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小 發表於 2011-8-16 02:19 PM (第 4609 天)
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when a person is asleep, due to gravity, it is difficult to lift up the rib cage by the contraction of the intercostal muscles.
moreover, less oxygen is needed for respiration to release energy during sleeping, thus although only the diaphragm contracts, the amount of oxygen inhaled can still support our body activities.