The human penis contains no bone, no cartilage, and no skeletal muscle. It is built almost entirely from specialized spongy tissue, blood vessels, connective tissue, nerves, and skin. Three internal cylinders of erectile tissue form the core structure, and the way blood flows into and gets trapped inside these cylinders is what produces an erection. Understanding what these tissues are and how they work together explains a lot about how the penis functions, why erections happen, and why things change with age.
The Three Cylinders of Erectile Tissue
If you could see a cross-section of the penis, you’d find three tube-shaped chambers running its length, bundled together like a small cluster of pipes. Two of these sit side by side on top: the corpora cavernosa (one on each side). Below them, centered along the underside, sits a single smaller cylinder called the corpus spongiosum.
The corpora cavernosa are the main structural players. They’re filled with a network of tiny, interconnected spaces, almost like a sponge. These spaces are lined with smooth muscle and surrounded by blood vessels. When blood floods into these spaces, the tissue expands and stiffens, producing an erection. The corpora cavernosa are responsible for most of the rigidity.
The corpus spongiosum has a different job. It surrounds the urethra, the tube that carries both urine and semen. During an erection, the corpus spongiosum fills with blood too, but it stays softer and more flexible than the corpora cavernosa. This keeps the urethra open so fluid can pass through. The corpus spongiosum also flares out at the tip of the penis to form the glans, or head.
The Tough Outer Casing
Wrapping around each of the corpora cavernosa is a dense, fibrous sheath called the tunica albuginea. This layer is what gives the penis its shape and structural integrity during an erection. It’s made almost entirely of collagen, with type I collagen accounting for about 95.5% of its protein content, along with smaller amounts of types III, VI, and XII. Elastic fibers are woven throughout as well, allowing the tissue to stretch and then return to its resting state.
The tunica albuginea isn’t uniformly stretchy. Mechanical testing shows it is significantly stiffer along its length than around its circumference. This means during an erection, the penis expands more in girth than it elongates, and resists bending or buckling. This structural property is central to how an erection works: the expanding spongy tissue inside pushes outward against a casing that can contain and direct that pressure.
The tunica also plays a critical role in trapping blood. As the internal spongy spaces fill and expand, they press the small veins that normally drain blood out of the penis flat against the inside of the tunica. This compression blocks the exit of blood, keeping the penis rigid for as long as arousal signals continue. When those signals stop, the smooth muscle contracts again, blood drains out, and the penis returns to its soft state.
Blood Supply and How Erections Work
The penis has an unusually rich blood supply for its size. The main source is the internal pudendal artery, which branches into three smaller arteries on each side. One of these, the cavernosal artery, runs through the center of each corpus cavernosum and gives off dozens of tiny corkscrew-shaped branches called helicine arteries. These helicine arteries open directly into the spongy spaces of the erectile tissue.
In its resting state, smooth muscle cells in the artery walls and within the spongy tissue itself stay contracted, limiting blood flow. The penis remains soft. When the brain sends arousal signals through the nervous system, chemical messengers cause that smooth muscle to relax. The arteries widen, blood rushes in, the spongy spaces expand, and the veins get pinched shut against the tunica. The whole sequence, from nerve signal to full erection, depends on healthy smooth muscle, intact blood vessels, and a tunica that can withstand the internal pressure.
Nerves and Sensation
The penis is one of the most densely innervated structures in the body, with two types of nerve function: autonomic nerves that control erection and ejaculation without conscious input, and sensory nerves that relay touch and pressure signals to the brain.
The glans is particularly sensitive due to a high concentration of specialized touch receptors. The foreskin, in uncircumcised men, also contains sensory receptors called Meissner’s corpuscles, which detect light touch and fine texture. The density of these receptors varies considerably from person to person and changes over a lifetime. Studies tracking Meissner’s corpuscle density in foreskin tissue show it peaks somewhere between adolescence and the late twenties, then declines steadily, dropping by roughly 90% by age 45 to 50.
Skin, Fascia, and Outer Layers
The outermost layer of the penis is skin, but it’s unlike skin elsewhere on the body. It’s notably thin, loosely attached, and highly elastic, which allows it to accommodate the significant size change between flaccid and erect states. There’s very little fat beneath it.
Just under the skin lies the dartos fascia, a thin layer of smooth muscle fibers that merges with the deeper skin layer without a clear dividing line. The dartos contracts in response to cold or touch, giving the penile skin its ability to wrinkle and tighten. Beneath the dartos are deeper fascial layers that bundle the three erectile cylinders together into a single unit, along with the arteries, veins, and nerves that serve them.
No Bone, Unlike Most Mammals
Many mammals, including dogs, bears, walruses, and most primates, have a bone inside the penis called a baculum. Humans do not. At some point in evolutionary history, a genetic change eliminated the baculum in human ancestors, and this trait became fixed across all human populations. The human erection relies entirely on blood pressure within the erectile tissue rather than any skeletal support, making it a purely hydraulic system. This is one reason cardiovascular health is so directly linked to erectile function.
How These Tissues Change With Age
The composition of penile tissue shifts gradually over a lifetime. Research on the corpus cavernosum shows a statistically significant decline in smooth muscle content with age, paired with an increase in collagen. The ratio of smooth muscle to collagen drops steadily as men get older. Since smooth muscle relaxation is what allows blood to flow in and fill the erectile tissue, less smooth muscle means less effective blood trapping and weaker erections.
Declining testosterone levels accelerate these changes. Lower testosterone is associated with structural changes in the nerves within the erectile tissue, reduced smooth muscle content, increased connective tissue deposits, and abnormal remodeling of the tunica albuginea. Specifically, elastic fibers get replaced by disorganized collagen, which reduces the tunica’s flexibility and tensile strength. These tissue-level changes are a major reason erectile function tends to decline with age, separate from any psychological or relationship factors.
Size: What the Data Shows
A 2023 meta-analysis in the World Journal of Men’s Health pooled data from 75 studies spanning 1942 to 2021, covering 55,761 men. The averages: 8.7 cm (about 3.4 inches) flaccid, 12.9 cm (5.1 inches) stretched, and 13.9 cm (5.5 inches) erect. There is wide natural variation around these numbers. Interestingly, the analysis found that average erect length increased by about 24% over the past 29 years, though the reasons for that trend remain unclear.