Muscle Contraction and Contractility Factors
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Questions and Answers

What primarily influences the increase in heart rate during exercise?

  • Increased sympathetic and decreased parasympathetic discharge (correct)
  • Increased diastolic pressure
  • Decreased sympathetic activity
  • Decreased blood volume

    • Which of the following factors contributes to an increase in cardiac output?

  • Increased diastolic blood pressure
  • Increase in heart rate (correct)
  • Decrease in stroke volume
  • Decreased venous return

    • How does endurance training affect maximum heart rate?

  • It has no effect on maximum heart rate.
  • It raises resting heart rate.
  • It increases maximum heart rate by up to 15 b.p.m.
  • It lowers maximum heart rate by up to 15 b.p.m. (correct)

    • Which measurement is likely to be the highest during dynamic exercise?

    <p>Systolic blood pressure</p> Signup and view all the answers

    What initiates the muscle chemoreflex during heavy exercise?

    <p>Increase in muscle lactate</p> Signup and view all the answers

    What is the primary purpose of the cardiorespiratory system?

    <p>Transport oxygen and nutrients to tissues</p> Signup and view all the answers

    Which of these statements about cardiac output is true?

    <p>Cardiac output is defined as the amount of blood pumped by each ventricle each minute</p> Signup and view all the answers

    During exercise, which of the following adjustments occurs in blood flow?

    <p>Increased stroke volume and cardiac output</p> Signup and view all the answers

    What is one of the benefits of regular exercise on the heart?

    <p>It improves the heart's antioxidant capacity</p> Signup and view all the answers

    What is the expected resting cardiac output for a healthy adult?

    <p>~ 5 l/min</p> Signup and view all the answers

    Which factor would increase cardiac contractility?

    <p>Increased Ca²⁺ in extracellular fluid</p> Signup and view all the answers

    What is the effect of increased afterload on stroke volume?

    <p>Decreases stroke volume</p> Signup and view all the answers

    What is the normal cardiac output at resting conditions?

    <p>5.5 L/min</p> Signup and view all the answers

    Which of the following contributes to the regulation of arterial blood pressure?

    <p>Cardiac output and peripheral resistance</p> Signup and view all the answers

    Which factor can decrease cardiac output?

    <p>Parasympathetic stimulation</p> Signup and view all the answers

    What does cardiac index measure?

    <p>Cardiac output relative to body surface area</p> Signup and view all the answers

    Under what condition does increased preload enhance stroke volume?

    <p>Under normal conditions</p> Signup and view all the answers

    Which of these could result in ischemia affecting contractility?

    <p>Decreased blood flow</p> Signup and view all the answers

    What is the primary function of the SA node in the heart?

    <p>To serve as the primary pacemaker</p> Signup and view all the answers

    Which factors can positively affect autorhythmicity?

    <p>Fever</p> Signup and view all the answers

    What is the role of conductivity in cardiac muscle?

    <p>To transmit action potential between cells</p> Signup and view all the answers

    What triggers contraction in cardiac muscle?

    <p>Increased intracellular calcium</p> Signup and view all the answers

    Which part of the heart's conductive system is responsible for transmitting impulses after the SA node?

    <p>Atrioventricular node (AVN)</p> Signup and view all the answers

    What type of fibers are categorized as fast response fibers?

    <p>Purkinje fibers</p> Signup and view all the answers

    Which of the following represents a negative chronotropic factor?

    <p>Parasympathetic stimulation</p> Signup and view all the answers

    What is contractility in the context of cardiac muscle?

    <p>The conversion of chemical energy to tension</p> Signup and view all the answers

    What is one of the main functions of nitric oxide in blood flow regulation during exercise?

    <p>Promotes smooth muscle relaxation</p> Signup and view all the answers

    How does blood flow to working skeletal muscles change during maximal exercise?

    <p>Increases to 80-85% of cardiac output</p> Signup and view all the answers

    Which mechanism is chiefly responsible for venous return during exercise?

    <p>Muscle and thoracic pumps</p> Signup and view all the answers

    What is the primary result of sympathetic neural outflow during exercise?

    <p>Increased cardiac output</p> Signup and view all the answers

    Which statement best describes the role of chemical factors in blood flow redistribution during exercise?

    <p>They can override vasoconstrictor effects of norepinephrine.</p> Signup and view all the answers

    What is the effect of endurance training on the heart over the long term?

    <p>Strengthens the heart muscle</p> Signup and view all the answers

    What role do vasodilatory metabolites play in exercising muscles?

    <p>They promote increased blood flow to working skeletal muscles.</p> Signup and view all the answers

    Which organs experience decreased blood flow during exercise?

    <p>Liver and gastrointestinal tract</p> Signup and view all the answers

    Study Notes

    Sarcoplasmic Reticulum (SR)

    • SR is involved in muscle contraction.
    • SR stores and releases calcium ions (Ca⁺⁺), which are essential for muscle contraction.

    Factors Affecting Contractility

    • Positive Inotropic Factors: Increase the force of heart muscle contraction.
      • Sympathetic stimulation: Increases heart rate and contractility.
      • Catecholamines: Hormones like epinephrine and norepinephrine increase heart rate and contractility.
      • Digitalis: A medication that increases the force of heart contractions.
      • Mild heat: Increases the rate of chemical reactions, potentially leading to increased contractility.
      • ↑Ca⁺⁺ in ECF: Increased calcium in the extracellular fluid can enhance muscle contraction.
    • Negative Inotropic Factors: Decrease the force of heart muscle contraction.
      • Parasympathetic stimulation: Slows heart rate and reduces contractility.
      • Ether, chloroform, and bacterial toxins: These substances can weaken heart muscle function.
      • Ischemia: A lack of blood flow to the heart muscle can significantly reduce contractility.
      • Mild cold: Slows down chemical reactions, potentially reducing contractility.
      • ↑K⁺ in ECF: Increased potassium in the extracellular fluid can interfere with muscle function.

    Contractility

    • Preload: The amount of stretch on the heart muscle before contraction.
      • Frank-Starling Law: The heart will pump out more blood with increased preload.
      • ↑Preload → More shortening of cardiac muscle.
    • Contractility: The intrinsic strength of the heart muscle.
    • Afterload: The resistance the heart has to pump against to eject blood.
      • ↑Afterload → Less shortening of cardiac muscle.

    Cardiac Output (CO)

    • The volume of blood pumped by the heart per minute (usually 5.5L/min at rest).
    • Cardiac Index (CI): CO divided by body surface area (3.2 L/min./m²).
    • Determinants of CO: Stroke volume (SV) and heart rate (HR).
      • SV: Amount of blood ejected per beat.
      • ↑Preload, ↑Contractility → ↑SV.
      • ↑Afterload → ↓SV.

    Arterial Blood Pressure (ABP)

    • ABP = CO x PR (Cardiac Output x Peripheral Resistance).
    • Represents the pressure of blood flowing through the arteries.

    Components of the Cardiovascular System (CVS)

    • Heart: A four-chambered organ responsible for pumping blood throughout the body.
    • Blood Vessels: A network of tubes (arteries, veins, capillaries) that transport blood.

    Structural Functional Relationship of the Heart

    • The structure of the heart is optimized for its function as a pump.
    • The heart's chambers, valves, and muscle tissue work together to ensure efficient blood flow.

    SA Node

    • The Sinoatrial (SA) node is the pacemaker of the heart.
    • It initiates electrical impulses that stimulate heart contractions.

    Fast and Slow Response Fibers

    • Fast Response Fibers: Found in the atria, ventricles, and Purkinje fibers.
      • Have a steep upstroke in their action potential, which allows for rapid conduction of electrical impulses.
    • Slow Response Fibers: Found in the SA node and AV node (atrioventricular node).
      • Have a slower upstroke in their action potential which leads to slower conduction.

    Factors Affecting Autorhythmicity

    • Positive Chronotropic Factors: Increase the heart rate.
      • Sympathetic stimulation: Releases norepinephrine, increasing the heart rate.
      • Fever: Raises body temperature, speeding up heart rate.
      • Mild alkalosis: Increased pH level can slightly increases heart rate.
      • Mild hypoxia: A slight decrease in oxygen levels can stimulate the heart to beat faster.
    • Negative Chronotropic Factors: Decrease the heart rate.
      • Parasympathetic stimulation: Releases acetylcholine, slowing the heart rate.
      • Hypothermia: Low body temperature slows the heart rate.
      • Mild acidosis: Decreased pH level can slightly slow the heart rate.
      • Severe hypoxia: A significant decrease in oxygen can reduce or stop heart contractions.

    Conductivity

    • The heart's ability to transmit electrical impulses from one cell to the next, allowing for coordinated heart muscle contractions.
    • The conductive system of the heart:
      • SA Node (pacemaker)
      • AV Node: Relays signals from the atria to the ventricles after a brief delay.
      • Bundle of His: Conducts impulses to the right and left bundle branches.
      • Purkinje Fibers: Carry impulses throughout the ventricles, stimulating contraction.

    Contractility

    • The ability of the heart to convert chemical energy into mechanical energy, resulting in muscle contraction.
    • Increased intracellular Ca⁺⁺ triggers contraction:
      • Extracellular fluid (ECF): Some calcium enters from the outside of the cell.
      • The Sarcoplasmic reticulum (SR): Stores and releases Ca⁺⁺.

    Cardiopulmonary System

    • Includes the heart and the lungs.
    • Regulates the transport of oxygen and nutrients to tissues and the removal of carbon dioxide waste.
    • Helps regulate body temperature.

    Two Major Adjustments of Blood Flow During Exercise

    • Increased Cardiac Output
    • Redistribution of blood flow.

    Organization of the circulatory system

    • The heart is a double pump.
    • The right side of the heart pumps blood through the pulmonary circulation (lungs).
    • The left side of the heart pumps blood through the systemic circulation (rest of the body).

    Exercise Training Protects the Heart

    • Reduces the incidence of heart attacks.
    • Improves survival from heart attacks.
    • Less myocardial damage from heart attacks.
    • Improvements in the heart's antioxidant capacity.
    • Improved function of ATP-sensitive potassium channels.

    Cardiac Output (CO)

    • The amount of blood pumped by the heart per minute.
    • Q = HR (Heart Rate) x SV (Stroke Volume)
    • CO increases during exercise to meet the increased demands of the body.

    Factors that Regulate Cardiac Output

    • Preload, contractility, afterload, heart rate influence cardiac output.

    Resting Cardiac Output

    • Typically ~ 5 L/min.
    • Can be significantly higher during exercise (~ 25 to 45 L/min in elite athletes).

    Dynamic Exercise

    • ↑ Muscle pump + ↑ Sympathetic vasoconstriction → ↑ Venous return → ↑ Stroke volume → ↑ Cardiac output.
    • Muscle "pump": Contracting muscles squeeze veins, increasing venous return.
    • Increased cardiac output during exercise is achieved by both increased stroke volume and increased heart rate

    Cardiac Output Increases

    • Steady-state heart rate increases linearly with work rate from rest to VO2 max.
    • Increased sympathetic nervous system activity and reduced parasympathetic activity increase heart rate.
    • Signals from working muscles, blood-borne metabolites, and increased body temperature contribute to heart rate increase.

    Heart Rate

    • Maximum HR is predicted by the rule: HR (b.p.m.) = 220 - age.
    • Endurance training lowers resting and maximum heart rate.

    Blood Pressure

    • Systolic Pressure (SBP): Rises during exercise (150-170 mmHg).
    • Diastolic Pressure (DBP): Remains relatively stable during dynamic exercise but can rise during isometric exercise.

    Muscle Chemoreflex

    • ↑ Muscle lactate during heavy exercise stimulates muscle chemoreceptors and afferent nerves.
    • This leads to increased sympathetic nervous system activation, boosting heart rate, cardiac output, and vasoconstriction in the viscera and non-working muscles.
    • Vasodilation occurs in the working skeletal muscles.

    Endurance and Strength Training

    • Both types of exercise have different impacts on the cardiovascular system.
    • Endurance training improves cardiovascular fitness, while strength training strengthens muscles.

    Blood Flow Redistribution

    • Regulated by sympathetic nervous system activity and chemical factors.

    Redistribution of Blood Flow During Exercise

    • Increased blood flow to working skeletal muscle:
      • At rest, 15-20% of cardiac output goes to muscle.
      • This increase to 80-85% during maximal exercise.
    • Decreased blood flow to less active organs:
      • Liver, gastrointestinal tract.
    • This redistribution is determined by metabolic rate and exercise intensity.

    Increased Blood Flow to Working Skeletal Muscle

    • Vasodilatory metabolites like AMP, adenosine, H+, and K+ dilate pre-capillary sphincters, overriding vasoconstriction from norepinephrine.

    Changes in Muscle and Splanchnic Blood Flow During Exercise

    • Blood flow changes during exercise vary depending on the muscle group involved and the type of exercise.

    The Muscle Thoracic Pump Helps Venous Return

    • Contracting muscles press on veins, propelling blood towards the heart.
    • The thoracic or respiratory pump also contributes to venous return by compressing veins in the chest and abdomen during breathing.

    Nitric Oxide is an Important Vasodilator

    • Produced in the endothelium of arterioles.
    • Promotes smooth muscle relaxation, leading to vasodilation and increased blood flow.
    • Plays a key role in autoregulation alongside other local factors.
    • Improves muscle blood flow during exercise.

    Coronary Artery and Blood Flow During Exercise

    • Coronary blood flow increases substantially during exercise.
    • This is due to increased cardiac output, shear stress, and endothelial-dependent vasodilation.
    • Nitric oxide and prostacyclin contribute to vasodilation.

    Circulatory Responses to Exercise

    • Heart rate and blood pressure responses differ during arm and leg exercise.

    The Long-Term Effect of Exercise: The Heart

    • Aerobic exercise strengthens the heart.

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    Description

    Explore the roles of the sarcoplasmic reticulum in muscle contraction and the various factors affecting the contractility of heart muscles. This quiz covers both positive and negative inotropic factors, highlighting how different stimuli influence heart performance. Test your knowledge on how calcium levels and hormonal changes impact contractility.

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