【英语时差8,16】心脏为什么不会疲倦？

Your heart muscle does something  truly incredible–it expands and contracts, non-stop, every moment of every day of your entire life.   By comparison, if you tried to squeeze and release the muscles in your hand, they would grow fatigued¹ and need to rest probably within an hour. Yet the heart never rests–it can't let up beating for even one day of your life. How is this feat² achieved?

One answer is that the "cardiac" muscle that comprises the heart is of a different kind than the "skeletal" muscle comprising the hand. Skeletal muscles are attached to bone structures and cannot stay long in a flexed⁴ position without depleting⁵ their energy reserves.Those energy reserves come from mitochondria: structures inside the cells that use the energy taken in from food. Thus the more mitochondria it has, the greater the available energy for the muscle.

Because it has not been necessary in the course of evolution for humans to be able to flex³ our skeletal muscles for prolongedperiods of time, the total volume of skeletal muscle contains an average of only 1 to 2% mitochondria. This is an entirely⁶ sufficient energy source for such intermittent⁷ muscular tasks as walking or running. The total volume of the heart, by contrast, is between 30 and 35% mitochondria.

That massive amount of energy-generators means cardiac muscle,in a healthy state, need never rest: there is always some energy being transferred to the muscle at the same time that more energy is being derived⁸ from caloric intake⁹. And always just in time for that next beat.

1 fatigued   

adj. 疲乏的

参考例句：

• The exercises fatigued her. 操练使她感到很疲乏。
• The President smiled, with fatigued tolerance for a minor person's naivety. 总统笑了笑，疲惫地表现出对一个下级人员的天真想法的宽容。

2 feat   

n.功绩；武艺，技艺；adj.灵巧的，漂亮的，合适的

参考例句：

• Man's first landing on the moon was a feat of great daring.人类首次登月是一个勇敢的壮举。
• He received a medal for his heroic feat.他因其英雄业绩而获得一枚勋章。

3 flex   

n.皮线，花线；vt.弯曲或伸展

参考例句：

• We wound off a couple of yards of wire for a new lamp flex.我们解开几码电线作为新的电灯花线。
• He gave his biceps a flex to impress the ladies.他收缩他的肱二头肌以吸引那些女士们的目光。

4 flexed   

adj.[医]曲折的，屈曲v.屈曲( flex的过去式和过去分词 )；弯曲；（为准备大干而）显示实力；摩拳擦掌

参考例句：

• He stretched and flexed his knees to relax himself. 他伸屈膝关节使自己放松一下。 来自辞典例句
• He flexed his long stringy muscles manfully. 他孔武有力地弯起膀子，显露出细长条的肌肉。 来自辞典例句

5 depleting   

使大大的减少，使空虚( deplete的现在分词 )； 耗尽，使枯竭

参考例句：

• Regulations are outlawing certain refrigerants, such as chlorofluorocarbons, which contain ozone-depleting chemicals. 随后出台的政策禁用了部分制冷剂，如破坏臭氧层的氟氯碳化合物。
• Aging, being a series of continual losses, can be keenly depleting. 老龄化，作为一个系列的连续亏损，可以清楚地消耗。

6 entirely   

ad.全部地，完整地；完全地，彻底地

参考例句：

• The fire was entirely caused by their neglect of duty. 那场火灾完全是由于他们失职而引起的。
• His life was entirely given up to the educational work. 他的一生统统献给了教育工作。

7 intermittent   

adj.间歇的，断断续续的

参考例句：

• Did you hear the intermittent sound outside?你听见外面时断时续的声音了吗？
• In the daytime intermittent rains freshened all the earth.白天里，时断时续地下着雨，使整个大地都生气勃勃了。

8 derived   

vi.起源；由来；衍生；导出v.得到( derive的过去式和过去分词 )；（从…中）得到获得；源于；（从…中）提取

参考例句：

• Many English words are derived from Latin and Greek. 英语很多词源出于拉丁文和希腊文。 来自《简明英汉词典》
• He derived his enthusiasm for literature from his father. 他对文学的爱好是受他父亲的影响。 来自《简明英汉词典》

9 intake   

n.吸入，纳入；进气口，入口

参考例句：

• Reduce your salt intake.减少盐的摄入量。
• There was a horrified intake of breath from every child.所有的孩子都害怕地倒抽了一口凉气。