Which of the following is NOT a stimulus for breathing?

• A. Decreased levels of oxygen (\(\mathrm{O_2}\)) in the blood
• B. Increased levels of carbon dioxide (\(\mathrm{CO_2}\)) in the blood
• C. Increased blood glucose levels
• D. Decreased blood pH

Answer: (C)

Verified step by step guidance

1. Understand the primary stimuli for breathing: Breathing is regulated by the respiratory control centers in the brain, primarily in response to changes in blood levels of oxygen (\(\mathrm{O_2}\)), carbon dioxide (\(\mathrm{CO_2}\)), and pH.
2. Recognize the role of carbon dioxide (\(\mathrm{CO_2}\)): Increased levels of \(\mathrm{CO_2}\) in the blood stimulate breathing because \(\mathrm{CO_2}\) combines with water to form carbonic acid, which lowers blood pH. This triggers chemoreceptors to signal the brain to increase respiratory rate.
3. Understand the role of oxygen (\(\mathrm{O_2}\)): Decreased levels of \(\mathrm{O_2}\) in the blood can also stimulate breathing, although this is a secondary mechanism compared to \(\mathrm{CO_2}\) and pH changes. Peripheral chemoreceptors detect low \(\mathrm{O_2}\) levels and signal the brain to adjust breathing.
4. Learn about blood pH: Decreased blood pH (acidosis) is a direct stimulus for breathing. When blood becomes more acidic, chemoreceptors in the brain and arteries detect this change and increase respiratory rate to expel \(\mathrm{CO_2}\) and restore pH balance.
5. Identify the exception: Increased blood glucose levels are not a stimulus for breathing. Glucose levels are regulated by insulin and glucagon, and they do not directly affect the respiratory control centers or chemoreceptors involved in breathing regulation.