+$), allowing the cell to relax.

2. Autorhythmic (Pacemaker) Cells

Unlike contractile cells, these cells do not require an external stimulus to fire; they generate their own rhythm:

• Pacemaker Potential (Phase 4): A unique "funny" $Na^+$ leak allows the membrane potential to drift upward toward the threshold.
• Depolarization (Phase 0): Driven by the influx of Calcium ($Ca^{++}$) rather than sodium.
• Repolarization (Phase 3): Driven by the outward movement of Potassium ($K^+$).

The Hierarchy of Pacing

The heart possesses a built-in hierarchy of electrical command. If one part of the system fails, the others can take over, though at a slower rate:

⚠️ Exam Alert: Ion Differences

A common exam question involves distinguishing between the ions used in depolarization.

• Contractile cells use $Na^+$ for rapid depolarization.
• Autorhythmic cells use $Ca^{++}$ for depolarization.