题目内容
The cycle of _____ really begins when the phrenic nerves stimulate the diaphragm to contract and flatten.
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Oxygen is carried in the blood bound to ______ in red blood cells.
Too little carbon dioxide in the blood can lead to a condition called ___
Which one of the followings cannot stop foreign particles from entering the respiratory system?
_____are cells that can swallow up foreign particles in the bronchial tree.
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