Human erections run on blood pressure, while a bird's run on lymph
Penile erection is a hydraulic trick run by the nervous system. Nerve signals release nitric oxide, arteries widen, spongy chambers flood with blood and small muscles squeeze the exit veins shut. Much of it happens without conscious control, which is why it also occurs during sleep, and why other animals do it very differently.
Two tubes of erectile tissue, the corpora cavernosa, run along the shaft, with a third, the corpus spongiosum, beneath them carrying the urethra. The trigger comes from the parasympathetic branch of the autonomic nervous system. Its nerve endings release acetylcholine, which prompts cells lining the arteries to give off nitric oxide, a gas that relaxes smooth muscle. As blood pours in, the ischiocavernosus and bulbospongiosus muscles press on the veins, trapping it. When that signal fades, baseline sympathetic tone narrows the arteries again and blood drains out, a reversal clinicians call detumescence.
The brain can start the process from sights, sounds, smells or imagination alone, acting through centres in the lower spinal cord, and it can equally suppress it. Erections during sleep, known medically as nocturnal penile tumescence, are routine; their absence helps doctors tell physical causes of erectile dysfunction from psychological ones. Around one in ten men face recurring problems at some point, and conditions such as diabetes, kidney disease and vascular disease account for roughly 70 percent of cases. The medical specialty involved is andrology, part of urology.
Variation is normal. One survey put the average erection angle for a standing man at 74.3 degrees, and 63 percent had no curvature. Flaccid length is a poor guide to erect length.
Other species take different routes. Dogs and most placental mammals have a bone, the baculum, so a dog can penetrate before swelling fully. A bull's penis contains little erectile tissue and is already stiff, extending mainly by straightening an S-shaped bend. A stallion's roughly doubles in length and thickness. Birds that have a penis erect it with lymph rather than blood, and in the lake duck it can match the length of the bird's own body.
Source: Erection
