TEM of arteriole in human testis. An arteriole is a small diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries. Arterioles have muscular walls (usually only one to two layers of smooth muscle) and are the primary site of vascular resistance. The testicle is the male gonad in animals. Magnification @ 6000x, 8x10 print.