prob_set8_solns - Prob_set7_solns Page 1 of 9 End of...

End of Problem Set #7
9. Because all of the baroreceptors are located either in the aortic arches or in the carotid sinuses, the
input to all regulatory reflex loops comes only from those two locations. Thus, regulation of
cardiovascular output occurs in response to changes in the amount of blood leaving the left ventricle and
changes in the amount of blood going to the head (i.e., the brain). If the blood supply to parts of the
body other than the head becomes compromised, there is no reflex response to bring it back to normal.
10. When you are lying down, the pressures in your head and in your feet are about equal--and both are
equal to the pressures in the aorta. When you stand up, gravity pulls blood away from your head and
toward your feet. Thus the average blood pressure in the head will drop, while at the same time the
blood pressure in the legs and feet rises. (Regulatory mechanisms cause the heart to generate enough
pressure to counteract the hydrostatic forces pulling blood away from the head. If these mechanisms are
sluggish or incapacitated, the blood supply to the brain becomes insufficient and the person faints.)
11. a. When skeletal muscle contracts, it pushes against veins running through the muscle, narrowing the
lumen of each vein and thus squeezing blood along the veins. Veins contain valves that prevent back
flow of the blood, so muscle contraction pushes venous blood back toward the heart. This effect has
been called "the muscle pump.
b. More than half of the blood in the circulatory system is in the venous side of the vascular tree. When
skeletal muscle contracts actively, it pushes some of this blood back to the heart, increasing venous
return.
c. The stroke volume is the amount of blood ejected from a ventricle in a single contraction.
d. Increased venous return will mildly stretch the ventricular fibers, and this increased stretch will cause
an increase in the vigor of ventricular contraction (the Frank-Starling effect). As a result, the stroke
volume will increase. (PLEASE NOTICE: The correct answer to "why?" in this question describes the
mechanism behind the change.)
e. The increase in contractility of the heart will cause the heart to empty more completely, increasing the
ejection fraction as well as increasing the stroke volume.
f. Even if the atria were disabled, increased venous return would still increase the ventricular stroke
volume because increased venous return would stretch the ventricular fibers.
12. A normal heart pumps about 60-70 ml/beat at a rate of about 60 beats/minute. This means that a
normal heart in a resting person will pump:
70 ml/beat x 60 beats/min x 25 min = 105 liters in 25 minutes.
During exercise, your heart ejects 90% of its end-diastolic volume with each beat, or 160 x 0.9 = 144
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5/28/2009 http://biology.ucsd.edu/classes/bipn100.SP09/prob_set8_solns.htm

ml/beat.