**Core Concept**
Carbon dioxide (CO2) is a byproduct of cellular respiration and is transported in the blood, primarily in the form of bicarbonate ions (HCO3-). The regulation of CO2 levels in the blood is critical for maintaining acid-base balance and proper pH levels within the body.

**Why the Correct Answer is Right**
CO2 causes vasodilation of cerebral blood vessels, leading to an increase in cerebral blood flow. This is mediated by the stimulation of chemoreceptors in the carotid and aortic bodies, which sense changes in CO2 levels in the blood. The resulting vasodilation is a key component of the cerebrovascular response to changes in CO2 levels.

**Why Each Wrong Option is Incorrect**
**Option A:** This is not a direct effect of CO2. Although CO2 can influence various physiological processes, vasodilation is not a primary effect of CO2 on the cardiovascular system.

**Option B:** This is not a direct effect of CO2. While CO2 can influence respiratory drive, it is not primarily responsible for the regulation of respiratory rate.

**Option C:** This is not a direct effect of CO2. Although CO2 can influence various physiological processes, it does not directly stimulate the release of neurotransmitters.

**Clinical Pearl / High-Yield Fact**
The cerebrovascular response to CO2 is a critical component of maintaining proper cerebral blood flow and is often used as a measure of cerebrovascular health.

**Correct Answer: D.**