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MCAT

Unit 3: Lesson 1

Chemical and physical sciences practice passage questions

Understanding cardiac pressure-volume curves

Problem

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
Which of these following reasons best describes why the volume remains constant despite pressure increase during section BC?
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