正常人心率超过180次／分时，引起心输出量减少的主要原因是（）
A.心室充盈时间明显缩短，充盈量减少
B.心室舒张期过长，心室充盈早已接近限度
C.快速射血期缩短
D.慢速射血期缩短
E.迷走神经活动加强
参考答案：A
解析：心率增快，心输出量增加，但有一定限度。如果心率过快，超过170～180次/分，心室充盈时间明显缩短，充盈量减少，搏出量可减少一半左右，心输出量亦开始下降
I have __1__ my hometown for a few years. It has changed a lot __2__ the years. New roads and beautiful buildings have been __3__ . People in my hometown have realized the __4__ of protecting the environment. They have done many things __5__ planting trees, keeping the roads clean, and not __6__ plastic bags. Now the __7__ is much better than before, and people’s life is __8__ than before. The old are doing all kinds of __9__ such as playing __10__ , dancing and playing TaiJi in the park. There are __11__ places for people to relax such as KTVs and bars in the town. People’s life is not __12__ any more. They can relax in many different ways __13__ just watching TV at home. There are also some places __14__ further learning , especially for the __15__ who want to find good jobs.
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