Cardiac pressure-volume curves 2
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Cardiac pressure-volume curves 2
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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.
The mechanism by which the heart reliably pumps blood to the body can be understood by looking solely at the mechanics of the left ventricle, the thickest chamber of the heart. From a fluid motion perspective, the left ventricle can be modelled as a single-chamber pump (Figure 1) attached in series to two pressure-activated, one-way valves. Using radiometric techniques, the volume and shape of the heart can be imaged in real time, as can the volume of blood that passes through the heart during each part of the cardiac cycle. This data can be combined to generate a pressure-volume diagram for the left-ventricle, corresponding to a single, full cardiac cycle (Figure 2).
During the diastolic phase (segment AB in the Figures), the heart refills with blood from its contracted state at the end of the previous cycle. Thus, this phase is essentially a return stroke, in which blood slowly fills the heart, increasing its volume without changing the pressure due to the flexibility of the walls. The mitral valve closes at B, at which point the heart begins to contract around the trapped blood. The aortic valve opens at C, but the heart continues to contract---resulting in a rapid efflux of blood at a constant pressure. The aortic valve closes at D, and the heart tissue expands, relieving pressure, until the mitral valve re-opens at A, blood rushes back in, and the cycle repeats.
Figure 1: A simple diagram of the four phases of the cardiac cycle
Figure 2: A pressure-volume diagram for the four phases of the cardiac cycle
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[question] => Which of these following reasons best describes why the volume remains constant despite pressure increase during section BC?
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[description] => Reason for the Correct Answer:
The ideal gas law, if it applied to blood, would imply a steady volume reduction in response to a pressure increase.
The two closed valves prevent blood from exiting the heart during this phase
The incompressibility of liquids (like blood) explains the constant volume during section BC
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[each_answer] => A. The ideal gas law
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[each_answer] => B. Boyle’s law
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[each_answer] => C. Liquid incompressibility
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[each_answer] => D. Blood efflux
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[question] => During the pressurization phase BC, what is the pressure in the narrow valve entrance compared to the wider chamber?
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[answer] => 1
[description] => Reason for the Correct Answer:
At point B, fluid is flowing out through the opening
Hydrostatic pressure does not reflect the pressure of a flowing fluid
The pressure in the opening is smaller due to the Venturi effect
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[0] => Array
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[each_answer] => A. Smaller
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[each_answer] => B. Larger
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[each_answer] => C. The same
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[quiz_unique_key] => 83407773
[question] => During the efflux phase CD, what is the pressure in the narrower valve entrance compared to the wider chamber?
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[answer] => 1
[description] => Reason for the Correct Answer:
At point C, no fluid flow is occurring in the heart
Pressure increases equilibrate very quickly when a fluid is stationary
The hydrostatic pressure in the valve and the chamber are EQUAL via Pascal’s principle
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[each_answer] => A. Equal
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[each_answer] => B. Larger
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[question] => During which segment of the cycle does the mechanical potential energy of the heart increase?
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[answer] => 2
[description] => Reason for the Correct Answer:
Mechanical potential energy is often stored as either gravitational potential energy or elastic (spring) potential. The former is not relevant to this system.
During blood influx and outflux, the volume of the chamber is able to change in response to pressure, and so the tissue does not undergo heavy stretching or deformation.
Pay attention to the source of the pressure increase in the heart; since gas laws don’t apply, it must be due to a stored mechanical force (like the flexible rubber membrane of a tire)
Mechanical potential is stored during the pressurization phase BC
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[each_answer] => A. CD
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[each_answer] => B. BC
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[each_answer] => C. DA
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[each_answer] => D. AB
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[quiz_unique_key] => 574431310
[question] => Which of the following quantities is given by the area enclosed by the loop?
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[answer] => 4
[description] => Reason for the Correct Answer:
The area within the loop has units of (pressure) x (volume)
Analyzing the units, (pressure) x (volume) = (Force/(Distance²)*(Distance³) = (Force)(distance) = Work
Alternatively, remember that the area within a closed loop on a pV diagram always gives the work performed by a heat engine.
The area of the loop gives the work performed during one cardiac cycle
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[each_answer] => A. The total tissue displacement during one cardiac cycle
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[each_answer] => B. The total number of moles of blood in the heart times the temperature
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[each_answer] => C. The net force exerted during one cardiac cycle
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[each_answer] => D. The total work performed during one cardiac cycle
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[558129|3] => A
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Figure 2: A pressure-volume diagram for the four phases of the cardiac cycle
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