Which of the following does NOT stimulate peripheral chemoreceptors:
**Core Concept**
Peripheral chemoreceptors, located in the carotid and aortic bodies, play a crucial role in detecting changes in the chemical composition of arterial blood, particularly in the levels of oxygen (O2), carbon dioxide (CO2), and hydrogen ions (H+). These receptors are responsible for triggering the respiratory response to maintain acid-base balance and oxygenation of the body.
**Why the Correct Answer is Right**
Peripheral chemoreceptors are stimulated by decreases in arterial oxygen levels, increases in carbon dioxide levels, and rises in hydrogen ion concentrations. The decrease in oxygen levels triggers the release of neurotransmitters, such as dopamine and serotonin, which stimulate the chemoreceptor nerve endings. This leads to an increase in the firing rate of the chemoreceptor neurons, which in turn activates the respiratory centers in the brainstem, resulting in an increase in respiratory rate and depth. On the other hand, **Option A** does not directly stimulate peripheral chemoreceptors.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is a direct stimulus for peripheral chemoreceptors. Decreased oxygen levels trigger the release of neurotransmitters, which stimulate the chemoreceptor nerve endings.
* **Option B:** This option is incorrect because it is a direct stimulus for peripheral chemoreceptors. Increased carbon dioxide levels increase the acidity of the blood, which stimulates the release of neurotransmitters, which in turn stimulate the chemoreceptor nerve endings.
* **Option C:** This option is incorrect because it is a direct stimulus for peripheral chemoreceptors. Increased hydrogen ion concentrations increase the acidity of the blood, which stimulates the release of neurotransmitters, which in turn stimulate the chemoreceptor nerve endings.
**Clinical Pearl / High-Yield Fact**
Peripheral chemoreceptors are also sensitive to changes in pH, and a drop in pH below 7.35 can stimulate the chemoreceptors, leading to an increase in respiratory rate and depth. This is known as the "pH-mediated" response.
**Correct Answer:** A.