Why do muscles to get rid of carbon dioxide? ?

It is not just the muscle cells that respire. All cells in the body need oxygen for respiration and all cells need to get rid of the carbon dioxide that they produce. In the lungs oxygen is taken from air into the blood and the blood transports the oxygen to the cells in all tissues of the body. As the muscles use the oxygen up,carbon dioxide is produced and absorbed into the blood. And this de-oxygenated blood is pumped back to the heart then on to the lungs. The lungs get rid of this carbon dioxide, and breathe in more oxygen.

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