2

Penis get erect due to stimulation because when the receptors get stimulus The tissues ( it has 2 dorsal corpus cavernosa and 1 ventral corpus spongiosum ) get filled by blood and the penis get eracted.

IT IS A VERY LONG SEQUENCE OF PROCESSES IN WHICH FINALLY NO2 IS RELEASED WHICH CAUSES DILATION OF BLOOD VESSELS PRESENT IN PENIS

WE ALL KNOW PENIS ERRECTION IS DUE TO DILATION OF BLOOD VESSELS