题目内容
The trachea is kept open by 16-20 C-shaped rings of cartilage separated by _____
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At the end of the bronchioles are clusters of air sacs called______
Each lung is enveloped in a double-folded membrane called _____
The outer layer of the pleura is called ______
The cycle of _____ really begins when the phrenic nerves stimulate the diaphragm to contract and flatten.
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