If you have the same question i have then im thinking cardio but am not too sure...
Answer:
Oxygen is a chemical element that constitutes 21 percent of the Earth's atmosphere, that is capable of combining with all elements except some noble gases, that is active in physiological processes of almost all known organisms, and that is involved especially in combustion — see Chemical Elements Table.
Explanation:
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I think the circulatory system is the best body system because we need the heart to stay alive, many people live with only one lung. but we need the respiratory system to let oxygen in, and let carbon dioxide out.
