How is the work of the digestive and circulatory systems related to the respiratory system?

The digestive system breaks down (catabolizes) food into its nutrient parts. The circulatory system brings those nutrients to the cells where they are needed. One of the main requirements of the cells is for energy, which the body manufactures by combing nutrients (especially carbohydrates, glucose, glycogen, and glycogon) with oxygen. The body takes in oxygen via the respiratory system and distributes it via the circulatory system.

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