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Questions and Answers
What causes the rapid depolarization phase (Phase 0) in cardiac muscle action potential?
What is the primary ionic movement responsible for Phase 1 (Small Repolarization)?
During Phase 2 (Plateau), what is happening in terms of ionic movement?
What happens during Phase 3 (Rapid Repolarization) of the cardiac action potential?
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What characterizes the Absolute Refractory Period (ARP) in cardiac muscle action potential?
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What is the primary function of the aorta and big arteries during systole?
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What key role do arterioles play in the circulatory system?
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Where does the exchange of gases and nutrients primarily occur in the circulatory system?
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Which statement correctly describes venules and veins?
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What drives the systemic circulation in the human body?
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Which component is responsible for electrical separation between atria and ventricles?
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What type of muscle fibers are responsible for contraction in the heart?
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What happens to deoxygenated blood after it is collected into venules and veins?
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What is the significance of the refractory period in cardiac muscle?
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What is the primary function of the right side of the heart?
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Which structure serves as the primary pacemaker of the heart?
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What role do cardiac valves play in the cardiovascular system?
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How does the duration of the mechanical response compare to that of the action potential in cardiac muscle?
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Which characteristic distinguishes pacemaker cells from ordinary cardiac muscle fibers?
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Which valve is located between the left atrium and left ventricle?
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Why is the left ventricle wall thicker than the right ventricle wall?
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What is the heart rate of the atrioventricular (AV) node?
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What prevents regurgitation of blood from the ventricles to the atria?
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What effect does increased heart rate have on the absolute refractory period (ARP)?
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What is the consequence of the failure of papillary muscles to contract?
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At what membrane potential do pacemaker cells typically stabilize?
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What type of valves prevent backflow of blood from the arterial trunks to the ventricles?
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What happens during the late part of phase 3 of the cardiac action potential?
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How do cardiac valves respond to pressure differences?
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What primarily contributes to the pacemaker potential of autorhythmic cells during Phase 4?
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Which factor increases the heart rate by steepening the slope of diastolic depolarization?
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What is the primary pacemaker of the heart under normal conditions?
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During which phase of the action potential does an influx of Ca++ occur through long-lasting L-channels?
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What is the normal discharge rate of the SA node?
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Which effect does vagal (parasympathetic) stimulation have on heart rate?
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What happens to the heart rate if both the vagus nerve and sympathetic inputs are cut?
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What is the primary consequence of increased K+ outflow in autorhythmic cells?
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Study Notes
Physiological Anatomy of the Heart
- The cardiovascular system is composed of the heart and blood vessels.
- The heart is a muscular organ weighing approximately 300 grams.
- The heart consists of two separate pumps: the right side (low-pressure volume pump) and the left side (high-pressure pump).
- The right side pumps blood through the lungs against low resistance.
- The left side pumps blood through all body organs against high resistance.
- Each pump is composed of atria and ventricles.
- Atria act as reservoirs and contract weakly to pump 30% of blood to the ventricles.
- Ventricles have thicker muscular walls and pump blood through arteries.
- Left ventricle wall is three times thicker than the right ventricle wall as it pumps blood against higher pressure.
- Cardiac valves allow blood flow in one direction only.
- From atria to ventricles.
- From the left ventricle to the aorta.
- From the right ventricle to the pulmonary artery.
Types of Cardiac Valves
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Atrioventricular (AV) valves: Found between atria and ventricles
- Mitral (Bicuspid) valve: Between the left atrium and left ventricle.
- Tricuspid valve: Between the right atrium and right ventricle.
- AV Valves prevent regurgitation of blood from the ventricle to the atrium.
-
Semilunar (SL) valves: Found between ventricles and arterial trunks.
- Aortic valve: Between the left ventricle and aorta.
- Pulmonary valve: Between the right ventricle and pulmonary artery.
- Semilunar valves prevent backflow of blood from the arterial trunks to the ventricles.
- Papillary muscles contraction leads to the closure of AV valves.
- If papillary muscles fail to contract, the AV valves will not close properly.
Closed System of Blood Vessels
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Arteries (Elastic arteries):
- Include aorta and big arteries.
- Expand during systole (to receive blood ejected) and prevent excessive rise in systolic blood pressure.
- Recoil during diastole (to push blood to tissues) and prevent excessive drop in diastolic blood pressure.
-
Arterioles (Resistance vessels):
- Can constrict or dilate to regulate local blood flow.
- Maintain peripheral resistance essential for regulating arterial blood pressure.
-
Capillaries (Exchange vessels):
- Have very thin walls to allow exchange.
- Have the greatest cross-sectional area.
-
Venules and Veins (Reservoir or Capacitance vessels):
- Can change their capacity by contraction or relaxation of their muscular walls.
- The largest part of the blood volume is found in veins.
Circulation
-
General or Systemic circulation:
- The left ventricle ejects oxygenated blood into the aorta at high pressure.
- Blood distributes through arteries, arterioles, and capillaries.
- Exchange of gases, nutrients, and waste products occurs at capillaries.
- Deoxygenated blood collects into venules and veins, reaching the superior vena cava (SVC) and inferior vena cava (IVC), which open in the right atrium.
- The right atrium contracts to eject blood to the right ventricle.
-
Pulmonary circulation:
- The right ventricle pumps deoxygenated blood to the pulmonary artery, reaching the lungs for oxygenation and CO2 removal.
- Oxygenated blood returns through pulmonary veins to the left atrium, which pumps blood to the left ventricle.
Functional Histology of Cardiac Muscle
-
Fibrous skeleton of the heart:
- Dense connective tissue causing electrical separation between atria and ventricles.
- Contains four rings forming the openings of AV valves, pulmonary, and aortic valves.
- Acts as a skeleton by providing points of attachment for the cardiac muscle.
Ordinary Cardiac Muscle Fibers
- Responsible for contraction.
- Branched, striated, involuntary fibers.
- Shorter and smaller than skeletal muscle fibers.
- Contain myofibrils (actin, myosin, troponin, and tropomyosin).
- Titin: A very large protein extending from the Z-disk to the M-line, keeping myosin centered in the sarcomere.
- Dystrophin: Provides structural support and connects actin to the extracellular matrix. Congenital deficiency leads to dystrophy and weakness.
Ordinary Cardiac Muscle Action Potential (Fast Response) and Its Ionic Basis
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Phase 0 (Rapid Depolarization and reversal of polarity): From -90mV to +20mV.
- Due to rapid Na+ inflow caused by opening of fast voltage-gated Na+ channels.
- Fast-voltage Na+ channels can be blocked by tetrodotoxins.
-
Phase 1 (Small Repolarization): From +20mV to +10mV.
- Due to the closure of Na+ channels.
- Opening of K+ channels causing K+ outflow.
- Opening of Cl- channels causing Cl- inflow.
-
Phase 2 (Plateau): Repolarization slows down (around 0mV).
- Due to a balance between K+ outflow and slow Ca++ inflow through slow calcium long lasting (L-channels).
- Ventricular AP shows a prolonged plateau lasting for 300 msec, while atrial fibers have a less prominent plateau lasting for 150 msec.
-
Phase 3 (Rapid Repolarization): Till reaching resting membrane potential (-90mV).
- Caused by K+ outflow and closure of Ca++ channels (no more calcium influx).
-
Phase 4 (Resting membrane potential): -90mV.
- The Na+-K+ pump drives out excess Na+ and brings in K+.
Refractory Period
-
Absolute Refractory Period (ARP):
- Period during which any stimulus reaching the heart will not produce a response.
- Coincides with phases 0, 1, 2, and part of 3 (all period of systole).
- Longer (due to plateau) than ARP in skeletal muscles.
- Prevents tetanus.
- Prevents fatigue.
-
Relative Refractory Period (RRP):
- A strong stimulus (supra-threshold) is needed to produce a weak response.
- Coincides with the rest of phase 3.
- Excitability returns gradually.
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Supernormal Phase (Vulnerable period):
- A weak (sub-threshold) stimulus can produce a response.
- Coincides with the late part of phase 3.
- Excitability is higher than normal.
- Stimulation of the heart during this period could lead to ventricular fibrillation.
- Refractory period of ventricles is longer than that of atrial muscles.
Relationship Between Mechanical Response and Action Potential
- Mechanical response lasts 1.5 times as long as the action potential.
- Contraction (systole):
- Starts just after depolarization.
- Ends at the end of the plateau.
- Relaxation (diastole):
- Starts with rapid repolarization.
- Ends by the end of the first half of diastole.
Rhythmicity
- Ability of cardiac muscle to beat regularly and initiate its own regular impulses.
- Found in self-excitable, autorhythmic, or pacemaker cells:
- Sino-atrial (SA) node: 90-105/minute (normal pacemaker).
- Atrio-ventricular (AV) node: 60/minute.
- Purkinje fibers: 30-40/minute.
- These fibers differ from ordinary cardiac muscle fibers:
- Their resting membrane potential is less negative (-60mV).
- They have an unstable potential during rest to discharge impulses spontaneously (do not require external stimulation).
- There is no plateau in their action potential.
- Repolarization is a single phase.
Pacemaker Potential of Autorhythmic Cells (Slow Response Fibers)
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Phase 4 (Slow spontaneous depolarization, diastolic depolarization, prepotential):
- Due to decreased K+ outflow, increased Na+ inflow (funny current), and increased Ca++ inflow (through T-channels).
- Prepotential depolarization reaches the firing level (-40mV).
-
Phase 0 (Depolarization):
- Influx of Ca++ through long-lasting L-channels.
-
Phase 3 (Repolarization):
- Efflux of K+.
- The slope of diastolic depolarization (prepotential) determines the heart rate.
Factors Affecting Autorhythmic Cells
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Positive chronotropic factors (increasing heart rate):
- Increasing the slope of diastolic depolarization.
- Increasing Na+ permeability of the SA node.
- Sympathetic stimulation secreting noradrenaline.
- Catecholamines, thyroxine.
- Fever.
-
Negative chronotropic factors (decreasing heart rate):
- Decreasing the slope of diastolic depolarization.
- Increasing K+ permeability of the SA node.
- Parasympathetic (vagal) stimulation secreting acetylcholine.
- Cholinergic drugs (as methacholine).
- Hypothermia.
Pacemaker of the Heart
- The part of the heart with the highest rhythm, which all other parts obey.
- Normally, the SA node is the pacemaker.
- If the SA node stops functioning, the AV node becomes the pacemaker.
- SA node discharging rate is 90/minute.
- Normal heart rate is 70/minute (due to vagal tone).
- Heart rate is 120/minute if the vagus is cut or blocked (due to sympathetic tone).
- Heart rate is 90/minute if the vagus and sympathetic nerves are cut (denervated heart).
- During rest, both parasympathetic and sympathetic tones are present, but vagal tone is more powerful.
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Test your knowledge on the anatomy of the heart and its role in the cardiovascular system. This quiz covers the structure and function of the heart, including the pumps, valves, and blood flow dynamics. Perfect for students studying anatomy and physiology!
