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Questions and Answers
What is the term used to describe the relationship between ventricular work and the extent of ventricular stretch?
What is the primary factor that determines the force of contraction in the heart?
What is the term used to describe the pressure in the right or left atrium?
What is the primary mechanism by which force increases with stretch in cardiac muscle?
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What is the term used to describe the volume of blood in the ventricle at the end of diastole?
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What is the term used to describe the relationship between cardiac work and the extent of ventricular stretch?
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What is preload in cardiac physiology?
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What is the main factor that determines afterload for the left ventricle?
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What is the name of the law that states that the total energy liberated at each heartbeat is determined by the diastolic volume of the heart?
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What is the normal range for right ventricular end-diastolic pressure?
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Who showed that the relationship between diastolic volume and isovolumetric pressure development was present in an intact circulation?
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What is the term for the pressure developed by a ventricle during systole?
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What is the term for the force against which a ventricle pumps to eject blood?
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Who showed that isovolumetric pressure development in a frog heart depended on diastolic distension?
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What is the effect of titin on myosin binding protein C?
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What determines CO according to the Frank-Starling Law?
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What is the result of increased volume of blood in the pulmonary veins?
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What is the definition of cardiac contractility?
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What regulates cardiac contractility?
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What is the effect of sympathetic stimulation on cardiac contractility?
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What is the effect of increased afterload on CO?
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What is the result of increased stroke volume?
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What is the primary effect of noradrenaline on cardiac contractility?
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What is the hallmark of a characterised heart failure curve?
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What is the effect of a fall in cardiac output on renal excretion of fluid?
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What is the primary response of the sympathetic nervous system to reduced blood pressure?
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What is the result of the Frank-Starling mechanism in response to decreased cardiac contractility?
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What is the definition of heart failure?
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What is the primary effect of aortic valve stenosis on cardiac output?
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What is the main reason for the low pressure gradient between the microcirculation and the right heart?
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What is the effect of venoconstriction on central venous pressure (CVP)?
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What is the role of the skeletal muscle pump in the venous system?
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What is the effect of inspiration on the pressure within the vena cavae?
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What is the primary mechanism by which CVP is regulated?
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What is the effect of increased afterload on stroke volume?
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What is the primary effect of increased CVP on cardiac output?
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Study Notes
Cardiac Output
- Cardiac output is directly affected by four things: heart rate, contractility, preload, and afterload.
Preload
- Preload is the degree of stretch of a ventricle immediately before it contracts.
- It is a function of the end-diastolic volume (EDV) and is related to the filling pressure of the ventricle.
- Preload for the right ventricle is typically 3-8 mmHg, equivalent to the right ventricular end-diastolic pressure (RVEDP), right atrial pressure (RAP), and central venous pressure (CVP).
- Preload for the left ventricle is equivalent to the left ventricular end-diastolic pressure (LVEDP) and left atrial pressure, which is also influenced by pulmonary venous pressure.
Afterload
- Afterload is the force against which a ventricle pumps to eject blood.
- For the left ventricle, afterload is mainly due to aortic blood pressure (approximately 95 mmHg).
- Afterload is also influenced by total peripheral resistance (TPR) and aortic stiffness.
- For the right ventricle, afterload is mainly due to main pulmonary artery pressure.
Frank-Starling Law of the Heart
- The Frank-Starling Law states that "the total energy liberated at each heartbeat is determined by the diastolic volume of the heart and therefore by the muscle fiber length at the beginning of contraction."
- This law indicates that the energy or pressure generated by a ventricle to eject blood is proportional to its output (stroke volume).
- The Frank-Starling relationship operates within a physiological range of preload.
Mechanisms of the Frank-Starling Law
- The Frank-Starling Law is explained by mechanisms involving actin, myosin, and calcium ions.
- Increased stretch of the ventricle leads to increased force of contraction, which is mediated by changes in calcium ion binding and troponin sensitivity.
Consequences of the Frank-Starling Law
- The Frank-Starling Law ensures that the stroke volumes of the left and right ventricles are perfectly matched.
- At a given heart rate and functional state of the heart, central venous pressure (CVP) determines cardiac output (CO).
- The law helps maintain cardiac output even in the face of increased afterload or decreased contractility.
Cardiac Contractility
- Cardiac contractility, also known as inotropy, is the strength of contraction of the heart.
- It is reflected by the amount and rate of cardiac tension development and the ability of the heart to eject a stroke volume at a given preload and afterload.
- Cardiac contractility is regulated by intracellular calcium ions in cardiac myocytes and is influenced by the sympathetic nervous system.
Compensatory Mechanisms in Heart Failure
- The Frank-Starling mechanism can compensate for decreased cardiac contractility in heart failure.
- Heart failure is characterized by a lower amplitude curve, and it can occur acutely or chronically.
- Compensatory mechanisms activated by heart failure include increased fluid retention, sympathetic nervous system activation, and the renin-angiotensin-aldosterone system.
Afterload and Cardiac Output
- Afterload has little effect on cardiac output in the normal range of blood pressures.
- However, a significant increase in afterload, such as in aortic valve stenosis, can reduce stroke volume and cardiac output.
Preload Regulation
- Preload is regulated by the amount of blood in the veins and vein capacitance.
- Venous constriction (e.g., during sympathetic nervous system stimulation) decreases venous capacitance and increases central venous pressure (CVP).
- This allows the output of the right and left ventricles to match and bring more blood to working muscles.
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Description
This quiz covers the factors that affect cardiac output, including preload, afterload, heart rate, and contractility. It also explores the relationship between the ventricles and the heart's pump function.
