Understanding Erection Physiology
An erection is triggered by a complex interplay between the sympathetic and parasympathetic nervous system, with local sensory stimulation of the genital area and/ or central psychogenic stimulation resulting from visual, tactile, auditory, olfactory, and/or imaginative input (Sadeghi-Nejad 2007; Kolodny 2011). The endothelial cells that line the blood vessels of the penis produce vasoactive factors that dilate blood vessels, one of the most important being nitric oxide.
Nitric oxide, by initiating production of another chemical messenger called cyclic guanosine monophosphate (cGMP), triggers a biochemical cascade leading to expansion (vasodilatation) of penile blood vessels and allows for increased blood flow into the corpus cavernosum, the two columns of spongy tissue that run along the top length of the penis (Kolodny 2011; Heidelbaugh 2010; AUA 2012; Sadeghi-Nejad 2007). As the corpus cavernosum fills with blood it stretches, compressing the primary site where blood exits the penis, called the subtunical venules. This compression causes resistance to blood flow out of the penis, producing and maintaining an erection (Sadeghi-Nejad 2007; Kolodny 2011; NIH 1992; Heidelbaugh 2010).