Part of the physiological process of vasodilatation involves the release of nitric oxide (NO) by vascular endothelial cells which then diffuses to nearby vascular smooth muscle cells. There, NO activates soluble guanylate cyclase which converts guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP), the main effector of the system. For example, in the penis, NO release at high levels from endothelial cells and penile nerves during sexual stimulation leads to relaxation of the smooth vasculature of the corpus cavernosum, causing vasocongestion and a sustained erection.