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新托福TPO24阅读原文(二):Breathing During Sleep

编辑:乐闻携尔 来源: 乐闻携尔 发布时间:2016-06-29

文章摘要: 乐闻携尔为考生们整理了新托福TPO阅读原文,供大家学习借鉴。本文是新托福TPO24第二篇文章的原文,新托福TPO24阅读原文(二):Breathing During Sleep(睡眠中的呼吸)

新托福TPO24阅读原文(二):Breathing During Sleep。托福TPO是ETS给的官方模考题,而托福TPO阅读原文对托福阅读备考很有参考价值。乐闻携尔为考生们整理了新托福TPO阅读原文,供大家学习借鉴。本文是新托福TPO24第二篇文章的原文,新托福TPO24阅读原文(二):Breathing During Sleep(睡眠中的呼吸)。

TPO24-2:Breathing During Sleep

Of all the physiological differences in human sleep compared with wakefulness that have been discovered in the last decade, changes in respiratory control are most dramatic. Not only are there differences in the level of the functioning of respiratory systems, there are even changes in how they function. Movements of the rib cage for breathing are reduced during sleep, making the contractions of the diaphragm more important. Yet because of the physics of lying down, the stomach applies weight against the diaphragm and makes it more difficult for the diaphragm to do its job. However, there are many other changes that affect respiration when asleep.

During wakefulness, breathing is controlled by two interacting systems. The first is an automatic, metabolic system whose control is centered in the brain stem. It subconsciously adjusts breathing rate and depth in order to regulate the levels of carbon dioxide (CO2) and oxygen (O2), and the acid-base ratio in the blood. The second system is the voluntary, behavioral system. Its control center is based in the forebrain, and it regulates breathing for use in speech, singing, sighing, and so on. It is capable of ignoring or overriding the automatic, metabolic system and produces an irregular pattern of breathing.

During NREM (the phase of sleep in which there is no rapid eye movement) breathing becomes deeper and more regular, but there is also a decrease in the breathing rate, resulting in less air being exchanged overall. This occurs because during NREM sleep the automatic, metabolic system has exclusive control over breathing and the body uses less oxygen and produces less carbon dioxide. Also, during sleep the automatic metabolic system is less responsive to carbon dioxide levels and oxygen levels in the blood. Two things result from these changes in breathing control that occur during sleep. First, there may be a brief cessation or reduction of breathing when falling asleep as the sleeper waxes and wanes between sleep and wakefulness and their differing control mechanisms. Second, once sleep is fully obtained, there is an increase of carbon dioxide and a decrease of oxygen in the blood that persists during NREM.

But that is not all that changes. During all phases of sleep, several changes in the air passages have been observed. It takes twice as much effort to breathe during sleep because of greater resistance to airflow in the airways and changes in the efficiency of the muscles used for breathing. Some of the muscles that help keep the upper airway open when breathing tend to become more relaxed during sleep, especially during REM (the phase of sleep in which there is rapid eye movement). Without this muscular action, inhaling is like sucking air out of a balloon—the narrow passages tend to collapse. Also there is a regular cycle of change in resistance between the two sides of the nose. If something blocks the "good" side, such as congestion from allergies or a cold, then resistance increases dramatically. Coupled with these factors is the loss of the complex interactions among the muscles that can change the route of airflow from nose to mouth.

Other respiratory regulating mechanisms apparently cease functioning during sleep. For example, during wakefulness there is an immediate, automatic, adaptive increase in breathing effort when inhaling is made more difficult (such as breathing through a restrictive face mask). This reflexive adjustment is totally absent during NREM sleep. Only after several inadequate breaths under such conditions, resulting in the considerable elevation of carbon dioxide and reduction of oxygen in the blood, is breathing effort adjusted. Finally, the coughing reflex in reaction to irritants in the airway produces not a cough during sleep but a cessation of breathing. If the irritation is severe enough, a sleeping person will arouse, clear the airway, then resume breathing and likely return to sleep.

Additional breathing changes occur during REM sleep that are even more dramatic than the changes that occur during NREM. The amount of air exchanged is even lower in REM than NREM because, although breathing is more rapid in REM,it is also more irregular, with brief episodes of shallow breathing or absence of breathing. In addition, breathing during REM depends much more on the action of the diaphragm and much less on rib cage action.

TPO24-2译文:睡眠中的呼吸

关于人类睡觉和清醒时生理状态的差异在过去的十年里已被发现,在所有的这些差异中,呼吸系统控制方面的变化尤其引人注目。不仅是呼吸系统运作水平有差异,在如何运作方面也出现了变化。胸腔所做的呼吸运动在睡觉时会减少,使得横膈膜的收缩变得更为重要。然而由于躺下来的物理作用,胃部压迫横膈膜使得横膈膜难以工作。不管怎样,睡眠时还有很多其他的变化影响着呼吸。

清醒的时候,呼吸受到两个互相影响的系统的控制。第一个是自动的新陈代谢系统,它的控制中心在脑干。它会潜意识的调整呼吸频率和深度来控制二氧化碳和氧气的浓度以及血液中的酸碱比。第二套系统是自发行为系统。它的控制中心在前脑,调节说话、唱歌、叹息等行为时的呼吸。它能忽略或无视自动新陈代谢系统并且产生无规律的呼吸模式。

在NMER(睡觉时没有快速眼部活动的阶段)这个阶段中,呼吸会变得更深更有规律,但是呼吸频率会降低,导致总体空气交换减少。发生这个是因为在NREM睡眠阶段中,自动的新陈代谢系统会独自控制呼吸,身体会利用更少的氧气产生更少的二氧化碳。同时,自动的新陈代谢系统对血液中二氧化碳和氧气的含量反应并不灵敏。在睡眠中呼吸控制的变化会导致两个结果。第一,睡着时呼吸可能会有短暂的停止或减少,因为睡眠者在睡眠和清醒之间徘徊,而这两种状态的控制系统不一样。第二,一旦得到了充足的睡眠,血液中二氧化碳含量升高而氧气含量降低,在NREM阶段也会持续这样。

但这并不是全部的变化。在睡眠的所有阶段中,气道的一些变化已经被观察到了。睡眠时需要付出两倍的努力去呼吸,因为呼吸道气流的阻力会比较强并且用来呼吸的肌肉的效率会有变化。一些在呼吸时帮助保持上呼吸道通畅的肌肉在睡觉的时候会变得松弛,特别是在REM阶段(就是有快速眼部运动的睡眠阶段)。没有这种肌肉运动,呼吸空气就像从气球里吸气一样,狭窄的通道会面临崩溃。而且鼻子两侧的阻力也会周期性改变。如果有时候堵塞了“好”的一边,比如过敏和感冒引起的堵塞,阻力就会大大增加。与这些因素一起的是那些能够改变从鼻子到嘴巴的气流路径的肌肉之间失去了复杂的交互。

其他呼吸调节机制在睡眠时显然要停止运作。比如说,在清醒时如果呼吸变得困难的话就会有一个立即自动适应性的呼吸增强(比如戴上面具呼吸)。但在NREM状态时完全不存在这样反射性的调节。在这种情况下,只有几次不充分的呼吸后使得血液中二氧化碳的含量显著提升以及氧气的含量降低,呼吸才会被调整过来。最后,咳嗽反应在应对呼吸道中刺激物时产生的不是睡觉时咳嗽而是呼吸停止。如果刺激物足够严重,睡着的人会醒来清理气道,然后继续呼吸很可能再度入睡。

发生在REM时期的多余的呼吸变化比发生在NREM时期的呼吸变化更显著。REM的空气交换量要比NREM低,因为尽管REM中呼吸更加急促,但也更加没有规律,包括一些简短的浅呼吸或呼吸暂停。另外,REM时期的呼吸更多取决于横膈膜而不是胸腔的作用。

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