Cardiac pressure-volume curves 1
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Cardiac pressure-volume curves 1
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
As a pump, the heart continually is increasing and decreasing the pressure it exerts on blood. During diastole, or the relaxation phase, blood flows into the various chambers (called the atria and ventricles) of the heart, and during systole blood is pumped out. The total mechanical energy involved in this process can be calculated simply by multiplying the pressure of the blood by the volume of blood ejected from a given chamber (stroke volume). Stroke volume is distinct from the total volume of blood in a heart chamber, because not all blood is ejected from a chamber during a contraction. This amount of energy transferred by the heart during a contraction is often referred to as ‘stroke work’. Much like work done by an internal combustion engine, we can treat the heart as a thermodynamic system, and visually represent stroke work with the use of a pressure-volume diagram, as seen in Figure 1.
Figure 1. Generalized Cardiac Pressure-Volume Loop Diagram
Let’s say that this diagram describes the relationship of volume and pressure specifically for the left ventricle of a heart. Position 1 of the graph would then mark the opening of the mitral valve, which allows blood to fill the chamber from the left atrium (diastole). Position 2 marks the closing of the mitral valve, and the beginning of systole. The chamber begins to contract, increasing blood pressure until it raises high enough to push open the aortic valve, leading out of the heart (Position 3). Once blood is ejected from the chamber (Position 4), pressure will decrease and the cycle is completed, which begins diastole again.
There are various ways which we can influence how much work the heart does in a given stroke. One way we can increase stroke work is by increasing the volume of blood in the chamber during diastolic filling, known as preload. In other words, because the volume of preload is directly proportional to both stroke volume and blood pressure, an increase or decrease of preload will also increase or decrease stroke work respectively.
[post_title] => Cardiac pressure-volume curves 1
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[question] => Which area of the graph would be equal to the amount of work done to the system after systole?
[value] => Array
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[answer] => 3
[description] => Reason for the Correct Answer:
Net stroke work is found by adding the work done by the system (curve 3-4) to the work put into the system (curve 1-2) since the work done by the system is negative, we can subtract the larger area from the smaller area to get the net work done by the system, which is the shaded area of the graph.
The area underneath curve 3-4 is representative of the work done by the system.
The area underneath curve 1-2 is representative of the work put into the system.
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[answers] => Array
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[0] => Array
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[each_answer] => A. The area underneath line C entirely from points 3-4
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[1] => Array
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[each_answer] => B. The area contained within the 4 points of the cycle
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[2] => Array
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[each_answer] => C. The area underneath line A from points 1-2
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[3] => Array
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[each_answer] => D. The area underneath the curve from 2-3
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => From step 4 to step 1, no work is being done either to or by the system. What type of thermodynamic process would this be?
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[answer] => 2
[description] => Reason for the Correct Answer:
An isobaric process involves a process at constant pressure. from 4 to 1, pressure is changing.
An isothermal process is one that occurs at a constant temperature.
An adiabatic process is one that occurs so quickly that no energy is lost as heat, cardiac muscle cannot function that quickly.
Because volume is being held constant as pressure changes, the process from 4 to 1 is an isochoric process (also known as a constant-volume process, an isovolumetric process, or an isometric process)
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[answers] => Array
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[0] => Array
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[each_answer] => A. Adiabatic process
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[each_answer] => B. Isochoric process
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[each_answer] => C. Isobaric process
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[each_answer] => D. Isothermal process
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => How would a decrease in preload affect the shape of the Cardiac Pressure-Volume Loop?
[value] => Array
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[answer] => 4
[description] => Reason for the Correct Answer:
If preload is decreased, then the net stroke work would decrease, and the area within the cycle would decrease.
Increasing preload would result in an increase in pressure and stroke volume, shifting the cycle to the right.
Decreasing preload would result in a decrease in pressure and stroke volume, shifting the cycle to the left.
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[answers] => Array
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[0] => Array
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[each_answer] => A. It would increase the area within the cycle, and shift it to the right
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[1] => Array
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[each_answer] => B. It would increase the area within the cycle, and shift it to the left
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[2] => Array
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[each_answer] => C. It would decrease the area within the cycle, and shift it to the right
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[each_answer] => D. It would decrease the area within cycle, and shift it to the left
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[quiz_unique_key] => 2261298308
[question] => Aortic stenosis is a condition which stiffens the aortic valve. This results in the need for more pressure to cause ejection during systole. Which of the following would represent a cardiac cycle of someone with aortic stenosis?
[value] => Array
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[answer] => 2
[description] => Reason for the Correct Answer:
The graph should indicate an increased area because the heart has to work harder to push the blood past the stenotic valve.
Aortic stenosis would not result in an increased volume within the ventricle.
The graph should indicate a large increase in pressure during systole, which is evidenced in the second graph.
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[answers] => Array
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[0] => Array
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[each_answer] => A.
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[1] => Array
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[each_answer] => B.
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[2] => Array
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[each_answer] => C.
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[each_answer] => D.
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[quiz_unique_key] => 574431310
[question] => An adiabatic process is a thermodynamic process that involves maintaining a constant entropy, that is, no heat is lost from the system during the process. Would the cardiac cycle be considered an adiabatic process?
[value] => Array
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[answer] => 3
[description] => Reason for the Correct Answer:
Heart valves open due to the pressure of blood pushing on them, therefore they are opened by a passive process.
An adiabatic process happens in an extremely short time, much faster than a heartbeat.
Energy will be lost as heat during muscle contraction due to the metabolic processes involved in fueling cardiac tissue.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Yes, only mechanical energy from the heart is transferred directly to the blood
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[1] => Array
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[each_answer] => B. Yes, mechanical energy and elastic potential energy are transferred without an increase in entropy
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[2] => Array
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[each_answer] => C. No, some energy is lost as heat due to the chemical processes of cardiac muscle contraction
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[each_answer] => D. No, some energy is lost as heat due to the active process of opening heart valves
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