Final answer:
The carotid body primarily responds to changes in carbon dioxide levels. It detects the level of carbon dioxide in the blood and signals the brain to adjust the rate of breathing. This helps maintain normal blood gas and pH levels.
Step-by-step explanation:
The carotid body primarily responds to changes in oxygen levels. Specifically, the peripheral chemoreceptors in the carotid body are sensitive to large dips in arterial oxygen concentration. When blood oxygen levels fall significantly, approximately to 60 mm Hg or lower, the carotid body stimulates an increase in respiratory activity. Although carbon dioxide levels and pH changes are also detected by chemoreceptors, the primary response of the carotid body is to changes in oxygen levels in the blood.