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If you mean 'respiratory system'.....then it starts at the nose (breathing in is the first process before the air can reach the lungs for filtering out of oxygen into the blood stream). Breathing in is referred to as inhalation (of air), and breathing out is exhalation. Try browsing through the Biology categories (Human Anatomy, etc.) for the technical details, if you need them.

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13y ago

What else can I help you with?