What's inside: the hydraulics of a hard-on

Three spongy tubes, a tough wrapping, a valve that traps the blood and a gas that opens the tap. No bone, and a third of it hidden inside you

From the outside a cock is one thing. Inside it is three long tubes of spongy tissue, a wrapping tougher than it looks, a set of valves and a chemical switch that drug companies have made a fortune out of. No bone holds it up: an erection is hydraulics, blood let in and kept in.

This chapter opens the hood. How hard you get and how often is in Erection; here is how it works.

Three tubes, not one

The shaft is built from three chambers. Two corpora cavernosa run side by side along the top and do almost all the stiffening. Below them, in the groove between the two, runs the corpus spongiosum, which carries the urethra and swells at the end into the glans.

Each cavernosum is wrapped in the tunica albuginea, a sheath of collagen in two layers, an inner circular one and an outer longitudinal one. In cadavers it measured between 1.3 and 3.3 mm, and it stretches thin when you are hard, down to 0.25 to 0.5 mm by one estimate.

The spongiosum has only a thin tunica and the glans has virtually none, so during an erection the pressure in them is a third to a half of that in the cavernosa. That is why the head stays softer than the shaft, and why the urethra is not squeezed shut: the way out has to stay open.

How the plumbing locks

Soft is the default, and it takes effort. The smooth muscle inside the cavernosa and the small coiled helicine arteries that feed it are kept contracted, letting in just enough blood to keep the tissue alive.

When arousal flips the switch, that muscle relaxes and blood pours into the spongy spaces. As they fill they press the small veins that drain them against the tunica, and the stretched tunica squeezes the veins that cross it. The exits close just as the inlets open. Doctors call it the veno-occlusive mechanism, which is a long name for a cork.

Pressure inside a full erection settles around 100 mmHg. Squeeze the muscles at the base, the ischiocavernosus, and it briefly climbs to several hundred. The tunica can take it: in cadaver tests it held about 1500 mmHg before it bulged.

The chemistry that opens the tap

The order to relax comes from parasympathetic nerves that leave the spinal cord at its lowest segments, S2 to S4. Medical students remember it as "parasympathetic points, sympathetic shoots".

The messenger at the end of those nerves is nitric oxide, a gas. Its role was nailed down in 1992, when blocking the enzyme that makes it abolished erections in rats. Released from the nerve endings during arousal, nitric oxide raises a second messenger, cGMP, inside the muscle cells, and cGMP relaxes them.

An enzyme called PDE5 breaks cGMP down and ends the party. Sildenafil, sold as Viagra, blocks PDE5, so cGMP builds up. The logic matters: the pill amplifies a signal that has to start with arousal. It does not create one.

The part you never see

The cock does not start where your pubes do. Behind the visible shaft the two cavernosa split into the crura, which anchor onto the pubic and ischial bones. Press behind your balls during an erection and you can feel it.

Two bands hold the base in place: a superficial fundiform ligament and, behind it, the suspensory ligament proper, running from the pubic bone to the tunica. MRI during erection shows it as the main pivot for the mobile part of the shaft. How much is hidden? In the famous Dutch study that put couples having sex inside an MRI scanner, the erect penis was shaped like a boomerang, and a third of its length was root.

Why you cannot pee with a hard-on

The neck of the bladder is a valve that has to close at ejaculation, so semen goes out instead of back into the bladder. During arousal the body starts shutting it early. In the one man in whom it was measured through a whole erection, pressure at the bladder neck climbed from 5 to 115 cmH2O before he came. He had a spinal injury, so take the number with care, but the direction matches what every man knows at seven in the morning. Wait a few minutes, or think about tax returns.

When it breaks

A penile fracture is not a broken bone, since there is none. It is a tear in the tunica of an erect cock, usually when it bends hard, typically when it slips out and slams into the partner's body. The classic story is a crack or pop, pain, the erection collapsing at once, then swelling and bruising. It is rare, about 1 in 175,000 hospital admissions.

Across 3,213 cases in 58 studies, intercourse caused 48% and masturbation or forced bending 39%. In a Brazilian series of 288 men, sex was behind 88.5%, mostly from behind or with the man on top. Three quarters heard the snap, and the urethra was torn too in 18.7%. The positions in these series are described for sex with women, but the physics of a missed thrust is the same with any hole.