Kristen is running a half marathon. By her tenth minute of running, her breathing rate has increased from her resting rate.Which statement best describes how her tissues are responding to her running?

Running muscles need more energy, so their cells use more oxygen for cellular respiration and produce more carbon dioxide. Her breathing rate increases to bring in more oxygen and remove the extra carbon dioxide, so gas exchange increases. The best choice is “They need more oxygen to function, so gas exchange needs to increase.”

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