If you could slice a penis in cross-section, you’d see three cylindrical chambers of spongy tissue, a central tube (the urethra), a network of blood vessels and nerves, and tough outer wrapping holding it all together. It looks nothing like solid muscle. The interior is closer to a dense, blood-filled sponge surrounded by a thick fibrous casing.
The Three Cylinders
The core structure of the penis is built around three columns of erectile tissue, each running the length of the shaft. Two of these sit side by side along the top (when the penis is viewed from above) and are called the corpora cavernosa. The third runs along the underside and surrounds the urethra, acting as a protective cushion for it. In a cross-section, these three cylinders look like a triangular arrangement: two circles on top, one on the bottom.
The tissue inside these cylinders isn’t smooth or solid. It’s a mesh of tiny blood-filled spaces, often compared to the inside of a sponge. When the tissue is not engorged, these spaces are mostly collapsed and the tissue appears dense, pinkish-red, and somewhat rubbery. During an erection, the spaces fill with blood and the tissue becomes firm, darker in color, and visibly expanded.
The Outer Casing
Each of the two upper cylinders is wrapped in a tough, white, fibrous sheath called the tunica albuginea. This is the structure that gives the penis its rigidity during an erection, and it’s surprisingly engineered. It has two layers: an inner layer with fibers running in circles around the cylinder, and an outer layer with fibers running lengthwise along the shaft. This crisscross pattern provides flexibility when soft and remarkable strength when pressurized with blood.
From inside the tunica, small columns of tissue extend inward like support struts, connecting the walls and reinforcing the spongy tissue within. These internal pillars help maintain the shape of the cylinders under pressure. The tunica also has an elastic fiber network woven through it, allowing it to stretch during erection and snap back when blood drains out. This is the layer that tears during a penile fracture, which is why that injury produces a sudden pop, rapid swelling, and loss of rigidity.
The Urethra and Its Cushion
The urethra is a thin-walled tube that runs through the bottom cylinder. Its walls are made of a few delicate layers: a lining of specialized skin-like tissue, a layer of smooth muscle, and connective tissue around the outside. The spongy tissue surrounding it is softer and less rigid than the tissue in the two upper cylinders. This matters because the urethra needs to stay open during ejaculation, even when the rest of the penis is fully rigid. If it were encased in the same high-pressure tissue as the upper chambers, it would be squeezed shut.
The urethra’s section of the penis is the longest portion of the entire urethral tract, extending from the base of the shaft all the way to the opening at the tip.
Blood Supply and the Erection Mechanism
Running near the center of each upper cylinder is a single artery. These arteries branch off into smaller coiled vessels called helicine arteries, which open directly into the spongy spaces of the erectile tissue. The blood flow path goes: central artery, then helicine branches, then into the spongy spaces, then out through small veins near the outer wall, and finally through veins that pass through the tunica to exit the penis.
What makes erections work is a trapping mechanism. When the spongy spaces fill with blood, the expanding tissue compresses the small exit veins against the inside of the tunica albuginea, pinching them shut. At the same time, these tiny veins get stretched longer and narrower as the tissue expands, further restricting outflow. The result is that blood flows in freely but can barely escape, creating a pressurized, rigid chamber. This is why erections feel firm from the inside out: the entire cylinder is essentially a sealed hydraulic system.
Roughly 28 to 35 percent of people have an extra artery contributing to penile blood flow, which is a normal anatomical variation.
Nerves and Sensation
A major nerve runs along the top of each upper cylinder, just beneath the skin and above the tunica. These nerves carry sensation from the shaft and the glans. They contain two distinct groups of nerve fibers: one set travels straight to the tip, providing sensation to the glans, while the other branches sideways and downward into the lower cylinder and the urethral lining.
In about 73 percent of anatomical dissections, researchers found nerve branches that actually pierce through the tunica and enter the spongy tissue directly. This means sensation isn’t only skin-deep. The internal erectile tissue itself has nerve supply, which plays a role in the reflexes involved in ejaculation.
Ligaments That Anchor the Structure
The penis doesn’t just hang freely from the body. A strong ligament connects the shaft to the point where the left and right pelvic bones meet. This ligament attaches directly to the fibrous covering of the upper cylinders and holds the penis close to the pelvis. It’s also what supports the upward angle during an erection. A second, more superficial ligament wraps around the base and connects to the pelvic bones from a slightly different angle, adding further support.
Together, these ligaments mean that a significant portion of the penile shaft actually extends inside the body, anchored to the pelvic bone. What’s visible externally is only part of the total structure.
What Goes Wrong Internally
The most well-known internal injury is a penile fracture, which is a tear in the tunica albuginea. Since this sheath is what contains the pressurized blood during an erection, a rupture causes immediate loss of rigidity, significant internal bleeding, and swelling. In some cases, the tear extends into the urethra as well. This requires surgical repair with stitches to close the torn tunica.
A more gradual condition called Peyronie’s disease involves scar tissue (plaque) forming within the tunica albuginea itself. This plaque can develop anywhere along the length of the penis, typically after minor injuries that cause bleeding inside the tunica wall. As the scar tissue hardens, it pulls on surrounding tissue and creates a bend or curve during erection. The plaque replaces the normally elastic fibers with rigid scar tissue, so that section of the tunica can no longer stretch evenly with the rest.