六级冲刺练习阅读（68）
<p> <span word="When">When</span> <span word="blood">blood</span> <span word="is">is</span> <span word="sent">sent</span> <span word="to">to</span> <span word="the">the</span> <span word="lungs">lungs</span> <span word="by">by</span> <span word="the">the</span> <span word="heart">heart</span>, <span word="it">it</span> <span word="has">has</span> <span word="come">come</span> <span word="back">back</span> <span word="from">from</span> <span word="the">the</span> <span word="cells">cells</span> <span word="in">in</span> <span word="the">the</span> <span word="rest">rest</span> <span word="of">of</span> <span word="the">the</span> <span word="body">body</span>. <span word="So">So</span> <span word="the">the</span> <span word="blood">blood</span> <span word="that">that</span> <span word="goes">goes</span> <span word="into">into</span> <span word="the">the</span> <span word="wall">wall</span> <span word="of">of</span> <span word="an">an</span> <span word="air">air</span> <span word="sac">sac</span> <span word="contains">contains</span> <span word="much">much</span> <span word="dissolved">dissolved</span> <span word="carbon">carbon</span> <span word="dioxide">dioxide</span> <span word="but">but</span> <span word="very">very</span> <span word="little">little</span> <span word="oxygen">oxygen</span>. <span word="At">At</span> <span word="the">the</span> <span word="same">same</span> <span word="time">time</span>, <span word="the">the</span> <span word="air">air</span> <span word="that">that</span> <span word="goes">goes</span> <span word="into">into</span> <span word="the">the</span> <span word="air">air</span> <span word="sac">sac</span> <span word="contains">contains</span> <span word="much">much</span> <span word="oxygen">oxygen</span> <span word="but">but</span> <span word="very">very</span> <span word="little">little</span> <span word="carbon">carbon</span> <span word="dioxide">dioxide</span>.</p><p> <span word="You">You</span> <span word="have">have</span> <span word="learned">learned</span> <span word="that">that</span> <span word="dissolved">dissolved</span> <span word="materials">materials</span> <span word="always">always</span> <span word="diffuse">diffuse</span> <span word="from">from</span> <span word="where">where</span> <span word="there">there</span> <span word="is">is</span> <span word="more">more</span> <span word="of">of</span> <span word="them">them</span> <span word="to">to</span> <span word="where">where</span> <span word="there">there</span> <span word="is">is</span> <span word="less">less</span>. <span word="Oxygen">Oxygen</span> <span word="from">from</span> <span word="the">the</span> <span word="air">air</span> <span word="dissolves">dissolves</span> <span word="in">in</span> <span word="the">the</span> <span word="moisture">moisture</span> <span word="on">on</span> <span word="the">the</span> <span word="lining">lining</span> <span word="of">of</span> <span word="the">the</span> <span word="air">air</span> <span word="sac">sac</span> <span word="and">and</span> <span word="diffuses">diffuses</span> <span word="through">through</span> <span word="the">the</span> <span word="lining">lining</span> <span word="into">into</span> <span word="the">the</span> <span word="blood">blood</span>. <span word="Meanwhile">Meanwhile</span>, <span word="carbon">carbon</span> <span word="dioxide">dioxide</span> <span word="diffuses">diffuses</span> <span word="from">from</span> <span word="the">the</span> <span word="blood">blood</span> <span word="into">into</span> <span word="the">the</span> <span word="air">air</span> <span word="sac">sac</span>. <span word="The">The</span> <span word="blood">blood</span> <span word="then">then</span> <span word="flows">flows</span> <span word="from">from</span> <span word="the">the</span> <span word="lungs">lungs</span> <span word="back">back</span> <span word="to">to</span> <span word="the">the</span> <span word="heart">heart</span>, <span word="which">which</span> <span word="sends">sends</span> <span word="it">it</span> <span word="out">out</span> <span word="to">to</span> <span word="all">all</span> <span word="other">other</span> <span word="parts">parts</span> <span word="of">of</span> <span word="the">the</span> <span word="body">body</span>.</p><p> <span word="Soon">Soon</span> <span word="after">after</span> <span word="air">air</span> <span word="goes">goes</span> <span word="into">into</span> <span word="an">an</span> <span word="air">air</span> <span word="sac">sac</span>, <span word="it">it</span> <span word="gives">gives</span> <span word="up">up</span> <span word="some">some</span> <span word="of">of</span> <span word="its">its</span> <span word="oxygen">oxygen</span> <span word="and">and</span> <span word="takes">takes</span> <span word="in">in</span> <span word="some">some</span> <span word="carbon">carbon</span> <span word="dioxide">dioxide</span> <span word="from">from</span> <span word="the">the</span> <span word="blood">blood</span>. <span word="To">To</span> <span word="keep">keep</span> <span word="diffusion">diffusion</span> <span word="going">going</span> <span word="as">as</span> <span word="it">it</span> <span word="should">should</span>, <span word="this">this</span> <span word="carbon">carbon</span> <span word="dioxide">dioxide</span> <span word="must">must</span> <span word="be">be</span> <span word="gotten">gotten</span> <span word="rid">rid</span> <span word="of">of</span>. <span word="Breathing">Breathing</span>, <span word="which">which</span> <span word="is">is</span> <span word="caused">caused</span> <span word="by">by</span> <span word="movements">movements</span> <span word="of">of</span> <span word="the">the</span> <span word="chest">chest</span>, <span word="forces">forces</span> <span word="the">the</span> <span word="used">used</span> <span word="air">air</span> <span word="out">out</span> <span word="of">of</span> <span word="the">the</span> <span word="air">air</span> <span word="sacs">sacs</span> <span word="in">in</span> <span word="your">your</span> <span word="lungs">lungs</span> <span word="and">and</span> <span word="brings">brings</span> <span word="in">in</span> <span word="fresh">fresh</span> <span word="air">air</span>. <span word="The">The</span> <span word="breathing">breathing</span> <span word="muscles">muscles</span> <span word="are">are</span> <span word="controlled">controlled</span> <span word="automatically">automatically</span> <span word="so">so</span> <span word="that">that</span> <span word="you">you</span> <span word="breathe">breathe</span> <span word="at">at</span> <span word="the">the</span> <span word="proper">proper</span> <span word="rate">rate</span> <span word="to">to</span> <span word="keep">keep</span> <span word="your">your</span> <span word="air">air</span> <span word="sacs">sacs</span> <span word="supplied">supplied</span> <span word="with">with</span> <span word="fresh">fresh</span> <span word="air">air</span>.</p><p> <span word="Ordinarily">Ordinarily</span>, <span word="you">you</span> <span word="breathe">breathe</span> <span word="about">about</span> <span word="twenty">twenty</span>-<span word="two">two</span> <span word="times">times</span> <span word="a">a</span> <span word="minute">minute</span>. <span word="Of">Of</span> <span word="course">course</span>, <span word="you">you</span> <span word="breathe">breathe</span> <span word="faster">faster</span> <span word="when">when</span> <span word="you">you</span> <span word="are">are</span> <span word="exercising">exercising</span> <span word="and">and</span> <span word="slower">slower</span> <span word="when">when</span> <span word="you">you</span> <span word="are">are</span> <span word="resting">resting</span>. <span word="Fresh">Fresh</span> <span word="air">air</span> <span word="is">is</span> <span word="brought">brought</span> <span word="into">into</span> <span word="your">your</span> <span word="lungs">lungs</span> <span word="when">when</span> <span word="you">you</span> <span word="breathe">breathe</span> <span word="in">in</span>, <span word="or">or</span> <span word="inhale">inhale</span>, <span word="while">while</span> <span word="used">used</span> <span word="air">air</span> <span word="is">is</span> <span word="forced">forced</span> <span word="out">out</span> <span word="of">of</span> <span word="your">your</span> <span word="lungs">lungs</span> <span word="when">when</span> <span word="you">you</span> <span word="breathe">breathe</span> <span word="out">out</span>, <span word="or">or</span> <span word="exhale">exhale</span>.</p>
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