The Living World 8th Edition by George Johnson

تمرين 15

Do Big Hearts Beat Faster?
Small animals live at a much faster pace than large animals. They reproduce more quickly and live shorter lives. As a rule, they tend to move about more quickly and so consume more oxygen per unit body weight. Interestingly, small and large mammals have about the same size heart relative to body size (about 0.6% of body mass). But do all mammalian hearts beat at the same rate? the heart of a 7,000-kilogram (a kilogram is 1,000 grams) African bull elephant must push a far greater volume of blood through its body than the heart of a 20-gram mouse, but the elephant is able to do it through much-larger-diameter arteries, which impose far less resistance to the blood's flow. Does the elephant's heart beat faster? Or does the mouse's, in order to deliver more oxygen to its muscles? Or perhaps the mouse's heart beats more slowly because of increased resistance to flow through narrower blood vessels.
The graph to the right displays the pulse rate of a number of mammals of different body sizes (the pulse rate is the number of heartbeats counted per minute, a measure of how rapidly the heart is beating).
Interpreting Data
a. What is the resting pulse rate of a 7,000-kilogram African bull elephant?
b. What is the resting pulse rate of a 20-gram mouse? How many complete heartbeats is that per second?
c. The pulse rate of an adult human at rest is about 70 beats per minute, while the heart of a cat, with a body weight less than a tenth of humans, beats 120 times per minute, almost twice as fast. What general statement can be made regarding the effect of body size on heart rate in mammals?