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Questions and Answers
What is the effect of sympathetic stimulation on heart rhythmicity?
What is the effect of sympathetic stimulation on heart rhythmicity?
How does temperature affect heart rate?
How does temperature affect heart rate?
What is the effect of acidosis on heart rhythmicity?
What is the effect of acidosis on heart rhythmicity?
During which period is the excitability of cardiac muscle zero?
During which period is the excitability of cardiac muscle zero?
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What is the conduction speed through the atrioventricular node (AVN)?
What is the conduction speed through the atrioventricular node (AVN)?
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What is the characteristic resting membrane potential of pacemaker cells in the SAN?
What is the characteristic resting membrane potential of pacemaker cells in the SAN?
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Which ion influx primarily causes the early part of the pacemaker potential?
Which ion influx primarily causes the early part of the pacemaker potential?
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What occurs during phase 2 of the cardiac muscle action potential?
What occurs during phase 2 of the cardiac muscle action potential?
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Which of the following correctly describes the pacemaker action potential?
Which of the following correctly describes the pacemaker action potential?
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What is the main cause of rapid repolarization in phase 3 of the contractile action potential?
What is the main cause of rapid repolarization in phase 3 of the contractile action potential?
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Study Notes
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)
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Description
Explore the vital functions of the heart within the circulatory system. This quiz covers the heart's anatomy, the flow of blood, and the properties of cardiac muscle fibers. Test your understanding of how oxygenated and deoxygenated blood circulates in the body.
