Erection of the penis results from increase in blood flow into the corpora. The blood flows through the corpus spongiosum and glans which increase in volume, whereas the blood becomes trapped in the corpus cavernosum which becomes rigid as the pressure increases. The protrusion of the penis may be aided by relaxation of the retractor penis muscle. The major erectile fibres lie in the pelvic nerve and anti-erectile fibres in the sacral sympathetic outflow. The hypogastric nerves may contain both nerve types but there is considerable species and individual variation. The neurotransmitters mediating erection have yet to be determined. There is some evidence that acetylcholine is involved in the increase in blood flow through the corpus spongiosum but not in the corpus cavernosum. Vasoactive intestinal peptide may also have a role. It is possible that these and other substances interact to control the complete process. Erection is inhibited by noradrenaline released from sympathetic nerves, and this acts mainly on alpha-1 adrenoceptors within the penis and on the retractor penis muscle. During tumescence blood flows into the sinusoids from the helicine arterioles which supply them. The sinusoids become dilated due to relaxation of smooth muscle within the trabeculae. Blood may also be redirected from anastomoses between the dorsal arteries and corpus spongiosum through other helicine arterioles supplying the sinusoids of the corpus cavernosum. The significance of polsters (smooth muscle projections into the blood vessel lumen) remains controversial. Occlusion of venous drainage from the corpora cavernosa is both passive (due to increased corpus cavernosum pressure) and active. Relaxation of trabecular smooth muscle may also modify blood flow through the corpora cavernosa.