In spider nevi, dilatation of blood vessels is due to (AIIMS May, 2010)
a. A spider angioma (also known as a nevus araneus,spider nevus, vascular spider, and spider telangiectasia) is a type of telangiectasis found slightly beneath the skin surface, often containing a central red spot and reddish extensions which radiate outwards like a spider's web.
b. Spider angiomas are due to failure of the sphincteric muscle surrounding a cutaneous arteriole.
c. The central red dot is the dilated arteriole and the red "spider legs" are small veins carrying away the freely flowing blood. The dilation is caused by increased estrogen levels in the blood.
d. Many pregnant women, or women using hormonal contraception, have spider angiomas, due to high estrogen levels in their blood. People who have significant hepatic disease also show many spider angiomas, as their liver cannot detoxify estrogen from the blood, resulting in high levels of estrogen. About 33% of patients with cirrhosis have spider angiomas.