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Answers to Study Questions

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  • 1–1. The lungs always receive more blood flow than any other organ because 100% of the cardiac output always passes through the lungs.
  • 1–2. False. Flow through any vascular bed depends on its resistance to flow and the arterial pressure. As long as this pressure is maintained constant (a critical point that is dependent upon adjustments in cardiac output), alterations in flow through any individual bed will have no influence on flow through other beds in parallel with it.
  • 1–3. A leaky aortic valve will cause a diastolic murmur. Normally, the aortic valve is closed during diastole when there is a large reverse pressure difference between the aorta and left ventricle.
  • 1–4. False. Slowing conduction through the AV node will have no effect on the heart rate but will increase the interval between atrial and ventricular excitation. The heart rate is normally slowed by decreases in the rate of action potential initiation by pacemaker cells in the SA node.
  • 1–5. a. Saying that the diameter of a vessel increases by 10% is equivalent to saying that its radius increases by 10%; that is, the radius after the change is 1.1 times the radius before the change. The Poiseuille equation says that, other factors equal, resistance is proportional to 1 over radius to the fourth power:

    R ∝ 1/r4

    Thus,

    Rbefore ∝ 1/(rbefore)4

    Rafter ∝ 1/(1.1rbefore)4 = [1/(1.1)4][1/(rbefore)4] = (1/1.46)Rbefore = 0.68 Rbefore

    Therefore, a 10% increase in vessel radius will reduce its resistance by 32%.

    b. Because Q̇ = ΔP/R, and Rafter = 0.68 Rbefore

    Q̇after = ΔP/0.68 Rbefore

    … and for a given ΔP…

    Q̇after = 1/0.68 Rbefore = 1.46 Q̇before

    Therefore, for a given ΔP, a 10% increase in vessel diameter will increase flow by 46%.

    [Note: One must always use change factors NOT percentage changes in these equations.]

  • 1–6. a. Wrong! The pulmonary and systemic circuits are arranged in series and therefore must have the same through-flow.

    b. Wrong! The right and left hearts are served by a common electrical excitation system and therefore beat at the same rate.

    c. Wrong! Although it is true that the right ventricle is less muscular than the left, that fact does not explain why pulmonary pressure is so low. Actually, it is the other way around: because pulmonary arterial pressure is low, the right ventricle does not have to be very muscular to pump blood into the lungs.

    d. Right! ΔP = Q̇ × R. Because Q̇ is the same (= CO) through the lungs and the systemic circulation, the only way pulmonary arterial pressure could be relatively low is for pulmonary vascular resistance to be relatively low.

    e. Wrong! CO = HR × SV. CO and HR are the same for both the right ...

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