Final answer:
The human respiratory rate is mainly regulated by chemoreceptors that monitor the blood's carbon dioxide levels, along with pH and oxygen levels to a lesser extent.
Step-by-step explanation:
The human respiratory rate is primarily regulated by chemoreceptors that monitor a variety of factors. The most important factor they sense is the level of carbon dioxide (CO2) in the blood. When carbon dioxide levels rise, it leads to increased levels of hydrogen ions, decreasing pH, which triggers chemoreceptors to signal the respiratory centers to increase breathing rate and depth. Additionally, peripheral chemoreceptors in the aortic arch and carotid arteries are sensitive to blood oxygen levels, especially when they drop significantly. Although oxygen levels are also important, they play a secondary role compared to carbon dioxide in the regulation of respiratory rate.