The regional arterial resistance of the mesentery and kidney vessels is reduced by –
**Core Concept**
The regulation of regional arterial resistance is crucial for maintaining blood flow to vital organs. The mesentery and kidney vessels are sensitive to various vasoactive substances that help in modulating their vascular tone. The sympathetic nervous system plays a key role in regulating these vessels through the release of specific neurotransmitters.
**Why the Correct Answer is Right**
The correct answer involves the release of **adenosine**, a potent vasodilator that reduces regional arterial resistance in the mesentery and kidney vessels. Adenosine acts by stimulating **adenosine receptors**, particularly A2 receptors, which are present on the smooth muscle cells of these vessels. This leads to the activation of **guanylate cyclase**, resulting in an increase in **cGMP** levels, which in turn causes smooth muscle relaxation and vasodilation. This mechanism is essential for matching blood flow to the metabolic demands of these organs.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it doesn't specify a known vasoactive substance that reduces regional arterial resistance in the mesentery and kidney vessels.
* **Option B:** This option is incorrect because it mentions a vasoconstrictor, which would increase regional arterial resistance, not reduce it.
* **Option C:** This option is incorrect because it doesn't accurately describe a mechanism or substance that reduces regional arterial resistance in the mesentery and kidney vessels.
**Clinical Pearl / High-Yield Fact**
Adenosine is a key player in the regulation of regional arterial resistance, and its release is crucial for maintaining blood flow to vital organs. It's essential to remember that adenosine acts on specific receptors to cause vasodilation, which is a vital mechanism for matching blood flow to tissue demand.
**Correct Answer:** A. Adenosine