Cardiac Output and Its Variations
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Questions and Answers

Which of the following factors directly increases preload?

  • Increased pulmonary vascular resistance
  • Increased contractility
  • Increased aortic pressure
  • Increased venous return (correct)

    • Increased afterload results in a decreased stroke volume.

    True (A)

    What is the ejection fraction (EF) and what is its significance?

    Ejection fraction (EF) is the percentage of blood ejected from the ventricle with each heartbeat. It is a measure of the heart's pumping efficiency. A normal EF is about 55-75%.

    The ______ pump and the ______ pump are two main body pumps that contribute to venous return and thus preload.

    <p>Respiratory, Skeletal Muscle</p> Signup and view all the answers

    Match the following factors with their impact on cardiac output:

    <p>Increased contractility = Increased stroke volume Increased heart rate = Increased cardiac output Increased afterload = Decreased stroke volume Increased preload = Increased stroke volume</p> Signup and view all the answers

    What is the primary factor affecting afterload?

    <p>Arterial pressure (A)</p> Signup and view all the answers

    Explain how systemic hypertension impacts afterload.

    <p>Systemic hypertension elevates the pressure in the arteries, making it more difficult for the left ventricle to pump blood out into the systemic circulation. This increased resistance to blood flow is the increased afterload.</p> Signup and view all the answers

    Aortic stenosis increases afterload on the right ventricle.

    <p>False (B)</p> Signup and view all the answers

    What is the typical value of cardiac output in a normal adult?

    <p>5 L/min (D)</p> Signup and view all the answers

    Cardiac index is the cardiac output per square meter of body surface area.

    <p>True (A)</p> Signup and view all the answers

    What two factors determine cardiac output?

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

    A pathological decrease in cardiac output can be caused by __________.

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

    Which of the following is a pathological increase in cardiac output?

    <p>Fever (C)</p> Signup and view all the answers

    Match the following terms with their definitions:

    <p>End diastolic volume = Volume of blood in the ventricles at the end of diastole Total peripheral resistance = Frictional resistance in the arteries Stroke volume = Amount of blood pumped by the left ventricle in one contraction Cardiac output = Amount of blood the heart pumps from each ventricle per minute</p> Signup and view all the answers

    Cardiac output is vital for supplying __________ and nutrients to tissues.

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

    List two factors that can affect cardiac output.

    <p>Body metabolism and exercise level</p> Signup and view all the answers

    What effect does stimulation of the cardio-acceleratory center have on heart rate?

    <p>Increases heart rate (A)</p> Signup and view all the answers

    The chemoreceptor reflex is more important than the baroreceptor reflex in regulating cardiac function.

    <p>False (B)</p> Signup and view all the answers

    What neurotransmitter is released by the cardio-acceleratory center to increase heart rate?

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

    Increases in intrathoracic pressure can lead to decreased _______ and cardiac output.

    <p>venous return</p> Signup and view all the answers

    Match the following centers with their effects on heart rate:

    <p>CAC = Increases heart rate CIC = Decreases heart rate Baroreceptor Reflex = Regulates BP based on stretch Chemoreceptor Reflex = Responds to low O2 and high CO2</p> Signup and view all the answers

    Which of the following factors decreases left ventricular end-diastolic volume (LVEDV)?

    <p>Volume contraction (D)</p> Signup and view all the answers

    The release of acetylcholine by the cardio-inhibitory center increases heart rate.

    <p>False (B)</p> Signup and view all the answers

    What is the primary role of the autonomic nervous system in cardiac function?

    <p>To control heart rate and strength of contraction</p> Signup and view all the answers

    Signup and view all the answers

    Study Notes

    Cardiac Output

    • Cardiac output (CO) is the volume of blood pumped by each ventricle per minute.
    • CO is roughly the same for both ventricles, approximately 5 liters/minute in a normal adult.
    • 75% of CO is distributed to vital organs (liver, kidney, brain, lungs, and heart).
    • Cardiac index is CO per square meter of body surface area. A typical value is about 3 L/min/m².

    Physiological Variations in Cardiac Output

    • Age: Cardiac output is higher in adults than in children due to higher blood volume.
    • Gender: Cardiac output is usually higher in males than females.
    • Altitude: Cardiac output increases at high altitudes.
    • Pregnancy: Cardiac output increases during pregnancy.
    • Exercise: Cardiac output increases during exercise.
    • Emotion: Cardiac output increases during emotional stress.

    Pathological Variations in Cardiac Output

    • Increase: Hyperthyroidism and fever.
    • Decrease: Hypothyroidism, hypovolemia, hemorrhage, and myocardial infarction.

    Importance of Cardiac Output

    • Cardiac output is essential for energy production in tissues.
    • Adequate perfusion is critical for energy supply and proper cellular function.
    • Sufficient cardiac output supplies oxygen and nutrients to tissues.

    Factors Affecting Cardiac Output

    • Body metabolism
    • Exercise status
    • Age
    • Body size

    Stroke Volume

    • Stroke volume (SV) is the amount of blood pumped by the left ventricle in a single contraction.
    • Normally, only about two-thirds of the blood in the ventricle is ejected with each beat.
    • SV, along with heart rate, determines the cardiac output.

    Stroke Volume Regulation

    • End-diastolic volume (EDV) (preload): This refers to the volume of blood in the ventricles at the end of the diastole. Larger EDV leads to larger SV due to the Frank-Starling mechanism.
    • Total peripheral resistance (afterload): Frictional resistance in the arteries. Higher resistance leads to lower stroke volume as the heart must work harder.
    • Contractility: This is the strength of ventricular contraction. Stronger contractions result in higher stroke volume.

    Regulation of Stroke Volume

    • Stroke volume increases with increased EDV.
    • Stroke volume is inversely related to total peripheral resistance.
    • Stroke volume increases with contractility.

    Ejection Fraction (EF)

    • Ejection fraction is the percentage of end-diastolic volume ejected per cardiac cycle.
    • Normal ejection fraction is approximately 55-75%.
    • EF = (SV / EDV) x 100

    Heart Rate (HR) and its Effect on Cardiac Output (SV)

    • Changes in heart rate alone affect stroke volume inversely.
    • SV increase occurs along with other factors (venous return, venous constriction, etc.) in cases of increased HR (example: exercise).
    • In the absence of these factors, elevated HR cannot maintain sufficient SV.

    Preload

    • Preload is the amount of stretch of the ventricular myocytes at the end of diastole.
    • High preload leads to high stroke volume and cardiac output.
    • Factors increasing preload:
      • Exercise
      • Increased blood volume (e.g., transfusion, polycythemia)
      • Neuroendocrine excitement (sympathetic tone)
      • Arteriovenous fistula

    Factors Affecting Preload:

    • Respiratory pump: Changes in intra-pleural and abdominal pressure move blood towards the heart during inspiration.
    • Skeletal muscle pump: Muscles of the legs compress veins, facilitating blood flow toward the heart.

    Afterload

    • Afterload is the pressure the heart must overcome to eject blood during systole (ventricular contraction).
    • Afterload is proportional to the average arterial pressure in the aorta and pulmonary artery.
    • Increased afterload increases the workload of the heart.
    • Factors increasing afterload:
      • Systemic and pulmonary hypertension.
      • Aortic and pulmonary stenosis/regurgitation.
      • Increased vascular resistance.

    Regulation of Cardiac Output

    • Maintaining a constant cardiac output around 5 liters/minute under normal conditions.
    • Cardiac output adjusts to meet physiological demands.

    Factors Affecting Cardiac Output

    • Primary Factors: Blood volume reflexes, autonomic innervation, and hormones.
    • Secondary Factors: Extracellular fluid ion concentration, body temperature, emotions, sex, and age.

    Blood Volume Reflexes

    • Atrial/Bainbridge Reflex: Triggered by increased venous return (e.g., increased blood volume). It increases heart rate.
    • Ventricular Reflex: The amount of blood ejected is dependent on the amount of blood filling the ventricles during diastole (Frank-Starling Law).

    Mechanisms of Increased Ventricular Filling and Stroke Volume

    • Increased venous return leads to an increase in end-diastolic volume
    • Stretching cardiac myocytes prior to contraction increases force of contraction.
    • Increased sarcomere length leads to an increase in force generation.
    • Increased ventricular filling results in increased stroke volume

    Factors Affecting Frank-Starling Physiology

    • Increase in LVEDV and CO: Volume expansion, avoiding increases in intrathoracic pressure, and augmenting venous tone/return.
    • Decrease in LVEDV and CO: Volume contraction (e.g., bleeding), increases in intrathoracic pressure, and decreased venous tone/return.

    Autonomic Innervation

    • The autonomic nervous system modulates heart rate, which is crucial for short-term blood pressure and cardiac output control.
    • Cardio-acceleratory center (CAC): Located in the medulla oblongata, it increases heart rate and contractility via norepinephrine release.
    • Cardio-inhibitory center (CIC): Located in the medulla oblongata, it decreases heart rate via acetylcholine release.

    Baroreceptor Reflex

    • Baroreceptors detect changes in arterial pressure.
    • Elevated arterial pressure results in negative chronotropic and inotropic effects.

    Chemoreceptor Reflex

    • Chemoreceptors detect changes in oxygen, pH, and CO2 levels.
    • Low oxygen, low pH, or high CO2 levels trigger positive chronotropic and inotropic effects.

    Hormones

    • Norepinephrine, epinephrine, and acetylcholine affect myocardial contractility and heart rate, therefore influence stroke volume.

    Secondary Factors

    • ECF ion concentration (Potassium or Sodium): High levels can decrease heart rate and stroke volume, while low levels can affect normal cardiac function.
    • Calcium ion concentration: Affects the strength and duration of cardiac contractions; high levels increase contraction, while low ones weaken contraction.
    • Temperature and Emotions: Temperature changes affect heart rate and contractility. Intense emotions can affect heart rate.
    • Sex and Age: Heart rate varies with sex and age; females usually have a faster heart rate than males, and heart rate is highest at birth but decreases with age.

    Table No. 1. Example Values in Healthy 70 kg Man

    • End Diastolic Volume (EDV): 120 ml (range 65–240 ml)
    • End Systolic Volume (ESV): 50 ml (range 16–143 ml)
    • Stroke Volume (SV): 70 ml (range 55–100 ml)
    • Ejection Fraction (EF): 65% (range 55–70%)
    • Heart Rate (HR): 75 bpm (range 60–100 bpm)
    • Cardiac Output (CO): 5 L/minute (range 4.0–8.0 L/minute)

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    Explore the concept of cardiac output, including its physiological and pathological variations. Understand how factors such as age, gender, exercise, and altitude impact cardiac output. Learn why maintaining proper cardiac output is vital for overall health.

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