Final answer:
The receptors stimulated by an increase in carbon dioxide are the peripheral chemoreceptors, which are located in the carotid arteries and aortic arch and signal the need to increase respiration when CO2 levels rise. The correct option is d. peripheral chemoreceptors.
Step-by-step explanation:
Mr. Smith hypoventilates and retains too much carbon dioxide. The receptors stimulated by an increase in carbon dioxide are the peripheral chemoreceptors.
Peripheral chemoreceptors are located in the carotid arteries and aortic arch and are responsible for detecting changes in carbon dioxide, hydrogen ions, and oxygen levels in the blood. When these chemoreceptors sense high levels of carbon dioxide, they stimulate an increase in ventilation to reduce the carbon dioxide concentration in the blood. This action helps maintain a stable pH level within the body by expelling carbon dioxide and increasing the uptake of oxygen. On the other hand, central chemoreceptors located in the brain also respond to carbon dioxide levels by stimulating the respiratory centers, but the question specifically refers to receptors, which implicates the peripheral ones.
Thus, the correct option in the final answer is d. peripheral chemoreceptors.