Under normal circumstances, carbon dioxide is excreted from the body by what system?

Carbon dioxide is primarily excreted from the body through the respiratory system. This occurs as a result of cellular respiration, where glucose is metabolized to produce energy, creating carbon dioxide as a waste product. The carbon dioxide then diffuses from the cells into the bloodstream. The circulatory system plays a role in transporting the carbon dioxide to the lungs, but the actual elimination of it from the body occurs when it is exhaled through the lungs.

In healthy individuals, this process is continuous and is essential for maintaining the body's acid-base balance. By removing carbon dioxide efficiently, the respiratory system ensures that the pH of the blood remains within a normal range, which is crucial for proper physiological function.

The other systems mentioned do not primarily handle carbon dioxide excretion. The digestive system is involved in processing food and waste, while the integumentary system relates to skin functions and does not excrete carbon dioxide to any significant extent. The circulatory system's main function is to transport, rather than excrete, gases. Thus, the respiratory system is the correct answer as it is directly responsible for the expulsion of carbon dioxide from the body.