If you could slice a penis in cross-section, you’d see three cylindrical chambers of spongy tissue, a central tube (the urethra), layers of tough wrapping, and a network of blood vessels and nerves. It looks nothing like solid muscle. The interior is closer to a dense, organized sponge designed to fill with blood under pressure.
The Three Erectile Chambers
The most prominent structures inside the penis are three long, tube-shaped chambers that run its full length. Two of these sit side by side along the top of the shaft. These are the corpora cavernosa, and they’re the main engines of an erection. Each one contains spongy tissue riddled with tiny hollow spaces, like a honeycomb. When blood rushes in and fills those spaces, the chambers expand and stiffen.
The two corpora cavernosa are separated by a thin wall called the septum, but it’s incomplete, so blood flows freely between them. They extend from the pubic bone all the way to the tip of the penis, and at the base they split apart and anchor to the pelvic bones for structural support.
The third chamber, the corpus spongiosum, runs along the underside of the shaft in a groove between the other two. It surrounds and protects the urethra, the tube that carries both urine and semen out of the body. The corpus spongiosum also fills with blood during an erection, but it stays softer and more flexible than the upper chambers. This matters: if it became fully rigid, it would squeeze the urethra shut and block ejaculation.
What the Spongy Tissue Is Made Of
The erectile chambers aren’t hollow balloons. They’re filled with a meshwork called trabecular tissue, a combination of smooth muscle, collagen, and elastic fibers. In young men without erectile problems, smooth muscle makes up roughly 40 to 52 percent of this tissue. That smooth muscle is what controls the process: when it relaxes, the tiny spaces within the mesh open up and blood pours in. When it contracts, blood drains out and the erection subsides.
The ratio of smooth muscle to connective tissue shifts with age, which is one reason erections change over a lifetime. Less smooth muscle means less expandability and a slower, weaker filling response.
The Protective Wrapping
Each erectile chamber is enclosed by a tough sheath called the tunica albuginea. This is a dense, fibrous coat made primarily of collagen and elastin, and it’s built in two layers. The inner layer runs in circles around the chamber and sends reinforcing pillars inward, like the struts of a bridge, to support the spongy tissue. The outer layer runs lengthwise along the shaft. This two-layer design makes the tunica both strong and slightly flexible.
The tunica around the two upper chambers averages about 2 millimeters thick, though it varies by location, ranging from 1 to over 3 millimeters. The tunica around the corpus spongiosum is much thinner, less than half a millimeter, which is what allows that chamber to stay softer during erection.
Outside the tunica, additional layers separate the erectile bodies from the skin. Buck’s fascia is a well-defined sheet that wraps tightly around all three chambers and carries the major blood vessels and nerves along its surface. Beyond that is the dartos fascia, a looser layer of tissue that slides freely over the structures beneath it. Then comes the dermis and skin. If you listed the layers from outside in, you’d have: skin, dermis, dartos layer, a thin fat layer, Buck’s fascia, tunica albuginea, and finally the erectile tissue itself.
Blood Supply and Hydraulic Pressure
An erection is a hydraulic event, and the internal plumbing reflects that. Arteries enter each corpus cavernosum through the base, running through the center of the spongy tissue. When the smooth muscle in the chamber walls relaxes, these arteries dilate and flood the tiny spaces with blood. The expanding tissue compresses the veins against the rigid tunica albuginea, trapping blood inside. This is what creates and maintains stiffness.
The pressure inside the chambers during a full erection reaches roughly 100 to 106 mmHg, which is comparable to normal systolic blood pressure. At rest, pressure sits around 30 to 35 mmHg. That jump from resting to full erection happens in under 30 seconds once the process starts. The dorsal vein, running along the top of the shaft just beneath Buck’s fascia, handles drainage when the erection ends.
The Urethra’s Path
The urethra runs concentrically through the center of the corpus spongiosum, like a tube within a tube. The segment inside the penis, sometimes called the spongy or penile urethra, is the longest portion of the entire male urethra. It begins where the urethra exits the pelvic floor, travels through the bulb at the base of the penis, continues along the full length of the shaft, and ends at the opening on the tip of the glans. Its position inside the protective, softer corpus spongiosum keeps it cushioned and open during both erection and ejaculation.
The Nerve Network
Several distinct nerves run through the interior of the penis, each with a different job. The dorsal nerves travel along the top of the shaft beneath Buck’s fascia and primarily handle sensation, carrying touch and pressure signals from the skin, glans, and urethra back to the brain and spinal cord.
The cavernous nerves enter the erectile chambers at the base of the penis and serve a completely different role. They trigger the chemical signal (nitric oxide) that tells smooth muscle to relax and allows blood to flow in. Without intact cavernous nerves, the mechanical process of erection can’t start, which is why surgeries near the prostate or pelvis carry a risk of erectile problems.
These two nerve systems aren’t entirely separate. At the base of the penis, the cavernous nerves send fibers that merge into the dorsal nerve, giving the dorsal nerve some autonomic control over blood flow to the glans and the spongy tissue around the urethra. This crossover means the nerve architecture inside the penis is more interconnected than it appears at first glance.