Inside the penis are three cylinders of spongy erectile tissue, a network of blood vessels and nerves, a tube for urine and semen, and several layers of tough protective tissue that hold everything together. From the outside, the penis looks simple, but its internal structure is surprisingly complex, engineered to switch between a soft, flexible state and a rigid one in a matter of seconds.
The Three Erectile Cylinders
The shaft of the penis is built around three columns of spongy tissue. Two of them, called the corpora cavernosa, run side by side along the top of the shaft. These are the main players in erection. Each one is filled with a honeycomb of tiny blood-filled chambers (sinusoids) separated by bands of smooth muscle and elastic fibers. Think of them as two long, expandable sponges.
The third cylinder, the corpus spongiosum, runs along the underside of the penis in the groove between the other two. It surrounds and protects the urethra, the tube that carries both urine and semen. At the tip, the corpus spongiosum flares out to form the glans, the rounded head of the penis. The glans caps the ends of the two larger cylinders, acting as a cushion during intercourse.
The Tough Outer Wrapper
Each erectile cylinder is wrapped in a dense, white fibrous sheath called the tunica albuginea. This is one of the strongest connective tissues in the body, with a tensile strength of roughly 1,200 to 1,500 mmHg before it would rupture. It has two layers: an inner layer of circular fibers and an outer layer of lengthwise fibers. The inner layer also sends pillars inward, like structural struts, that reinforce the spongy tissue from the inside.
This design lets the tunica do two very different jobs. When the penis is soft, the tunica stays supple and flexible. During an erection, it acts as a rigid pressure vessel, containing the expanding blood within. About 5% of the tunica is made of elastic fibers, which allow it to stretch evenly in length and width so the erection is straight. The tunica around the corpus spongiosum is noticeably thinner and contains smooth muscle fibers that help propel semen during ejaculation.
Over the tunica sits another fascial layer called Buck’s fascia, which wraps the corpora cavernosa and the corpus spongiosum in separate compartments. Above that is a looser layer of connective tissue containing the superficial veins you can sometimes see through the skin.
How Blood Creates an Erection
In its resting state, the smooth muscle inside the erectile tissue stays partially contracted, limiting blood flow. When you become sexually aroused, nerve signals trigger the release of a chemical messenger (nitric oxide) from nerve endings and blood vessel walls. This causes the smooth muscle lining the small arteries and the spongy chambers to relax and open up.
Blood rushes in through small, coiled arteries deep inside each corpus cavernosum. As the spongy chambers fill and expand, they press outward against the tunica albuginea. That pressure compresses the small veins that normally drain blood out of the penis, trapping the blood inside. This two-part process, increased inflow plus reduced outflow, is what produces and maintains rigidity. Muscles at the base of the penis further boost pressure by squeezing blood forward into the shaft.
When arousal fades, the smooth muscle contracts again, inflow drops, the veins reopen, and blood drains out. The whole cycle depends on a precise balance between the arterial supply, the spongy tissue, and the containment strength of the tunica.
The Urethra
Running through the center of the corpus spongiosum is the urethra, a narrow tube that serves double duty. It carries urine from the bladder and semen during ejaculation, exiting through a small opening (the meatus) at the tip of the glans. During an erection, the swollen corpora cavernosa press against the portion of the urethra connected to the bladder, effectively blocking urine flow so that only semen passes through at orgasm.
Nerves and Sensation
The penis is served by two distinct populations of nerve fibers bundled together in the dorsal nerve, which runs along the top of the shaft. One set of fibers branches out across the surface of the shaft and into the urethra, providing sensation along the length. The other set travels directly to the glans.
The glans is densely packed with sensory nerve endings, functioning as what researchers describe as a “sensory end organ” for sexual reflexes. This concentration is why the head of the penis is far more sensitive to touch than the shaft. Beyond sensation, other nerve fibers control the smooth muscle that regulates blood flow, making erection and ejaculation possible.
Ligaments That Anchor the Penis
A significant portion of the penis actually extends inside the body, anchored to the pelvic bones by a system of ligaments. The suspensory ligament is a triangle-shaped band that connects the shaft to the spot where the left and right pelvic bones meet. It holds the penis close to the pelvis and supports the upward angle during an erection.
This ligament has three components. The fundiform ligament is a stretchy outer band of fat and connective tissue that wraps around the base of the penis. The suspensory ligament proper splits around a large vein on the top of the penis and reattaches deeper in the erectile tissue. Behind both sits the arcuate sub-pubic ligament, a thick band that provides additional structural reinforcement. Together, these ligaments allow the penis to lift and move during sex while keeping it firmly attached to the body.