The penis contains no bone, no cartilage, and very little fat. It’s built almost entirely from spongy erectile tissue, wrapped in layers of connective tissue and covered by unusually mobile skin. Three cylindrical columns of tissue run the length of the shaft, and the interplay between these columns, a network of blood vessels, and a tough outer casing is what allows the penis to shift between its soft and rigid states.
The Three Cylinders of Erectile Tissue
The core of the penis is made up of three tube-shaped chambers. Two of these, called the corpora cavernosa, sit side by side along the top of the shaft and do most of the work during an erection. The third, called the corpus spongiosum, runs along the underside and surrounds the urethra, the narrow tube that carries both urine and semen out of the body. The corpus spongiosum also expands at the tip to form the glans (the head of the penis).
All three chambers are filled with a mesh of tiny, interconnected spaces, somewhat like a sponge. During arousal, blood rushes into these spaces and inflates the tissue. The corpora cavernosa are the primary drivers of rigidity. As they fill, they compress the veins that would normally drain blood away, trapping it inside and keeping the penis firm. The corpus spongiosum stays softer by comparison, which keeps the urethra open.
What the Spongy Tissue Is Actually Made Of
Under a microscope, the erectile tissue inside the corpora cavernosa is roughly a 50/50 split between smooth muscle and collagen. One study measuring this directly found the average content was about 47% smooth muscle and 53% collagen. That ratio matters: the smooth muscle cells are what relax to let blood flood in during an erection, while the collagen fibers provide structural support. Elastic fibers are woven throughout as well, giving the tissue the ability to stretch and then snap back to its resting size.
As men age, the balance between smooth muscle and collagen can shift. Collagen tends to increase while smooth muscle decreases, which is one reason erections often become less firm over time. This isn’t a sudden change but a gradual remodeling of the tissue itself.
The Tough Outer Casing
Each of the three erectile chambers is wrapped in a dense, white sheath called the tunica albuginea. This layer is made of tightly packed collagen fibers and acts like a pressure jacket. When the spongy tissue inside fills with blood, the tunica albuginea resists stretching beyond a certain point, which is what converts blood pressure into actual rigidity. Without it, the erectile tissue would simply swell without becoming firm.
The tunica albuginea around the corpora cavernosa is notably thicker than the layer around the corpus spongiosum. This difference is intentional: thicker walls create more pressure and more rigidity in the main chambers, while the thinner wall around the spongiosum keeps it from compressing the urethra shut during an erection.
Skin and Fascial Layers
The outermost layer is skin, but penile skin is unusual. It has almost no subcutaneous fat beneath it and is only loosely attached to the structures underneath. This allows it to slide freely over the shaft, stretching during an erection and retracting when the penis is flaccid.
Between the skin and the erectile chambers sit two distinct fascial layers. The outer one, called the dartos fascia, contains smooth muscle fibers that help with thermoregulation (similar to how scrotal skin tightens in the cold). Its loose connection to both the skin above and the deeper tissue below is what gives penile skin its characteristic mobility. The inner layer, called Buck’s fascia, is denser and more firmly anchored. It wraps the erectile chambers in separate compartments and houses the main arteries, veins, and nerves that run along the top of the shaft.
Blood Supply and Nerves
Erections are a vascular event, so the penis has an especially rich blood supply. Paired dorsal arteries run along the top of the shaft, supplying blood to the skin and glans. Deeper cavernous arteries run through the center of each corpus cavernosum, and these are the ones that open wide during arousal to flood the erectile tissue.
The nerve supply is equally important. Signals traveling from the brain or from local sensory stimulation trigger the smooth muscle inside the erectile tissue to relax. This relaxation is what opens the floodgates for blood to rush in. Dorsal nerves running along the top of the shaft carry sensation from the skin, particularly from the glans, which has one of the highest concentrations of sensory nerve endings in the body.
No Bone, Unlike Most Mammals
One of the more surprising facts about human anatomy: most male mammals, including most other primates, have a bone inside the penis called a baculum. Humans lost this bone over the course of evolution. Instead, human erections rely entirely on blood pressure trapped within the erectile chambers, a purely hydraulic system. In very rare cases, small bits of bone have formed in the soft tissue of a human penis, but this is considered an abnormality, not a normal structure.
The absence of a bone means that human erections depend completely on cardiovascular health. The smooth muscle, collagen, blood vessels, and nerves all need to work together for the hydraulic system to function. This is why conditions that affect blood flow or nerve signaling, like diabetes or cardiovascular disease, so often affect erectile function.
Other Structures at the Surface
The glans contains small sebaceous glands, sometimes called Tyson’s glands, that appear as tiny openings on either side of the frenulum (the small ridge of tissue on the underside of the head). These produce a small amount of natural lubricant. The foreskin, in uncircumcised men, is a double-layered fold of skin and mucous membrane that covers the glans and contains its own network of sensory nerve endings.
Small lymphatic vessels also run throughout the penile tissue, draining fluid back toward the lymph nodes in the groin. These are normally invisible, though they can occasionally become temporarily swollen after vigorous activity, appearing as a firm cord-like structure along the shaft that resolves on its own.
Typical Size and Proportions
A meta-analysis covering over 55,000 men across 75 studies found that the average flaccid length is about 8.7 cm (3.4 inches), while the average erect length is about 13.9 cm (5.5 inches). Stretched flaccid length, which doctors sometimes use as a proxy for erect length, averaged 12.9 cm (5.1 inches). There is wide natural variation in both directions, and flaccid size is a poor predictor of erect size since the ratio of smooth muscle to collagen and the elasticity of the tunica albuginea vary considerably from person to person.