Circulatory System - Heart Functions

Circulatory System - Part 2: Heart

• The circulatory system comprises the heart and blood vessels.
• The heart pumps blood, and blood vessels (arteries, capillaries, veins) carry blood.
• The right heart receives deoxygenated blood from the body, pumps it to the lungs, and then receives oxygenated blood returning from the lungs.
• The left heart receives oxygenated blood from the lungs, pumps it to the body tissues, and then receives deoxygenated blood returning from the body.

Properties of Cardiac Muscle Fibers

• Rhythmic: Initiate impulses (Sino-atrial node (SAN), atrioventricular node (AVN), Purkinje fibers).
• Conductive: Rapidly conduct impulses through the heart (His-Purkinje system).
• Contractile: Contract as two functional syncytia (separated by the AV fibrous ring).
• Auto-rhythmic: Ability to beat independently of external stimuli.
• Conductivity: Ability to transmit excitation waves.
• Excitability: Ability to respond to a stimulus.
• Contractility: Ability to convert energy into mechanical work.

Initiation of Cardiac Impulses in SAN

• Pacemaker Potential (Prepotential): A low, unstable resting membrane potential (-55 to -60 mV) characterized by a slow depolarization.
• Pacemaker Action Potential: Occurs around -40 mV characterized by a slow upstroke to +10 mV (due to L-type Ca2+ channels' opening) and gradual repolarization (K+ efflux).

Electrical Activity of Contractile Muscle Fibers

• Resting membrane potential: Stable at approximately -90 mV.
• Action Potential consists of 5 phases:
  • Phase 0: rapid depolarization (fast Na+ channels).
  • Phase 1: initial rapid repolarization (Na+ channel closure).
  • Phase 2: prolonged plateau (slow Ca2+ channel opening).
  • Phase 3: rapid repolarization (Ca2+ channel closure, enhanced K+ efflux).
  • Phase 4: Restores the resting membrane potential (Na+/K+ pump, Ca2+ pump).

Factors Affecting Rhythmicity (Chronotropism)

• Nervous Factors: Parasympathetic stimulation decreases rhythmicity, while sympathetic stimulation increases it.
• Physical Factors: Temperature increases rhythmicity; extremes (very high or very low temperature) suppress it.
• Mechanical Factors: Increased atrial distension increases heart rate (Bainbridge reflex).
• Chemical Factors: Hormones (e.g., catecholamines, thyroxine) and blood gases (e.g., hypoxia and hypercapnia) affect rhythmicity.

Electrical Activity of the Conducting System of the Heart

• The conducting system allows for coordinated contraction of the heart.
• Includes the Sinoatrial (SA) node, Atrioventricular (AV) node, AV Bundle of His, bundle branches and Purkinje fibers.
• The SA node initiates the heartbeat. The AV node delays the impulse to allow atrial emptying before ventricular contraction.

Factors Affecting Cardiac Excitability

• Nervous factors: Vagal stimulation increases, while sympathetic stimulation decreases, the refractory period.
• Mechanical Factors: Increased heart rate shortens the refractory period.
• Physical Factors: Higher temperature shortens the refractory period.
• Chemical factors: Cardiac ischemia or hypoxia, bacterial or chemical toxins lengthen repolarization time, thus increasing the refractory period.

Contractility

• Contraction is triggered by calcium ions (Ca2+).
• Ca2+ is released from the sarcoplasmic reticulum and extracellular fluid through L-type Ca2+ channels.
• Relaxation occurs via Ca2+ transport back into the sarcoplasmic reticulum.
• Contractility can be affected by various factors (neural, chemical, and physical).

Cardiac Output

• Stroke volume: Volume of blood pumped per beat.
• Cardiac output: Volume of blood pumped per minute (CO = SV x HR).
• Factors affecting stroke volume: Venous return, myocardial contractility, and afterload.

Cardiac Reserve

• Cardiac reserve: The ability of the heart to increase its output above basal levels.
• Factors contributing to cardiac reserve: increased heart rate, increased stroke volume, and increased cardiac muscle hypertrophy
• Factors affecting arterial blood pressure: Cardiac output, and peripheral resistance (R)