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Questions and Answers
What causes the first heart sound, or the 'lub', in the cardiac cycle?
In the action potential of the sinoatrial (SA) node, which ion influx is primarily responsible for the upward swing during Phase 0?
Which phase of the cardiac cycle occurs when the heart chambers are ejecting blood?
What is the cardiac output in a healthy adult with a heart rate of 70 bpm and a stroke volume of 75 mL?
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During which phase of the action potential does repolarization occur in the ventricular muscle due to the efflux of potassium ions?
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What primarily determines stroke volume?
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Which factor is NOT involved in determining preload?
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Which law describes the relationship between blood return to the heart and stroke volume?
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What does afterload primarily refer to?
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Which of the following factors does NOT influence total peripheral resistance (TPR)?
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Which equation correctly expresses the relationship between blood pressure, cardiac output, and total peripheral resistance?
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Which factor does NOT affect blood pressure?
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Which statement about preload is true?
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Study Notes
Heart Sounds
- The “lub-dub” sound of the heartbeat is caused by the closure of heart valves
- The “lub” sound occurs when the ventricles contract and the atrioventricular (AV) valves close.
- The “dub” sound occurs when the ventricles relax and the semilunar valves close
Sinoatrial (SA) Node Electrical Activity
- The SA node is the heart's natural pacemaker
- During Phase 4, the pacemaker potential, the cells of the SA node depolarize due to the influx of Na+ and Ca++ ions
- During Phase 0, the action potential occurs due to the influx of calcium through slow Ca++ channels.
- During Phase 3, repolarization, occurs due to the efflux of K+ ions.
Ventricular Muscle Electrical Activity
- The resting membrane potential, Phase 4, is very negative due to the high permeability of the K+ channels.
- During Phase 0, the action potential occurs due to the rapid influx of sodium through fast Na+ channels.
- During Phase 1, a brief repolarization occurs because of the closure of the Na+ channels.
- During Phase 2, a plateau occurs due to the influx of calcium through slow Ca++ channels.
- During Phase 3, repolarization occurs due to the efflux of K+ ions.
Cardiac Cycle
- The cardiac cycle consists of two phases: systole and diastole.
- During Systole: The chambers of the heart contract and eject blood.
- During Diastole: The chambers of the heart relax and fill with blood.
Cardiac Output
- Cardiac Output is the amount of blood pumped by the heart per minute.
- Cardiac Output is calculated by multiplying heart rate and stroke volume.
- Cardiac Output = Heart Rate x Stroke Volume
- For a healthy adult, Cardiac Output is approximately 5L/min
Factors Influencing Cardiac Output
- Stroke Volume: the amount of blood ejected from each ventricle with each heartbeat.
- Heart Rate: regulated by the autonomic nervous system (ANS).
Stroke Volume
- Stroke Volume is determined by three factors: preload, afterload, and contractility.
Preload
- Preload represents the volume of blood in the ventricle at the end of diastole (End Diastolic Volume).
- Factors affecting preload include:
- Blood Volume
- Venous Return
- Vasomotor Tone
Preload: Factors Influencing Venous Return
- Muscle Pump: Contraction of skeletal muscles surrounding veins during physical activity compresses veins, aiding blood flow towards the heart.
- Respiratory Pump: Inhalation reduces pressure in the thoracic cavity and increases pressure in the abdominal cavity, promoting venous return.
- Vein Valves: One-way valves prevent backflow of blood.
Starling’s Law of the Heart
- If blood returning to the heart increases, the ventricle will fill more during diastole.
- This stretching of the ventricular fibers results in a more powerful contraction on the subsequent systole, increasing stroke volume.
- The more fibers are stretched, the greater the force of the contraction (within reason).
Afterload
- The resistance against which the ventricle must work.
- Preload is a major determinant of myocardial contractile power, but afterload mainly influences mechanical performance.
Total Peripheral Resistance (TPR)
- This refers to the overall resistance to blood flow in the systemic circulation.
- The majority of resistance originates in the arterioles and smaller arteries, influencing blood pressure.
Factors Influencing TPR
- Vessel Length: Longer vessels offer more resistance.
- Vessel Diameter: Narrower vessels offer more resistance.
- Blood Viscosity: Thicker blood offers more resistance.
Blood Pressure
- Blood Pressure is the force exerted on a blood vessel wall by blood.
- The heart is the pump that circulates blood, creating pressure differences needed for blood flow.
Factors Influencing Blood Pressure
- Cardiac Output (CO): Higher Cardiac Output leads to higher blood pressure.
- Total Peripheral Resistance (TPR): Higher TPR leads to higher blood pressure.
- Blood Volume and Viscosity: Increased blood volume or viscosity increases blood pressure.
- Elasticity of Blood vessels: Arteries with decreased elasticity lead to higher blood pressure.
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Description
Test your knowledge of heart sounds and the electrical activity of the heart. This quiz covers the functions of the SA node, the phases of action potential in ventricular muscle, and the significance of heart valve closure. Perfect for students of physiology and cardiology.
