The penis is made of spongy erectile tissue, smooth muscle, blood vessels, connective tissue, nerves, and skin. There are no bones inside a human penis. Instead, it relies entirely on blood pressure within specialized chambers to become rigid during an erection. Understanding what these tissues are and how they work together explains both normal function and why problems like erectile dysfunction happen.
Three Cylinders of Erectile Tissue
The core structure of the penis consists of three long cylinders of spongy tissue bundled together. Two of them, 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 tube that carries both urine and semen out of the body.
Inside the corpora cavernosa, the tissue looks less like a solid muscle and more like a dense sponge. It contains countless tiny blood-filled spaces (sinusoids) separated by columns of smooth muscle and connective tissue fibers. In a healthy adult, smooth muscle makes up roughly 35% to 40% of the total tissue area in these chambers. The rest is a scaffolding of collagen and elastic fibers that gives the tissue its shape.
The corpus spongiosum has a similar spongy structure but is built differently in one important way: its outer wall is thinner (less than 0.5 mm, compared to about 2 mm for the corpora cavernosa) and contains smooth muscle fibers that help propel semen during ejaculation. It also expands at the tip to form the glans, or head of the penis. Because its wall is thinner, the corpus spongiosum never becomes as rigidly pressurized as the two main chambers. This keeps the urethra open so fluid can pass through even during an erection.
The Tough Outer Casing
Each cylinder of erectile tissue is wrapped in a dense sheath called the tunica albuginea. This is one of the strongest connective tissues in the body, able to withstand internal pressure of roughly 1,200 to 1,500 mmHg before tearing. It consists of two layers: an outer layer with fibers running lengthwise and an inner layer with fibers running in a circular pattern. Both layers are made primarily of wavy collagen fibers with elastic fibers woven between them. Elastin makes up about 5% of the tunica and is what allows the penis to stretch and lengthen.
The wavy structure of these fibers is key to how the penis transitions between soft and hard. When the penis is flaccid, the collagen fibers are crimped and relaxed, letting the organ bend and move freely. As blood fills the chambers during arousal, the fibers gradually straighten and lock into place, creating a rigid structure that resists bending. Think of it like a fabric that’s loose and floppy when crumpled but stiff and strong when pulled taut.
Layers Outside the Erectile Tissue
Beyond the tunica albuginea, several more layers wrap around the penis like concentric sleeves. Buck’s fascia is a tough, deep layer that sits just outside the tunica and holds all three erectile cylinders together in separate compartments. It also encloses the major blood vessels and nerves that run along the shaft. Outside of that lies the dartos fascia, a thinner layer of tissue that connects to the fascia of the lower abdominal wall and scrotum. Finally, the outermost layer is the penile skin, which is notably thin and loose to accommodate changes in size.
The two corpora cavernosa are separated from each other by an internal wall called the septum. This wall is incomplete, especially near the tip, with small openings that allow blood to flow between the two chambers. This shared circulation is why both sides of the penis become equally firm during an erection.
How Blood Creates an Erection
Since humans have no penile bone (unlike many other mammals, which have a rigid bone called a baculum), erections depend entirely on hydraulics. Arteries running through the center of each corpus cavernosum branch into smaller vessels that open directly into the spongy sinusoidal spaces. When the smooth muscle in the arterial walls and the spongy tissue relaxes during arousal, blood rushes in and the chambers expand.
As the chambers swell, they press outward against the tunica albuginea. This compression flattens the small veins that normally drain blood out of the penis, trapping blood inside. The tunica stretches to capacity and squeezes these exit veins even further, driving internal pressure higher. Muscles at the base of the penis then contract to add a final boost, producing full rigidity. The entire process is a balance between inflow and restricted outflow, which is why anything that damages blood vessels, smooth muscle, or the connective tissue framework can lead to erectile problems.
About 28% to 35% of men have an accessory artery contributing to penile blood supply, which is a normal variation but becomes clinically relevant during pelvic surgeries where those extra vessels could be damaged.
Nerves and Sensation
The penis is densely supplied with nerves that serve two distinct roles. Sensory nerves, concentrated especially in the glans, detect touch, pressure, and temperature. These signals travel to the spinal cord and brain, producing the sensations associated with sexual stimulation. A separate set of autonomic nerves controls the involuntary processes behind erection and ejaculation: relaxing smooth muscle to allow blood inflow, triggering the rhythmic contractions of ejaculation, and returning the penis to its flaccid state afterward.
The main sensory nerve runs along the top of the shaft, while the nerves controlling erection travel deeper, close to the erectile bodies themselves. This separation matters because injuries or surgeries that affect one set of nerves don’t necessarily affect the other. A person could, for instance, lose the ability to feel sensation on the skin while still being able to get an erection, or vice versa.
Why Humans Have No Penile Bone
Most primates and many other mammals have a baculum, a small bone inside the penis that provides structural support for mating. Humans are one of the few species that lost this bone entirely, relying instead on the blood-pressure system described above. The evolutionary explanation is still debated, but one hypothesis published in Mammal Review proposes that a boneless penis was actually an advantage: a bone inside a flaccid penis would have been vulnerable to fracture from blunt trauma during physical competition between males. Individuals born without the bone would have avoided those injuries, giving them a reproductive edge over time.
The practical result is that the human penis is remarkably flexible when soft and remarkably rigid when erect, all without any skeletal support. That versatility comes from the interplay of smooth muscle, elastic connective tissue, and a pressurized vascular system working within a casing strong enough to handle the load.